Table Of Contents

TABLE OF CONTENTS........................................................................................1

OVERVIEW...........................................................................................................3

INTRODUCTION TO HOST PLANT RESISTANCE TO INSECT PESTS OF RICE......................................................................................................................3

Introduction to Host Plant Resistance ot Insect Pests of Rice........................................3

Definition of Insect Host Plant Resistance......................................................................3

MECHANISMS OF RESISTANCE........................................................................4

Introduction to Mechanisms of Resistance.....................................................................4

Antixenosis.....................................................................................................................4

Antibiosis........................................................................................................................4

Tolerance........................................................................................................................6

BREEDING CROP VARIETIES WITH RESISTANCE TO INSECTS....................6

Introduction to Breeding Crop Varieties with Resistance to Insects...............................6

Identifying Sources of Resistance to a Target Pest........................................................7

Breeding Procedures......................................................................................................7

SUSTAINABLE USE OF RESISTANT VARIETIES.............................................9

Insect Adaptation to Resistant Varieties.........................................................................9

Major Genes and Minor Genes for Resistance............................................................10

Strategies for Using Major Genes.................................................................................10

INSECT RESISTANCE IN RICE.........................................................................10

Introduction to Insect Resistance In Rice.....................................................................10

Varietal Resistance to Brown Planthopper...................................................................11

Varietal Resistance to Asian Rice Gall Midge..............................................................14

Varietal Resistance to Stem Borers..............................................................................16

Varietal Resistance to Green Leafhopper....................................................................19

USING RESISTANT VARIETIES IN AN IPM PROGRAM..................................20

Responses to a Pest Outbreak.....................................................................................20

Farmer Knowledge and Use of Resistant Varieties......................................................21

EXERCISES, SUGGESTED READINGS, AND CONTRIBUTORS....................22

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Exercise: Seedbox Screening Test...............................................................................22

Establishing Insect Colonies............................................................................................22

Choice and Layout of Materials for Screening........................................................22

Insect Infestation and Damage Scoring......................................................................23

Suggested Reading......................................................................................................25

Contributors..................................................................................................................25

PRINT VERSION.................................................................................................26

Exercise: Choice and Layout of Materials for Screening...............................26

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OverviewIn this module, you will learn

· the advantages of host plant resistance as a component of integrated pest managment

· the three mechanisms of plant resistance to insects

· the basic steps in breeding rice varieties resistant to insects

· the differences between major resistance genes and minor resistance genes

· strategies to delay the adaptation of pests to resistant varieties

· the types of rice host plant resistance to planthoppers, stem borers, and gall midge

· aspects of using resistant varieties in an IPM program

· how to conduct a seedbox screening test for insect resistance

Introduction to Host Plant Resistance to Insect Pests of Rice

Introduction to Host Plant Resistance ot Insect Pests of RiceHost plant resistance to insects is an important component of integrated pest management (IPM) in rice and a topic that is studied by many entomologists and plant breeders. Most modern semidwarf varieties have been selected for increased resistance to one or more pests such as:

· brown planthopper (BPH) Nilaparvata lugens (Stål) (Homoptera: Delphacidae),· whitebacked planthopper (WBPH) Sogatella furcifera (Horvath) (Homoptera:

Delphacidae), · green leafhopper (GLH) Nephotettix virescens (Distant) (Homoptera: Cicadellidae), · Asian rice gall midge Orseolia oryzae (Wood-Mason) (Diptara: Cecidiomyiidae), and · yellow stem borer (YSB) Scirpophaga incertulas (Lepidoptera: Pyralidae).

Definition of Insect Host Plant ResistanceIn his classic textbook, R.H. Painter (1951) defined insect host plant resistance as: “the relative amount of heritable qualities possessed by the plant which influence the ultimate degree of damage done by the insect in the field.” Another definition is:"the ability of a variety to produce a larger crop of good quality than other varieties, under conditions of similar insect population levels and other environmental factors." Four important characteristics of host plant resistanceThe following are four important characteristics of host plant resistance:

· Resistance is heritable: it is controlled by one or more genes.· Resistance is measurable: its magnitude can be determined experimentally.· Resistance is relative: it can be measured only by comparison with a susceptible cultivar

of the same plant species.

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· Resistance is influenced by the environment: its magnitude is affected by abiotic factors (such as soil fertility and water availability) and biotic factors (such as infection by disease).

· Resistant varieties are a valuable component of an integrated pest management (IPM) program. Resistant varieties have several advantages over most chemical insecticides, including:o they have no additional cost to the farmero they are not harmful to the environmento they are usually compatible with biological control by natural enemies.

This course provides only a brief overview of host plant resistance to insects. If you are interested in more information on a particular topic, please refer to the Additional Reading section at the end of the course.

Mechanisms Of Resistance

Introduction to Mechanisms of ResistanceWhy are some cultivars more resistant to insects than others? Three mechanisms of resistance are recognized:

1. antixenosis

2. antibiosis

3. tolerance

To learn more about each of these mechanisms, click any of the above links or choose the appropriate mechanism from the navigation panel on the left side of the screen.

AntixenosisAntixenosis is the ability of a variety to repel insects, causing a reduction in oviposition or feeding. Antixenosis can be chemical or morphological. Some chemicals are volatile: they are released from plants into the air and are sensed by insects before they land. Other repellent chemicals are sensed or tasted by insects after they contact the plant or feed on it. Morphological features that can cause antixenosis include leaf hairiness and stem hardness. Antixenosis is generally measured by testing insect behavior, for example comparing the number of insects landing on or laying eggs on different test varieties. Antixenosis was formerly called nonpreference, and you may see this term in older books.

Antibiosis Antibiosis is the ability of a variety to reduce the survival, growth, or reproduction of insects that feed on it. Antibiosis is often caused by the production of toxic chemicals by the plant. Hundreds of chemicals that are toxic to insects have been identified from different species and cultivars of plants. Well-known examples include nicotine from tobacco (Fig. 1) and gossypol from cotton. Antibiosis may also be caused by large amounts of indigestible polymers such as lignin, which can reduce the nutritional value of the plant and increase the toughness of leaves and stems.

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Fig. 1 - Nicotine Only a few chemicals from rice have been demonstrated to contribute to antibiosis. One example is benzyl benzoate (Fig. 2), which is produced by some japonica varieties at the site of oviposition by planthoppers. This chemical can kill the eggs (Fig 3).

Fig. 2 - Benzyl benzoate

Fig. 3a - Healthy eggs of whitebacked planthopper in leaf sheath of the susceptible variety IR 24.

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Fig. 3b - Dead eggs in leaf sheath of the ovicidal variety Asominori. Photos copyright Dr. Masanori Yamasaki (yamasakm@agr.kyushu-u.ac.jp). Not to be reproduced without permission. Antibiosis can be measured by comparing the survival, size, fecundity, or rate of development of insects that have fed on different test varieties.

ToleranceTolerance is the ability of a variety to produce a larger crop of good quality than other varieties when being fed upon by similar numbers of insects. Tolerance can be conferred by general plant vigor or by an ability to produce new growth to replace damaged stems, leaves or roots. Tolerance is difficult to measure in experiments, because yield or plant growth must be compared while insect numbers or biomass are kept constant on all test varieties. Unlike antixenosis and antibiosis, tolerance does not affect the behavior or health of insects feeding on the plant.

Breeding Crop Varieties with Resistance to Insects

Introduction to Breeding Crop Varieties with Resistance to InsectsBreeding for insect resistance in rice has been an important activity at IRRI and many national institutions since the 1960s. Breeding resistant varieties begins with identifying sources of resistance in a collection of crop germplasm. Crop germplasm collections consist of traditional and improved varieties and wild and weedy species closely related to the crop. The International Rice Genebank at IRRI contains more than 82,000 accessions (entries) of Oryza sativa and more than 2000 accessions (entries) of 21 wild species of Oryza.

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Fig. 4 - Seeds in cold storage at the International Rice Genebank

Identifying Sources of Resistance to a Target PestTo identify sources of resistance to a target pest, it is sometimes necessary to screen large numbers of accessions from a germplasm collection. Many thousands of accessions have been screened to identify sources of resistance to major rice pests. Screening such large numbers of plants requires the development of efficient mass screening procedures, such as the seedbox tests used to screen seedlings for planthopper resistance (see section VII).

Breeding ProceduresDifferent breeding procedures are used for different types of crops, depending for example if they are self-pollinating or outcrossing. For rice, the pedigree method of breeding is most often used. Parents are carefully selected and crossed using hand pollination (Fig. 5). In a resistance breeding program, usually one parent is an improved variety and the other is a resistant variety or wild species. Several generations of crossing and selection are required to transfer the resistance to the improved parent and recover the desired characteristics of the improved parent (yield, grain quality, etc.). Tens of thousands of breeding lines are screened each year at IRRI for resistance to brown planthopper and green leafhopper.

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Fig. 5 - Making a cross between two rice varieties by hand pollination. Developments in plant biotechnology have led to new types of breeding procedures. In marker-assisted selection, DNA markers linked to traits of interest (such as genes for insect resistance) are used to improve the efficiency of breeding. DNA markers have also made it possible to analyze the number, effect, and chromosomal location of minor genes in plants, through a process called quantitative trait loci (QTL) analysis. The technique of genetic engineering allows any organism to serve as a source of resistance. For example, the bacterium Bacillus thuringiensis (Bt) is the source of gene encoding insecticidal toxins that have been used to produce transgenic rice resistant to stem borers.

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Fig. 6 - Production of Bt rice by the particle bombardment method. This is one of several methods that can be used to transfer novel genes to rice by genetic engineering.

Sustainable Use Of Resistant Varieties

Insect Adaptation to Resistant VarietiesJust as insect populations can become resistant or adapted to an insecticide, they can also become adapted to resistant varieties. A new resistant variety will usually provide good pest protection during the first few years after it is released to farmers. However, after several years, a resistant variety that has become popular in an area will sometimes suffer more and more insect damage. Why does this happen? When a variety with a new resistance gene is released, there is usually a small proportion of the pest population that has a genetic background which enables the insects to survive on the resistant variety. The offspring of these insects survive to reproduce and the proportion of adapted insects in the population increases each generation. In other words, the pest population evolves in response to selection by the resistant variety. The word biotype is sometimes used to describe insects of the same species that differ in adaptation to different resistant varieties. For example, “biotype 1” may be able to survive on

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variety A but not variety B, while “biotype 2” is able to survive on both varieties A and B.

Major Genes and Minor Genes for ResistancePlant resistance genes that confer high levels of insect resistance are called major genes. In contrast, minor genes confer lower levels of insect resistance. It is usually necessary to combine multiple minor genes in a variety to produce a moderate level of resistance. In varieties that contain several minor genes, each contributing a small amount of resistance, resistance is said to be polygenic. One obvious advantage of highly resistant varieties is that they will suffer less damage than moderately resistant varieties when pest populations are high. It is also much easier for plant breeders to produce a resistant variety with a single major gene than to produce a variety with numerous minor genes. However, polygenic resistance is usually more durable than major gene resistance, i.e. pest populations adapt more slowly to varieties with polygenic resistance. And in many cases, a moderate level of plant resistance is sufficient to keep pest populations from reaching levels that cause yield loss.

Strategies for Using Major GenesThe use of brown planthopper resistance genes in rice is an example of sequential release of resistance genes. Varieties with a new resistance gene were released when pest populations adapted to varieties containing an older resistance gene. Sequential release of genes can provide continued crop protection if varieties containing new resistance genes are available when older resistance genes are no longer effective. There are several disadvantages to the sequential release strategy. For example, new varieties are not always available in time to prevent yield losses from occurring when pests adapt to resistant varieties; and a new variety might not have all the desirable traits of the variety that is no longer effective. Therefore, various strategies have been proposed to delay the adaptation of pest populations to major resistance genes. One such strategy is pyramiding genes, i.e. combining two or more major resistance genes in single variety. Computer models indicate that, in most cases, pyramiding two genes will provide more durable resistance than will other possible strategies such as:

· releasing cultivars with each gene sequentially, · rotating cultivars with the two genes from year to year, or · growing a mosaic (i.e. planting some fields with a variety containing one gene and other

fields with a variety containing the second gene). Pests will adapt more slowly to a resistant variety (whether it contains one major gene or two or more pyramided major genes) if there are some susceptible plants in the area. These susceptible plants enable some non-adapted insects to survive in the pest population. If these non-adapted insects mate with the adapted insects (the resistant biotype), there will be a decrease in the rate at which the adapted insects increase in the population. The governments of the USA, Australia, and Canada require farmers who grow Bt cotton or maize to maintain fields of non-Bt cultivars (referred to as “refuges”)

Insect Resistance In Rice

Introduction to Insect Resistance In RiceBrown planthopper, green leafhopper, whitebacked planthopper, and Asian rice gall midge are

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rice pests for which major resistance genes have been found in rice germplasm and used in rice breeding programs. No major genes have been identified for resistance to stem borers, but much effort has been directed to increasing minor gene resistance to these pests. There has been comparatively little effort to breed for resistance to other pests such as leaffolders, whorl maggot, thrips, rice bug and hispa.

Varietal Resistance to Brown PlanthopperBPH resistant varieties released by IRRIAbout ten major genes for BPH resistance have been identified from traditional cultivars and wild rice species. Three of these major genes have been widely used in varieties produced by IRRI and other institutions.

Fig. 7a - Brown planthopper.

Fig. 7b - Rice field with hopperburn Table 1: BPH resistance in some IRRI varietiesVariety Year of release Major

resistance geneSource of gene

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Reaction to BPH biotype 1 2 3

IR8 1966 None S S SIR26 1973 Bph 1 Mudgo R S RIR36 1976 bph2 ASD 7 R R MRIR42 1977 bph2 ASD7 R R SIR56 1982 Bph3 Rathu

HeenatiR S R

IR64 1985 Bph 1 Mudgo R MR R S = susceptible, MR = moderately resistant, R = resistant Some varieties have added resistance caused by the presence of minor genes. Note that IR36 is moderately resistant to biotype 3 but IR42 is not, even though both varieties have the same major gene (bph2). Similarly, IR64 is moderately resistant to biotype2 but IR26 is not. IR36 and IR64 appear to show more durable resistance in the field. All three mechanisms of resistance (antibiosis, antixenosis, and tolerance) to BPH have been found in rice germplasm. Some cultivars have multiple mechanisms of resistance. The Bph3 gene is still frequently used in the elite rice lines that are distributed by IRRI to national breeding programs. The IRRI breeding program will soon begin to use one or more new BPH resistance genes. Evolution of BPH BiotypesThe reactions of a BPH population or greenhouse colony to the Bph1 and bph2 genes have been used to classify three biotypes of BPH (Table 1, Figs. 8a, 8b). Varieties with the Bph1 and bph2 genes were initially very effective when they were released in the 1970s. However, in some areas BPH populations began to cause damage to the new resistant varieties in as little as three years.

Fig. 8a - Resistance of rice to different biotypes of brown planthopper: Biotype 1 damages varieties with no major resistance genes. (From Pathak and Khan 1994)

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Fig. 8b - Resistance of rice to different biotypes of brown planthopper: Biotype 2 damages varieties with the Bph1 gene. Biotype 3 damages varieties with the bph2 gene. (From Pathak and Khan 1994)

Fig. 8c - Resistance of rice to different biotypes of brown planthopper: Biotype 3 damages varieties with the bph2 gene. (From Pathak and Khan 1994). Two factors contributed to the rapid adaptation of BPH populations to resistant varieties such as IR26 and IR42. First, in some areas the varieties were planted by almost all farmers, placing selection for adaptation on most of the local BPH population. Second, overuse of insecticide by farmers killed BPH natural enemies. As a result, BPH that were adapted to the varieties were able to rapidly increase in number. Which biotype occurs in my area?Many researchers and IPM specialists often wonder which BPH biotype occurs in their area. In most cases, there is no simple answer to this question. Varieties with Bph1, bph2, and Bph3 have been available in many rice growing areas for the past 20 years. After varieties with different resistance genes have been grown by farmers in an area, the local BPH population usually consists of a mixture of insects with different degrees of adaptation to different resistance genes. In such a situation, it is not possible to describe the population as being a particular biotype. However, it is possible to determine which resistance genes are still effective against the local BPH population (see sections VI and VII).

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Varietal Resistance to Asian Rice Gall MidgeMajor genes have been used in breeding for gall midge resistance since the 1970s. Resistant varieties with known major genes or uncharacterized sources of resistance have been widely grown in India, Sri Lanka, and southern China. Some of these varieties have been very popular with farmers and have provided up to ten years of excellent protection against gall midge damage.

Fig. 9a - Asian rice gall midge

Fig. 9b - Silver shoots caused by feeding of gall midge larvae. New biotypes of gall midge have arisen in areas where gall midge-resistant varieties have been popular. Six gall midge biotypes have been identified in India, four in China, and two in Sri Lanka. Other gall midge biotypes may exist in Thailand, Indonesia, Cambodia, Laos and Vietnam.

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Fig. 10 - Characterization of gall midge populations from four Asian countries. The prevailing biotype is indicated, where known. CB, Chinese biotype; IB, Indian biotype; SLB, Sri Lankan biotype. Grouping is based on DNA fingerprinting of gall midge populations. (Modified from Katiyar and Bennett 2001). Work is in progress to use marker-aided selection to pyramid multiple gall midge resistance genes in single cultivars. These gene pyramids should provide more durable resistance to gall midge. The number of years they remain effective will depend on several factors, including the proportion of fields that remain planted to susceptible varieties (refuges) in the areas where the varieties with gene pyramids are grown. Table 2. Gall midge resistance genes and their reaction against different biotypes in India and China (from Katiyar and Bennett 2001)a

aR = resistant, S = susceptible, MR = moderately resistant, - = reaction not known, * = gene not yet named.

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Varietal Resistance to Stem BorersMany thousands of rice accessions have been screened to identify donors with high levels of stem borer resistance, but none have been found. However, experiments have shown that some rice varieties have moderate levels of antibiosis or antixenosis to stem borers. Plant breeders and entomologists have worked for many years to transfer minor genes for stem borer resistance into improved varieties. The Indian variety TKM6 has been widely used as a donor for stem borer resistance.

Fig. 11a - Yellow stem borer adult

Fig. 11b - Stem borer larva

Fig. 11c - Whitehead caused by stem borer feeding. Semidwarf varieties generally have lower levels of stem borer damage in the field than do tall traditional varieties. Two factors that may partly explain this phenomenon are:

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· The stems of traditional varieties are longer and wider than those of semidwarf varieties,

and thus may provide a better habitat or greater food supply for stem borer larvae. · Semidwarf varieties have a greater number of tillers and panicles per plant. Therefore,

each damaged panicle is a smaller proportion of the grain produced. In addition, semidwarf varieties have a greater ability at the vegetative stage to replace damaged tillers by the fresh growth of new tillers. The ability to compensate for insect damage is a form of tolerance.

Fig. 12 - Traditional and semidwarf varieties Screening for stem borer resistance is more time consuming and less reliable than screening for resistance to planthoppers and leafhoppers for reasons including the following:

· Seedlings are too young for stem borer infestation, so older plants must be used. · Moderate levels of resistance are more difficult to quantify than high levels of resistance

caused by major genes.· Resistance at the reproductive stage is more important to preventing yield loss than is

resistance at the vegetative stage. When screening for resistance at the reproductive stage, different flowering times among test entries makes it difficult to make direct comparisons of resistance.

· Stem borer larvae have complex behavior. They bore into the plant at various locations (such as the leaf sheath and panicle neck), move between plant parts as they develop (such as from the leaf sheath to the stem lumen), and move from one tiller to another or from one plant to another as they develop. As a result, conclusions about relative resistance between entries are affected by the growth stage at which a plant is infested and the time after infestation at which data on plant damage or insect survival or development are made.

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Fig. 13 - Fresh stem borer entry hole with excreta. Bt riceBacillus thuringiensis (Bt), is a species of bacterium found in soil and other habitats in most parts of the world. Bt produces proteins that are highly toxic to some insects, and Bt has been used as a sprayable microbial insecticide for many years.

Fig. 14 - Photo of Bacillus thuringiensis spore and crystal, taken with an electron microscope. (Photo courtesy of Dr. Neil Crickmore). Using genetic engineering, Bt genes that code for the toxic proteins have been transferred to several crop plants, including rice. Greenhouse and field tests have shown that some Bt rice lines are highly resistant to stem borers. Thus, by use of genetic engineering it has been possible to achieve something that was not possible with conventional plant breeding. It is known that pest populations can evolve resistance to Bt toxins, just as they can become adapted to insecticides and varieties with major resistance genes. An important challenge for entomologists is to develop strategies to slow down the evolution of stem borer resistance to Bt toxins, so that Bt rice can be used sustainably. Most governments have established regulations that require extensive biosafety testing of genetically engineered crops. It is not yet known when Bt rice will be released to farmers. Field tests of Bt rice began in China in 1998 and in India in 2001.

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Varietal Resistance to Green LeafhopperNumerous major genes conferring resistance to GLH have been identified (Table). These genes confer antibiosis and/or antixenosis. GLH-resistant varieties have been highly effective in managing rice tungro disease (RTD), which is vectored by GLH. Almost all advanced breeding lines distributed by IRRI have resistance to GLH. RTD is caused by the joint action of two viruses, rice tungro bacilliform virus (RTBV) and rice tungro spherical virus (RTSV). When feeding on resistant varieties, GLH transmission of RTSV is greatly reduced. Transmission of RTBV may occur on resistant varieties, but both viruses must be transmitted for a plant to develop RTD. GLH populations can adapt to resistant varieties over time. The adapted GLH are able to transmit both viruses when feeding on the formerly resistant varieties. Numerous major genes conferring resistance to GLH have been identified (Table 3). These genes confer antibiosis and/or antixenosis. GLH-resistant varieties have been highly effective in managing rice tungro disease (RTD), which is vectored by GLH (Fig. 15). Almost all advanced breeding lines distributed by IRRI have resistance to GLH.

Fig. 15a - Green leafhopper.

Fig. 15b - Tungro-infected rice.

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Using Resistant Varieties in an IPM Program

Responses to a Pest OutbreakIPM specialists are often called upon to recommend resistant varieties when a pest outbreak occurs in their geographic area. This section describes how they can prepare for and respond to such situations. In response to a pest outbreak it is often useful to test the resistance of locally-available varieties, particularly those that are already popular with farmers or have characteristics (e.g., duration, grain quality) that might make them acceptable. In the case of BPH, WBPH, and GLH, the seedbox screening test (described in section VII) can be used to rapidly determine the degree of resistance of numerous varieties. Seedbox tests can also be conducted for gall midge resistance but may take longer to complete because this insect is difficult to rear and can be collected in sufficient numbers only at some times of the year. Surveys of pest damage in farmers’ fields during an outbreak can also provide valuble information on the relative resistance of varieties in local use. The results of a survey may indicate that some varieties have been damaged less than others.

Fig. 16 - Conducting a farmer survey. Once highly or moderately resistant varieties have been identified through seedbox tests or field surveys, recommendations can be provided to seed boards and farmers. However, it may take one or more seasons for sufficient seed supplies of the recommended varieties to become available. Therefore, in some areas it is useful to conduct seedbox screening tests of varieties against locally-important pests every year, as a routine activity. If this is done, then valuable information is immediately available if a pest outbreak occurs. If no suitable varieties are identified that are resistant to a local pest population, then it will be necessary to develop new varieties with new resistance genes. It can take several years to cross the resistance from the donor into a local variety, depending on whether the donor is an improved variety or a traditional variety or wild species. Donors with known major genes for resistance to BPH, GLH, and WBPH are shown in Table 3. For information on gall midge, see Table 2. Finally, it is important to note that resistant varieties are sometimes of limited use in managing a pest outbreak. The cause of the outbreak might not be the spread of a new biotype adapted to popular cultivars. Overuse of broad-spectrum insecticides (which kill natural enemies) or unusual weather patterns are frequent causes of pest outbreaks. IPM specialists should seek to identify

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the root cause of an outbreak, even if switching to resistant varieties appears to help control it. Table 3. Major genes for leafhopper and planthopper resistance in rice (from Pathak and Khan 1994)a

aDominant genes begin with an upper case letter; recessive genes with a lower case letter.

Farmer Knowledge and Use of Resistant VarietiesOne of the main objectives of producing resistant varieties is to provide farmers with an alternative to insecticides for management of insect pests. This objective will not be met if farmers do not know what resistant varieties are, or if they do not know which varieties are resistant to which pests. It is often observed that farmers continue to apply insecticides to resistant varieties to control the very pests that the varieties have been produced to resist. The topic of host plant resistance should be included in IPM training programs for farmers. Also, information should be made available on the pest resistance of locally-available varieties, and the information should be updated as needed.

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Fig. 17 - Farmers conducting an experiment at a farmer field school. Sometimes farmers will grow a susceptible variety even when a pest causes yield loss in their fields, and they know that a resistant variety is available. This can occur when the susceptible variety is strongly preferred because it has better grain quality, higher yield potential, or other important characteristics. In such situations, farmers might switch to a resistant variety only when the pest problem becomes severe.

Exercises, Suggested Readings, and Contributors

Exercise: Seedbox Screening TestThe section describes the standard seedbox screening test for resistance to leafhoppers and planthoppers. For a more detailed description of the procedure, see Heinrichs et al. (1984).

Establishing Insect ColoniesSeedbox tests should be infested with insects of the same age (usually second and third instar nymphs) and therefore it is more convenient to obtain the insects from a greenhouse colony than directly from the field. A greenhouse colony should be established from at least 50 females collected from the local area. Collect the insects from several fields. The objective is to establish a greenhouse colony that is representative of the local insect population, especially in terms of adaptation to different resistance genes. Fresh field-collected insects should be added to greenhouse colonies at least once per year. The colony should be reared on a variety that has no major genes for resistance. Taichung Native 1 (TN1) is often used to maintain colonies of BPH and GLH. Subcolonies of insects of almost uniform age can be obtained by infesting fresh plants (approximately 45 days old) in a cage with male and female adults for one or two days, then removing the adults.

Choice and Layout of Materials for ScreeningThe screening test should contain varieties being tested for their resistance to the local population and susceptible check (e.g., TN1) and resistant check (if known). The Indian variety Ptb33 is often used as a resistant check for BPH. The choice of test varieties will depend on the purpose of the experiment (see Section VI, Responses to a pest outbreak). Up to 39 test entries and a resistant and susceptible check can be conveniently tested in a seedbox of dimensions 60×40×10 cm. If possible, the experiment should be replicated three to four times, either by setting up multiple seedboxes or, if the number of entries is not too large, but having multiple rows of each entry within one box. The locations of the entries should be randomized differently in each

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replicate.

Fig. 18 - Placement of test (T) and check entries (R=resistant, S=susceptible) in a standard seedbox screening test.

Fig. 19a - Tool for marking soil in seedbox, to guide locations of seed sowing.

Fig. 19b - Soil after being marked.

Insect Infestation and Damage ScoringAt seven days after sowing, when the seedlings are at the three-leaf stage, transfer the seedboxes to a pan or galvanized iron tray filled with water to a depth of 5 cm, and infest the seedlings with second- and third-instar nymphs. For BPH, the rate of infestation should be 10 nymphs per seedling. For WBPH and GLH, the rate should be 8 and 3 nymphs per seedling,

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respectively. A convenient method to uniformly infest the seedbox is to hold a pot of infested plants by the base and pass the plants over the seedbox while blowing on them or tapping lightly.

Fig. 20 - Infesting seedlings with hoppers.

Fig. 21 - BPH damage scale of entries in a seedbox screening test. When the seedlings of the susceptible check are about 90% dead, rate the test entries according to the damage scales of the Standard Evaluation System for Rice (Table 4, Fig. 20). Table 4. Standard Evaluation System for Rice for seedbox tests with brown planthopper, whitebacked planthopper, and green leafhopper.

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Suggested ReadingGeneral

Heinrichs EA. 1994. Host plant resistance. In: Heinrichs, EA, editor. Biology and Management of Rice Insects. New Delhi: Wiley Eastern Limited. P 517-548. Heinrichs EA, Medrano FG, Rapusas HR. 1985. Genetic Evaluation for Insect Resistance in Rice. Manila (Philippines): IRRI. Painter RH. 1951. Insect Resistance in Crop Plants. New York: MacMillan. Panda N, Khush GS. 1995. Host Plant Resistance to Insects. Oxon (UK): CAB International, and Manila (Philippines): IRRI. Pathak MD and Khan ZR. 1994. Insect pests of rice. Manila (Philippines): IRRI.

Bt rice

Cohen MB, Gould F, Bentur JS. 2000. Bt rice: practical steps to sustainable use. International Rice Research Notes 25(2): 4-10. Tu J, Zhang G, Datta K, Xu C, He Y, Zhang Q, Khush GS, Datta SK. 2000. Field performance of transgenic elite commercial hybrid rice expressing Bacillus thuringiensis d-endotoxin. Nature Biotech. 18:1101-1104.

Marker-aided selectionKatiyar SK, Bennett J. 2001. Biotechnology for gall midge resistance: from molecular tagging to gene pyramiding. pp. 369-378 in Peng S, Hardy B, editors. Rice research for food security and poverty alleviation. Proceedings of the International Rice Research Conference, 31 March-3 April 2000, Los Banos, Philippines. Los Banos (Philippines): International Rice Research Institute. Resistance to GLHDahal G, Hibino H, Cabunagan RC, Tiongco ER, Flores ZM, Aguiero VM. 1990. Changes in cultivar reactions to tungro due to changes in “virulence” of the leafhopper vector. Phytopathology 80:659-665.

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ContributorsContribution PersonWriter Mike CohenContent Editor Mike CohenCourse Coordinator Zahirul Islam, Training Center, IRRI, PhilippinesProgramming and editing Albert Dean Atkinson

Print VersionHost Plant Resistance to Insect Pests of Rice may also be completely printed, provided you have a printer attached to your computer and Microsoft Word. Click here to launch the entire contents of this course in Microsoft Word.

Exercise: Choice and Layout of Materials for ScreeningThe screening test should contain varieties being tested for their resistance to the local population and susceptible check (e.g., TN1) and resistant check (if known). The Indian variety Ptb33 is often used as a resistant check for BPH. The choice of test varieties will depend on the purpose of the experiment (see Section VI, “Responses to a pest outbreak.”) Up to 39 test entries and a resistant and susceptible check can be conveniently tested in a seedbox of dimensions 60×40×10 cm (Figs. 18-19). If possible, the experiment should be replicated three to four times, either by setting up multiple seedboxes or, if the number of entries is not too large, but having multiple rows of each entry within one box. The locations of the entries should be randomized differently in each replicate.

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Index

A

Abiotic............................................................................................................................................ 3Additional Reading....................................................................................................................... 3

refer............................................................................................................................................ 3Aguiero VM.................................................................................................................................. 25And/or antixenosis...................................................................................................................... 19Antibiosis...................................................................................................................................... 4Antixenosis....................................................................................................................... 4, 11, 16AR................................................................................................................................................ 14ASD.............................................................................................................................................. 11ASD7............................................................................................................................................ 11Asian........................................................................................................................................ 3, 10Asian Rice Gall Midge................................................................................................................14

Varietal Resistance................................................................................................................. 14Australia...................................................................................................................................... 10

B

Bacillus thuringiensis.............................................................................................................7, 16Bacillus thuringiensis d-endotoxin...........................................................................................25

expressing............................................................................................................................... 25Bennett 2001............................................................................................................................... 14Bennett J..................................................................................................................................... 25Bentur JS..................................................................................................................................... 25Biomass......................................................................................................................................... 6Biosafety...................................................................................................................................... 16Biotype......................................................................................................................... 9, 10, 11, 20Biotype 1........................................................................................................................................ 9Biotype 2........................................................................................................................................ 9Biotype2....................................................................................................................................... 11Biotypes................................................................................................................................. 11, 14

BPH........................................................................................................................................... 11BPH.............................................................................................................................. 3, 11, 20, 22

biotypes................................................................................................................................... 11test............................................................................................................................................ 22

BPH biotype................................................................................................................................ 11BPH Biotypes.............................................................................................................................. 11Bph1............................................................................................................................................. 11Bph2............................................................................................................................................. 11Bph3............................................................................................................................................. 11Breeding........................................................................................................................................ 7

Procedures................................................................................................................................ 7Breeding Crop Varieties...............................................................................................................6

Introduction............................................................................................................................... 6Brown planthopper...............................................................................................................10, 11

Varietal Resistance................................................................................................................. 11Bt.................................................................................................................................. 7, 10, 16, 25

Producing.................................................................................................................................. 7tests.......................................................................................................................................... 16

27

C

CAB International....................................................................................................................... 25Cabunagan RC............................................................................................................................ 25Cambodia.................................................................................................................................... 14Canada......................................................................................................................................... 10Cecidiomyiidae............................................................................................................................. 3Characteristics..............................................................................................................3, 7, 20, 21China...................................................................................................................................... 14, 16Cicadellidae................................................................................................................................... 3Cohen MB.................................................................................................................................... 25Conduct......................................................................................................................................... 3

seedbox...................................................................................................................................... 3Crop germplasm........................................................................................................................... 6Crop Plants.................................................................................................................................. 25Cultivar.................................................................................................................................... 3, 25Cultivars.................................................................................................................4, 10, 11, 14, 20

releasing.................................................................................................................................. 10rotating..................................................................................................................................... 10

D

Dahal G........................................................................................................................................ 25Datta K......................................................................................................................................... 25Datta SK....................................................................................................................................... 25Definition....................................................................................................................................... 3

Insect Host Plant Resistance...................................................................................................3Delphacidae................................................................................................................................... 3Diptara........................................................................................................................................... 3DNA................................................................................................................................................ 7

E

Exercise....................................................................................................................................... 22Expressing.................................................................................................................................. 25

Bacillus thuringiensis d-endotoxin........................................................................................25

F

Farmer Knowledge...................................................................................................................... 21Farmers’....................................................................................................................................... 20Field............................................................................................................................................... 3Flores ZM..................................................................................................................................... 25

G

Genetic Evaluation...................................................................................................................... 25Insect Resistance....................................................................................................................25

Germplasm.................................................................................................................... 6, 7, 10, 11GLH.............................................................................................................................. 3, 19, 20, 25GLH-resistant.............................................................................................................................. 19Gossypol....................................................................................................................................... 4Gould F........................................................................................................................................ 25Green Leafhopper....................................................................................................................... 19

Varietal Resistance................................................................................................................. 19

H

Hardy B........................................................................................................................................ 25Heenati......................................................................................................................................... 11

28

Heinrichs..................................................................................................................................... 25Heinrichs EA............................................................................................................................... 25Hibino H....................................................................................................................................... 25Hispa............................................................................................................................................ 10Homoptera..................................................................................................................................... 3Hopperburn................................................................................................................................. 11Horvath.......................................................................................................................................... 3Host Plant Resistance................................................................................................................25

Insects...................................................................................................................................... 25Host Plant Resistance ot Insect Pests........................................................................................3

Introduction............................................................................................................................... 3

I

Identifying...................................................................................................................................... 7Sources...................................................................................................................................... 7

India....................................................................................................................................... 14, 16Indian........................................................................................................................................... 16Indonesia..................................................................................................................................... 14Insect Adaptation.......................................................................................................................... 9

Resistant Varieties.................................................................................................................... 9Insect Host Plant Resistance.......................................................................................................3

Definition.................................................................................................................................... 3Insect Resistance....................................................................................................................... 25

Genetic Evaluation.................................................................................................................. 25Insect Resistance In Rice...........................................................................................................10

Introduction............................................................................................................................. 10Insects..................................................................................................................................... 6, 25

Host Plant Resistance.............................................................................................................25Resistance................................................................................................................................. 6

International Rice Genebank.......................................................................................................6International Rice Germplasm Collection...................................................................................6International Rice Research Conference..................................................................................25International Rice Research Institute........................................................................................25International Rice Research Notes 25.......................................................................................25Introduction.....................................................................................................................3, 4, 6, 10

Breeding Crop Varieties...........................................................................................................6Host Plant Resistance ot Insect Pests....................................................................................3Insect Resistance In Rice.......................................................................................................10Mechanisms............................................................................................................................... 4

IPM............................................................................................................................... 3, 11, 20, 21IR26.............................................................................................................................................. 11IR36.............................................................................................................................................. 11IR42.............................................................................................................................................. 11IR56.............................................................................................................................................. 11IR64.............................................................................................................................................. 11IR8................................................................................................................................................ 11IRRI........................................................................................................................... 6, 7, 11, 19, 25

K

Katiyar.......................................................................................................................................... 14Katiyar SK.................................................................................................................................... 25Khan............................................................................................................................................. 20Khush GS.................................................................................................................................... 25Khush p....................................................................................................................................... 11

29

L

Laos............................................................................................................................................. 14Leaffolders.................................................................................................................................. 10Lepidoptera................................................................................................................................... 3Locally-available................................................................................................................... 20, 21Locally-important........................................................................................................................ 20Los Banos................................................................................................................................... 25

M

MacMillan..................................................................................................................................... 25Major Genes................................................................................................................................ 10Management................................................................................................................................ 25

Rice Insects............................................................................................................................. 25Manila........................................................................................................................................... 25Marker-assisted............................................................................................................................ 7Mechanisms.................................................................................................................................. 4

Introduction............................................................................................................................... 4Medrano FG................................................................................................................................. 25Midge-resistant........................................................................................................................... 14Minor Genes................................................................................................................................ 10

Resistance............................................................................................................................... 10MR.......................................................................................................................................... 11, 14Mudgo.......................................................................................................................................... 11

N

Nature Biotech............................................................................................................................ 25Nephotettix virescens...................................................................................................................3New biotypes............................................................................................................................... 14New Delhi..................................................................................................................................... 25New York..................................................................................................................................... 25Nilaparvata lugens........................................................................................................................ 3Non-Bt.......................................................................................................................................... 16Non-Bt cultivars.......................................................................................................................... 10

O

Orseolia oryzae............................................................................................................................. 3Oryza.............................................................................................................................................. 6Oryza sativa................................................................................................................................... 6Outcrossing................................................................................................................................... 7Overview........................................................................................................................................ 3Ovipositing.................................................................................................................................... 4Oviposition.................................................................................................................................... 4Oxon............................................................................................................................................. 25

P

Painter RH................................................................................................................................... 25Panda N....................................................................................................................................... 25Pathak.......................................................................................................................................... 20Peng S.......................................................................................................................................... 25Pest Outbreak............................................................................................................................. 20

Responses............................................................................................................................... 20Philippines................................................................................................................................... 25Phytopathology 80...................................................................................................................... 25Planthopper...................................................................................................................3, 7, 10, 22

30

Planthoppers....................................................................................................................... 3, 4, 16Polygenic..................................................................................................................................... 10Procedures.................................................................................................................................... 7

Breeding..................................................................................................................................... 7Producing...................................................................................................................................... 7

Bt................................................................................................................................................ 7Pyralidae........................................................................................................................................ 3

Q

QTL................................................................................................................................................ 7

R

Rapusas HR................................................................................................................................. 25Rathu............................................................................................................................................ 11Recommendations...................................................................................................................... 20Refer............................................................................................................................................... 3

Additional Reading....................................................................................................................3Refuges........................................................................................................................................ 10Releasing..................................................................................................................................... 10

cultivars................................................................................................................................... 10Resistance....................................................................................................................... 4, 6, 7, 10

Insects........................................................................................................................................ 6Minor Genes............................................................................................................................ 10Target Pest................................................................................................................................. 7

Resistant Varieties.................................................................................................................. 9, 21Insect Adaptation...................................................................................................................... 9

Responses................................................................................................................................... 20Pest Outbreak.......................................................................................................................... 20

Rice.......................................................................................................................................... 3, 25Rice Insects................................................................................................................................. 25

Management............................................................................................................................ 25Rotating....................................................................................................................................... 10

cultivars................................................................................................................................... 10RTBV............................................................................................................................................ 19RTD.............................................................................................................................................. 19RTSV............................................................................................................................................ 19

S

Scirpophaga incertulas................................................................................................................3Seedbox.............................................................................................................3, 7, 10, 11, 20, 22

conduct...................................................................................................................................... 3Seedbox Screening Test............................................................................................................22Semidwarf................................................................................................................................ 3, 16Sogatella furcifera........................................................................................................................ 3Sources.......................................................................................................................................... 7

Identifying.................................................................................................................................. 7Sprayable..................................................................................................................................... 16Sri Lanka...................................................................................................................................... 14Stål................................................................................................................................................. 3Stem Borers................................................................................................................................ 16

Varietal Resistance................................................................................................................. 16Stemborer.................................................................................................................................... 16Strategies.................................................................................................................................... 10

Using Major Genes..................................................................................................................10Suggested Reading....................................................................................................................25

31

Sustainably.................................................................................................................................. 16

T

Table............................................................................................................................................ 20Target Pest.................................................................................................................................... 7

Resistance................................................................................................................................. 7Tests...................................................................................................................................... 16, 22

BPH........................................................................................................................................... 22Bt.............................................................................................................................................. 16

Thailand....................................................................................................................................... 14Thrips........................................................................................................................................... 10Tiongco ER.................................................................................................................................. 25TKM6............................................................................................................................................ 16Tolerance....................................................................................................................................... 6Transgenic............................................................................................................................... 7, 25Tungro................................................................................................................................... 19, 25Tungro-diseased......................................................................................................................... 19

U

Uncharacterized.......................................................................................................................... 14USA.............................................................................................................................................. 10Use............................................................................................................................................... 21

Resistant Varieties..................................................................................................................21Using Major Genes.....................................................................................................................10

Strategies................................................................................................................................. 10

V

Valuable....................................................................................................................................... 20Varietal Resistance...................................................................................................11, 14, 16, 19

Asian Rice Gall Midge.............................................................................................................14Brown Planthopper................................................................................................................. 11Green Leafhopper................................................................................................................... 19Stem Borers............................................................................................................................. 16

Vietnam........................................................................................................................................ 14Virulence...................................................................................................................................... 25

W

WBPH....................................................................................................................................... 3, 20Which biotype............................................................................................................................. 11Whitebacked planthopper......................................................................................................3, 10Wiley Eastern Limited................................................................................................................. 25Wood-Mason................................................................................................................................. 3

X

Xu C.............................................................................................................................................. 25

Y

YSB................................................................................................................................................ 3

Z

Zhang G....................................................................................................................................... 25Zhang Q....................................................................................................................................... 25

32