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8 Important pestsImportant pestsImportant pestsImportant pestsImportant pestsof citrus in Asiaof citrus in Asiaof citrus in Asiaof citrus in Asiaof citrus in AsiaThere are a large number of pests which attack citrus. They rangein size from the large longicorn beetle, to tiny mites and aphidswhich can hardly be seen at all without a magnifying glass. There isnot enough space in this book to show all the pests which attackcitrus trees in Asian countries, but this chapter gives a list of themost important ones. All of them are major pests in several Asiancountries. Some of them, such as the green citrus aphid (left), arepests in every country where citrus trees are grown.

BEETLESWhite-spotted longicorn beetle Anoplophora macularia (Thomson)

BUGSCitrus stinkbug = shield bug Rhynchocoris humeralis Thunberg

THRIPSCitrus thrips Scirtothrips dorsalis Hood

APHIDSCitrus aphid = brown citrus aphid = tropical citrus aphid Toxoptera citricida (Kirkaldy)Green citrus aphid = spiraea aphid Aphis citricola van der GrootCotton aphid = melon aphid Aphis gossypii GloverBlack citrus aphid Toxoptera aurantii (Boyer

de Fonscolombe)PSYLLIDSAsian citrus psyllid Diaphorina citri Kuwayama

MEALYBUGSCitrus mealybug Planococcus citri (Risso)Spherical mealybug Nipaecoccus filamentosus

(Cockerell)

SCALESGreen scale Coccus viridis GreenCottony-cushion scale Icerya purchasi (Maskell)Florida red scale Chrysomphalus aonidum (Linnaeus)Purple scale = mussel scale Lepidosaphes beckii (Newman)Chaff scale Parlatoria pergandii Comstock

MOTHS AND BUTTERFLIESCitrus leafminer Phyllocnistis citrella StaintonCitrus leafroller Homona coffearia Nietner

MITESCitrus rust mite Phyllocoptruta oleivora (Ashmead)Citrus red spider mite Panonychus citri McGregor

FRUIT FLYOriental fruit fly Bactrocera dorsalis Hendel

Note to species names

Each species has a uniquename, shown in italics e.g.Anoplophora macularia. Thethird word at the end is thename of the author who wasthe first person to use thespecies name. If there has been a changein the species name since hefirst used it, the name of theauthor is shown inparentheses e.g. (Thomson)

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White-spotted Longicorn Beetle (Anoplophora macularia (Thomson))

Distribution

Widely distributed in China, Japan,Korea, Taiwan.

Damage

Adult females lay their eggs incracks at the base of the tree. Theeggs hatch after about ten days,and the larvae immediately boreinto the wood (Fig. 8-2). The holesand tunnels made in the wood bythe larvae cause tree decline, andeventually wilting. Weakenedbranches may break off.

Furthermore, moisture anddisease-causing fungi can enterthrough the damaged parts of thetree. Infested trees are thus proneto secondary attack by otherinsects, or by diseases.

The larvae cannot be seensince they are inside the wood, butsawdust and debris may be seenaround the base of an infested

tree. Egg-laying cavities eaten outby the females are found in thebark, and at the junction ofbranches and the trunk.

Life cycle

The typical life cycle for this pest is:

Egg —> Larva —> Pupa —> Adult

The beetle can overwinter as anegg, a larva or a pupa. Adultbeetles begin to emerge in theearly summer months throughsmall holes in the bark. Peakpopulations are found in themiddle of summer. Females live14-66 days, males live 3-50 days.

Description

The larvae of this beetle is a whitegrub with a dark brown or blackhead. Most of the damage to citrustrees is done by the larvae ratherthan the adult beetles.

The adult is black and shiny.It is a large, conspicuous insectwith very long antennae (“feelers”)(Fig. 8-1). The antennae of themales are 2.5 times their bodylength. The antennae of the femalebeetles are 1.3 times theirbody length.

Fig. 8-1. Adult white-spottedlongicorn beetle

Fig. 8-2. Larva of white-spotted longicornbeetle tunneling through woodof citrus tree

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Distribution

Widely distributed in SoutheastAsia.

Description

The citrus stinkbug is brightgreen and broadly oval in outline.The adult female is around 1.5cm in length, while the adult maleis smaller.

Stinkbugs are named fortheir ability to exude a stinkingsubstance from a pore on eachside of their body. They can alsobe recognized by the distinctivetriangular shape of their bodies.The adult has a prominent spineon each shoulder of the thorax.Because their backs are shapedlike a shield, stink bugs aresometimes given the name of“shield bugs”.

Damage

The mouth of the stinkbug is likea long, pointed beak. Citrusstinkbugs feed on plant fluids byinserting their mouthparts intothe stems, leaves and fruit. Thisdamages the leaves and fruit,rather as if they have beenstabbed repeatedly by a fineneedle (Fig. 8-3).

Both the adults andnymphs of this bug suck thejuices from young citrus shootsand fruit, especially at night.Young shoots become mottledand their growth is retarded.Feeding by stinkbugs oftencauses premature color breakand fruit drop, as well asproviding access for variousfungal diseases and insects.Fruits which have been attackedshow brown patches on the skin.These discolored patches makethe fruit difficult to market.

The stinkbug is mostdestructive to satsumas,tangerines, and early andmidseason oranges. Stinkbugdamage is seldom seen ongrapefruit or Valencia oranges,because of their thicker rind.

Life cycle

The typical life cycle for this pestis shown below:

Citrus stinkbug (=shield bug) (Rhynchocoris humeralis Thunberg)

Fig. 8-3. Adult and nymph stink bugs on citrus

Fig. 8-4. Eggs of stink bug

eggs (Fig. 8-4). The eggshatch out after about one week.

The young nymphsremain clustered together for ashort period of time and thenstart feeding on the youngshoots and fruits. They passthrough five nymphal stagesand become adults after about6 to 8 weeks.

Egg —> Nymph—> 1st Instar Larva —>2/3/4th Instar Larva —> Adult

The female deposits her eggs onthe leaves in clusters of 9 to 14

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Citrus thrips (Scirtothrips dorsalis Hood)

Fig. 8-6. Scarring on fruit caused by feeding by thrips

Fig. 8-7. Damage to leaves from citrus thrips

Distribution

Widely distributed in SoutheastAsia.

Description

Thrips are very small, slenderinsects, less than 0.1 cm long(Fig. 8-5). Adult thrips have twopairs of narrow wings which arefringed with hairs.

Damage

Thrips feed by puncturing theirhost plant and sucking up theplant juices. They prefer to feed

on young plant tissue, such asshoots, blossoms and youngfruit. On fruit, they like to feed inprotected areas such as under thecalyx.

Citrus thrips, particularly theimmature thrips, feed on the rindof the fruit and cause scarring (Fig.8-6). A typical pattern is a ring ofscarring at the stem end of thefruit. Second instar thrips damagethe rind by piercing and suckingjuices from the surface cells. Adultthrips do not harm the crop.

In addition to feeding ondeveloping fruit, thrips also feedon the young shoots (Fig. 8-7, Fig.

8-8). Although this does notreduce the yields of mature trees, itwill often delay the maturity ofyoung trees which are not yetbearing fruit.

Life cycle

The typical life cycle for this pestis shown below:

Fig. 8-8. Young shoot damaged by thrips

Fig. 8-5. Adult thrips (magnified)

Egg —> 1st Instar Larva—> 2nd Instar Larva—> Propupa —>Pupa —> AdultEggs are deposited in planttissue. Hatching occurs overabout five days during thesummer months. The immaturestages take around five to sevendays to complete theirdevelopment.

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Citrus aphid(Toxoptera citricida (Kirkaldy))

Fig. 8-11. Heavy infestation of citrus aphids

(Above left) Fig. 8-9. Citrus aphid:Winged (= alate) form

(Above right) Fig. 8-10. Citrus aphid:Wingless (= apterous) form

APHIDSAbout aphids

Aphids are very small insects about 2 mm long. Some aphids arefound only on specific hosts. The citrus aphid, for example, is almostalways found on citrus trees. A few species of aphid, such as thecotton aphid, are found on a wide range of crops and also on weeds.

Both the adults and nymphs of these pests suck the sap fromthe young shoots, causing them to curl up and become yellow incolor. As a result growth of the shoots is greatly impaired and theshoots are often distorted. The aphids also produce a sticky liquid onwhich sooty mold grows. This looks like a thin coating of blackpowder on the leaves, shoots and fruit.

However, the important damage inflicted by aphids is indirect.Aphids, particularly the citrus aphid, are the main vectors of citrustristeza virus. Tristeza virus is one of the most important citrusdiseases in Southeast Asia (see page 59). The aphids spread the virusby sucking the sap of an infected tree, and moving on to a healthy one,taking the virus with it.

Life cycle

Aphids do not lay eggs, but produce live young. The life cycle, from thebirth of the young aphids through numerous molts to the adult form,may be as short as one week. The adult aphid has both a winged and awingless form (Fig. 8-9, Fig. 8-10). Winged forms are born whencrowding occurs, to enable populations to disperse.

The typical life cycle for this pest is: Nymph —> Adult

Also called the brown aphid,the brown citrus aphid, or thetropical citrus aphid.

Distribution

Worldwide distribution. Thecitrus aphid is especiallycommon and noxious insubtropical and tropical areas.

Description

Citrus aphids are black, and areslightly larger than the otheraphids which feed on citrus.They are often found in dense,dark populations on the youngshoots of citrus trees (Fig. 8-11).

They do not attack olderleaves. The immature stages arereddish-brown in color.

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Green citrus aphid

Fig. 8-12. Green citrus aphid

Fig. 8-15. Cotton aphid: alate or winged form

Fig. 8-13. Citrus leaves infested with cotton aphids

Fig. 8-14. Cotton aphid: Apterous form (without wings)

Distribution

DescriptionThe adult green citrus aphid isapple-green in color, and about2 mm long (Fig. 8-12).

Cotton aphid(Aphis gossypii Glover)

Also called the melon aphid

Distribution

Worldwide distribution, in tropical,subtropical and temperate zones.

Description

The cotton aphid is smaller thanmost other aphid species. It isgenerally a greenish color, but thecolor can range from yellowishgreen to dark blue-green (Fig. 8-14,Fig. 8-15). It has a wide host range,and is a pest of melon, cotton,tomato, asparagus and many othercrops, as well as being found on awide range of weeds. This aphidtransmits more than 80 kinds ofvirus disease to various crops,including the infection of citrus treeswith citrus tristeza virus.(Fig. 8-13).

(Aphis citricola Van der Geet)Also called the spirea aphid

Worldwide distribution.

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Distribution

This aphid is widely distributedin tropical and subtropical Asia.It is cold-sensitive, and is notfound in areas where minimumwinter temperatures fall below15oC. Another psyllid, theAfrican citrus psyllid, is coldtolerant but is not found in Asia.

Life cycle

The typical life cycle for this pestis: Egg —> Nymph—> Adult

Description

This tiny insect is 3-4 mm long,mottled brown in color, withtransparent wings. The eggs ofpsyllids are bright yellow anddeposited on young leaves.Nymphs (immature psyllids) aregreen or dull orange, and can bedifficult to see because they

blend in with the leaf.The Asian citrus psyllid is

most likely to be found on newshoots (Fig. 8-16). For thisreason, their numbers tend toincrease when new leaves aresprouting. This is likely to be atthe beginning of spring, when theweather warms up after a coolwinter, or at the beginning of therainy season.

Flushes of new growth mayoccur several times a year insome areas. The Asian citruspsyllid cannot withstand lowwinter temperatures and is notfound in Korea and Japan, whichconsequently are both free ofgreening disease.

Damage

Psyllids reproduce quickly, andlarge populations can damagetrees when they feed on them(Fig. 8-17). Psyllid feeding causes

curled, distorted leaves andpremature leaf drop. However, itis the greening disease transmit-ted by the psyllids which doesmost damage to the region’scitrus orchards. Greening is themost important disease of citrustrees in tropical and subtropicalAsian countries.

The psyllid acquires thegreening organism when it feedson the sap of an infected tree,and carries it in its mouth partswhen it moves onto a healthyone. The ability of the psyllid totransmit greening disease variesfrom one place to another,depending on the biotype of thepsyllid. The Taiwan biotype, forexample, has a very lowtransmission rate of less than1%.

However, fieldtransmission of the pathogen bythe psyllid vector is significant,since it is very difficult to keeptrees completely free of psyllids.This means that citrus trees maybe exposed to infection in theopen for quite a long time.

Fig. 8-16. Citrus psyllids Fig. 8-17. Heavy infestation of citrus psyllids

Asian citrus psyllid(Diaphorina citri Kuwayama)

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Mealy bugsAbout mealybugs

Mealybugs are slow-moving insects with flat, oval bodies. They arecovered with a thin layer of white wax, and look as if they have beenrolled in white flour, hence their name of “mealybug”. They oftencrowd together in colonies on a sheltered part of the plant.

They feed on sap, using their sucking mouthparts. Their saliva istoxic to plants, and heavy infestations may cause distorted growthand premature leaf drop. Like aphids, mealybugs excrete a sugarysyrup known as honeydew. This serves as a food source for sootymold fungus, which grows on the honeydew.

Life cycleThe life cycle of this pest is different in males and females. Males gothrough an additional stage.

Egg —> crawler (=1st instar) —> 2nd/3rd/instar nymph (whichlooks like the adult) —> adult femaleEgg —> crawler —> 2nd/3rd/instar —> male pupa (=cocoon) —> adult male

Citrus mealybug(Planococcus citri Risso)

The adult citrus mealybug is 3-4mm long (Fig. 8-18). Up to 500yellow eggs are laid in a densewaxy white sac. The eggs hatchin a few days into tiny, mobileyellow crawlers with red eyes.The crawlers move onto the fruitand leaves. Later stages maymigrate down the trunk andreproduce on weeds. Citrusmealybugs can live on manyplants besides citrus. They oftenlive in green-houses onornamental crops.

Citrus mealybugs damageplants by sucking their sap. Fruitinfested with citrus mealybugalso show damage (Fig. 8-19).However, the most importanteconomic damagefrom mealybugs iscaused by the blacksooty mold, whichdisfigures the fruitand reduces theirmarket value.

Spherical mealybug(Nipaecoccus filamentosus(Cockerell))

Fig. 8-18. Citrus mealybug

Fig. 8-20. Spherical mealybugFig. 8-19. Fruit damaged by mealybug

The adult female sphericalmealybug is 2.5-4 mm long,slightly flattened in shape, andcovered with creamy white wax.Beneath the wax, the body is apurplish color. The adult femaleis usually almost hidden by herlarge white domed egg sac (Fig.8-20).

Spherical mealybugsinfest a number of otherimportant fruit crops besidescitrus, including mango, papayaand grape.

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SCALESAbout scales

Scale insects are small insectswhich suck the sap of plants.Usually, they have some kind ofprotective covering over theirbodies. Soft-scale insects havea toughened skin on the uppersurface of their bodies. Hardscale are very small, and arecovered by a waxy shield likearmor. They are sometimescalled “armored scale”.

Young scale crawlers aremobile. Adult females have nowings, and usually remain fixedin one place. If they move at all,they are very sluggish.

All citrus scales describedin this chapter, both soft scaleand hard scale, also infest arange of other plants,particularly tree crops andornamental crops. Forexample, Green scale infests awide range of plants as well ascitrus, including coffee andvegetable and ornamental crops

I. SOFT SCALES unfavorable. The mature femaledoes not move (Fig. 8-21).

Green scale are foundmainly on young leaves andgreen twigs of the tree. If there isa heavy infestation, they may alsobe found on the fruit.

Green scale insects areoften found together with antswhich feed on their honeydew.

Damage

Green scale are usually found onthe undersurface of the leaf, oftenin a line along both sides of themidrib and lateral leaf veins(Fig. 8-22). Green scale may alsoattack the young shoots, whichmay become almost hidden in amass of scales.

Large populations of greenscale may cause yellowing,defoliation, reduction in fruit setand loss of plant vigor. Citrusscale is especially damaging toyoung trees in the first two yearsafter transplanting.

Economic damage is alsocaused by the sooty mold, whichdisfigures the fruit and reducesphotosynthesis when it occurson leaves.

Fig. 8-22. Leaf damaged by green scaleFig. 8-21. Citrus green scale and parasitized citrus greenscales.

Life cycle:

The life cycle of this pest is:

Green scale(Coccus viridis Green)

Distribution

Widely distributed in the tropics.

Description

The adult female is a paleyellowish green. It is oval inshape, and 3-5 mm long. It canbe distinguished by the blackU-shape of its gut. Although thegut is internal, this marking canbe seen from above by thenaked eye.

The female green scalereproduces without mating. Eggsare whitish green, elongate-ovaland are laid singly. They mayhatch as soon as they are laid, ora few hours later.

Crawlers (1st instarnymphs) emerge and move outover the citrus tree. Once asuitable leaf or green shoot isfound, the crawlers settle andbegin to feed. They usuallyremain in this same spot unlesstheir position becomes

Egg —> crawler (=1st instarnymph) —> 2nd/3rd instarnymph —> adult

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Cottony-cushion scale(Icerya purchasi (Maskell))

Distribution

legs. The second- and third-instar nymphs are covered with awhitish, cottony substance whichdisappears after the insectmolts. Unlike most other scales,the cottony-cushion scale retainsits legs and remains mobilethroughout its life.

Florida red scale

(Above) Fig. 8-25. Florida red scale(Below) Fig. 8-26. Damage to fruit

from Florida red scale

Above: Fig. 8-23.Cottony-cushion scaleNote the scale which has been

attacked by a natural enemy,the lady-bug Rodalia cardinalis(see arrow)

Right: Fig. 8-24.Cottony-cushion scale

on citrus tree

!

Distribution

II. HARD SCALES

Damage

Like other scales, the cottonycushion scale sucks the sapfrom the leaves and twigs, thusreducing the yield. Heavyinfestations are uncommon, butif they occur, can result indefoliation and dieback of twigs(Fig. 8-24). The cottony cushionscale excretes honeydew, whichis often accompanied by sootymold growth and ants.

World-wide distribution.

Description

The body of the female cottony-cushion scale is orange, red,yellow or brown. However, it ismost easily recognized by thefluted, cottony white egg sac thatis attached to its body (Fig. 8-23).

The egg sac contains from600 to 800 red eggs, and isusually found on twigs. The eggsac may become two or threetimes longer than the body of thefemale, resulting in an overalllength of around 1 cm.

Crawlers are red with black

Widespread in Central and SouthAmerica, Europe and Asia.Particularly common and noxiousin tropical countries.

Description

The adult female is small andround, about 2 mm across. It is adark reddish-brown in color (Fig.8-25). The adult male is oval andabout half the size of the female.The Florida red scale, like otherhard scale, does not produce anyhoneydew, so it is not foundtogether with sooty mold andants.

Hard scales are alsoknown as “armored scales”.The scale or armor is made froma thin, hard waxy cover. Thefemales lay up to 300 eggs.When the crawlers hatch, theysettle close to the mother scaleon leaves and fruit (Fig. 8-26).

(Chrysomphalus aonidum(Linnaeus))

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Purple scale(Lepidosaphes beckii (Newman))

Distributionscales, the purple scale does notproduce honeydew.

Purple scales infest citrusleaves, fruit and bark. Theyprefer sheltered and shady partsof the tree. They may occursingly, or in scattered clumps.

Like the other scales,purple scales suck the sap, andcause further damage byinjecting toxins into the host plantwhile feeding.

Places where they havebeen feeding may show yellow orbrown areas around the feedingsite, followed by leaf and fruitdrop. Green spots may show onripe fruit where scales werepresent, thus reducing themarket value.

World-wide distribution.

Description

The hard cover of the adultfemale is brownish purple incolor, elongated and shaped alittle like a comma (Fig. 8-27).The cover of the male is similarin color to that of the female, butis shorter and narrower. Thefemale lays a mass of 50-100pearly white eggs under theirown scale cover. The eggs hatchafter a few weeks. The crawlersare very small, less than 0.25mm, and are pearly white incolor. Three or more generationsmay occur each year. Unlike soft

Also called the mussel scale

Fig. 8-27. Purple scale (magnified)

Chaff scale(Parlatoria Pergandii Comstock)

Distribution

Worldwide distribution.

Description

The scale of the adult female isan irregular oval shape, and lightyellowish brown in color (Fig. 8-28). The males are slightlysmaller than the females. Themale scale is white, and almostoblong in shape. The bodyunder the scale is purplish incolor.

The chaff scale oftenoccurs together with Florida redscale and purple scale on thesame tree. It is most commonon the tree trunk and mainbranches, but also infests theleaves and fruit (Fig. 8-29,Fig. 8-30).

Fig. 8-28. Chaff scale (magnified)

Fig. 8-29, Fig. 8-30. (Above andright). Examples of fruitdamaged by chaff scale.

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MOTHS ANDBUTTERFLIESMany kinds of moths andbutterflies are found on citrustrees. Generally speaking, thetrees are not damaged by theadult butterflies and moths. Thedamage is caused by the larvaewhen they feed on the leaves.

The typical life cycle forthis pest is: Egg —> Larva (=caterpillar) —> Pupa (=cocoon) —> Adult (moth or butterfly)

Citrus leafminer(Phyllocnistis citrella Stainton)

Distribution

Widely distributed worldwide,including temperate,subtropical and tropicalSoutheast Asia. This pest isfound only on citrus trees.

is brownish-grey in color, withhindwings fringed with longhairs. Because adults arenocturnal, they are seldom seen.They may sometimes fly outwhen foliage is disturbed.

Eggs are laid singly on theunderside of leaves near themidrib. The eggs are about 0.3mm across, and look like tinywater droplets. They hatch within3-5 days. The young larvaeimmediately start burrowingunder the surface of the leaf.They pupate, when full grown,near the margin of the minedleaf. The total life cycle is about2 - 3 weeks.

Damage

The characteristic damage fromleafminer is silvery or yellowish‘blisters’ or wavy lines on thesurface of the leaf (Fig. 8-31, Fig.8-32). This is produced by thelarvae, which tunnel or minethrough the leaf, following acharacteristic wavy path justbelow the leaf surface (Fig. 8-33).Each larva has its own tunnel.

Larvae never leave theirmines to make new mines, ormove into the upper surface ofthe leaf. Usually each leaf hasonly a single mine, but in casesof heavy infestation there may beseveral mines per leaf. Thismining of the leaves causesthem to curl up and look distorted(Fig. 8-34). Sometimesleafminers may attack the youngshoots and cause them to wilt.

Description

Fig. 8-31. Larva of leafminer(magnified) inside citrus leaf

Left and below: Damage to citrusleaves from leafminer

Fig. 8-32

Fig. 8-33 Fig. 8-34

The citrus leafminer is a small,delicate moth about 2 mm long. It

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Citrus leafroller(Homona coffearia Nietner)

Description

The adult citrus leafroller is asmall brown moth, with curvedwings which bear black spots.The larvae are slender andgreen, with a dark head (Fig.8-35). Adult moths are activeduring the night, and shelter inthe tree during the day. The palegreen eggs are laid singly,usually on the upper surface ofthe leaf near the midrib.

The newly hatched larvaespin a thin silk web on the leafsurface, and stay beneath it to

feed. As the larvae grow, theybegin to fold or roll the leaflongitudinally and feed within theroll. From time to time they moveto new sites and make a new leafroll. When the larvae are mature,they pupate inside a cocooninside the leaf roll (Fig. 8-36).

LFig. 8-35. Larva (caterpillar) of leafroller and damage to leaf Fig. 8-36. Pupa (cocoon) of leafroller on citrus

Distribution

The citrus leafroller is wide-spread in Asia, from India toJapan. It is found on other cropsbeside citrus, including coffeeand indigo, and is an importantpest of tea. When the samespecies of leafroller is found ontea, it is generally known as thetea tortrix.

Damage

Damage is mainly from the lossof young leaves, which reducesphotosynthesis and depressesthe yield.

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Oriental fruit fly(Bactrocera dorsalis Hendel)

Distribution

The Oriental fruit fly iswidespread in tropical Asiancountries. where fruit fly is found.

The adult fruit fly is around8 mm long, larger than anordinary housefly. The color ofthe fly varies, but there arealways yellow and dark brownmarkings on the thorax(Fig. 8-37).

The larvae are small whitemaggots, and are 7-11 mm long.The adults mate at dusk, and thefemale lays its egg just insidethe skin of a fruit. Under optimumconditions, a female can laymore than 3,000 eggs duringher lifetime.

The egg hatches into alarva which then burrows into thefruit (Fig. 8-38). The mature larvaemerges from the fruit, drops tothe ground, and forms a brownpupa about 5 mm in length.

Fig. 8-37. Adult Oriental fruit fly

Fig. 8-38. Guava infested with fruitfly larvae

Damage

Description

The Oriental fruit fly infestsguava, mango, papaya and manyother kinds of fruit besides citrus.To some extent, citrus fruit areprotected from fruitfly attack bytheir thick skins. Oriental fruit flytends to attack citrus fruit onlywhen the fruit are ripe.

It is the larvae of this pest whichcauses damage, rather than theadult fruit flies. They burrowthrough the fruit, making it unfitfor human consumption.

Fruit fly is economically themost important insect pest of fruitin Southeast Asia, and cancause losses of 90-100%.

It can also prevent thedevelopment of export markets.Countries which do not have fruitfly are anxious to remain free ofthe pest.

They usually imposestrong quarantine restrictions onimports of fruit from countrieswhere fruit fly is found.

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MITES

About mites

Mites are not insects, theybelong to a different group. Theyare very small, less than 1mmlong, and are quite difficult tosee with the naked eye. To seethem in any detail, you need amagnifying glass of some kind.Nymphs and adults have fourpairs of legs. Larvae have onlythree pairs of legs.

There are a wide range ofmite species which live oncitrus trees. Many of them arepests which attack the leavesand fruits. They have piercingand sucking mouth parts whichsuck the sap. Their feedingscars the fruit, reducing itsmarket value, and causesdiscoloring and distortion ofbuds and leaves.

However, not all mites arepests. Some species of mitesare beneficial to growers,because they prey on pestmites, while they themselves dono damage to the plants.

It takes an expert todistinguish the two kinds. In anycase, the beneficial mites willbe killed along with the pestspecies of mites when theyare sprayed.

Citrus rust mite(Phyllocoptruta oleivora(Ashmead))

Distribution

A serious pest worldwide,especially in the humid areas.This mite is found only on citrustrees.

Description

The citrus rust mite infests twigs,leaves, and fruit of all kinds ofcitrus. It particularly preferslemons and grapefruit. The mitehas an elongated, wedge-shaped body, with the head atthe broader end. It is pale yellowin color. However, the body is sosmall (0.1 mm long) that it isdifficult to see without amagnifying glass.

The female begins layingeggs as soon as she reachesmaturity, and continues laying 1-2 eggs per day as long as shelives. The eggs are about aquarter the size of the adult mite.The eggs are spherical, rangingin color from transparent to paletranslucent yellow.

Mite populations usuallybegin to build up when newgrowth appears. The mites moveon to the new flushes, and thenon to the fruit as they appear.Early in the season, most citrusrust mites are found on newleaves. Later in the season,higher populations are found onthe fruit. Generally, populationsare highest in the lowerbranches of the tree. The mitestend to prefer the sheltered sideof the citrus tree, away fromdirect sunlight.

Damage

Citrus rust mites feed with theirsucking-piercing mouth partsjust below the skin of the leavesand fruit. This turns the greentwigs and mature leaves areddish brown color, knownas “russeting”.

Fig. 8-39. Damage to leaves fromcitrus rust mite

Life cycle

The typical life cycle for thispest is:

Egg —> Larva —>Protonymph —>Deutonymph —>Adult

The larvae and nymphs(protonymphs anddeutonymphs) resemble theadults, except that they aresmaller and the larvae onlyhave three pairs of legs.

Most of the leaf damageoccurs when the mites feed onthe undersurface of the leaf (Fig.8-39). This causes “bronzing”,and interferes with photo-synthesis. Extensive damage tothe lower leaf can result in leafdrop, especially in dry orcold weather.

54

However, it is the injury tothe fruit which causes thegreatest economic damage.Injuries from feeding mitesproduces blemishes on the peelof the fruit, which reduces themarket value. Damage variesaccording to the type of citrus fruitand its maturity.

Citrus rust mites seem toprefer to feed on immature fruit.Injury to lemons when the fruitare very small causesdiscoloration of the peel, whichmay be severe. The skin has athick, grey to dirty brownappearance known as “sharkskin”.

Injury to young orange,mandarins and grapefruitproduces a greyish-browndiscoloration or silver-greyblemish on the surface of the fruit(Fig. 8-40). Heavy infestationsmay even turn the skin black.

If citrus fruit are attackedwhen they are fully grown, mitedamage shows as a brown stainon the peel surface. This lateinjury on the fruit is referred to as“bronzing” (Fig. 8-41).

Damage to fruit from heavymite infestation can sometimesbe confused with that caused bymelanose, chemical burn orweather staining. However, mitedamage tends to be found on thesurface of the fruit facing insidethe tree. Damage from weatheretc. tends to be on the exposed,outer surfaces of the fruit.

Growers who suspect miteinfestations are causing thediscoloration should also look forthe tiny white skins on the fruitwhich are shed by the nymphs ofthe mites when they molt.

In addition to the visiblesymptoms, citrus fruit attacked byrust mites has a slower growthrate, so that the average size atharvest may be smaller. There isalso a higher rate of fruit drop.

Fig. 8-40. Damage to pomelo fruit from citrus rust mite

Fig. 8-41. Damage to sweet orange fruit from citrus rust mite

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Citrus red spider mite(Panonychus citri McGregor)(Also called citrus red mite)

Damage

The citrus red mite feeds onleaves, green twigs and fruit, butprefers the fully expanded leaf.

Most damage occurs onthe upper surface of the leaf. Apale stippling is seen, which incases of severe infestationsenlarges to dry necrotic areas.Marks are also seen on theunderside of the leaves(Fig. 8-44).

Stippling or silvering alsooccurs on green fruit but theseoften disappear when the fruit

change color. Damaged matureoranges and lemons maybecome pale yellow in color.

Severe infestations resultin fruit drop and leaf drop. Citrusred mite is likely to be more of aproblem in hot, dry weather,especially if the trees aresuffering from water stress.

Populations of citrus redspider mite tend to be heavier inorchards where natural enemieshave been destroyed by the useof broad-spectrum insecticidessuch as methidathion anddimethoate.

Fig. 8-42. Male red spider mite (magnified) Fig. 8-43. Female red spider mite (magnified)

Fig. 8-44. Two leaves, seen from underneathThe leaf on the left is damaged by red citrus mite.A normal leaf is on the right.

Distribution

The citrus red mite is native toAsia, but now has a world-widedistribution. It is found on otherplants besides citrus, includingmulberry and some legumes.

Description

The adult citrus red mite has anoval body which is dark red incolor (Fig. 8-42, Fig. 8-43).