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Natural Resource Conservation:Management for a Sustainable FutureDaniel D. Chiras John P. Reganold

Tenth EditionN

atural Resource Conservation Chiras Reganold 10e

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In tegrated Pest Management

potential pests in check. Second, monocultures often presentvast expanses of genetically identical plants that provide anenormous supply of food for pests. In the simplified ecosys-tems, species that were once held in check by predators andby a limited food supply can and often do proliferate rapidly,

causing extensive damage. By reducing biological diversity,human civilization has inadvertently unleashed forces that itnow struggles to control. In the United States alone, 19,000agricultural pests exist. Of these, 1,000 are considered majorpests (Figure 2).

FIGURE 1 Monoculture—a pest’s paradise. Modern agriculture is basedon the planting of monotypes or monocultures, a vegetation plot made upof a single species of plant, such as the wheat fields shown in this photo-graph of eastern Washington. Monocultures often cover many hundredsof square kilometers.

Grasshopper

Cicada

Termite San Jose scaleand adult

Chinch bug

Harlequincabbage bug

Squash bug Cornbillbug

Round-headedapple tree borer

Grainweevil

Cotton bollweevil

Potatobeetle June beetle

Cucumberbeetle

Granarybeetle

Larderbeetle

Cutworm moth

Army wormmoth

Tent caterpillarmoth

Walnut mothCabbage butterfly

Carpenter ant

FIGURE 2 Insecticides are employed against theharmful species of insects shown here. Only 0.1%of the 800,000 species of insects in the world areconsidered pests.

USDA/ARS/Agriculture Research Service

194

Pests may also arise from accidental or intentional introduc-tion of insects or other harmful organisms. In their new habitat,these alien species often face little environmental resistance.Populations explode. Consider some examples.

In 1869, the pupae of the gypsy moth, a native of Europe,were shipped from France to Medford, MA, at the request ofFrench astronomer Leopold Trouvelot of Harvard University.Trouvelot was interested in developing a disease-resistant silk-worm moth. Unfortunately, a few of the gypsy moths escapedfrom captivity and made off into the woods, where they livedin relative obscurity for many years. Twenty years after theirescape, however, the town of Medford was crawling withgypsy moth caterpillars—and now so are many other areas(Figure 3).

Released from density-dependent control agents such aspredators and parasites, which keep this species in check intheir native habitat, the gypsy moth population exploded in itsnew environs. One observer wrote, “The street was black withthem. . . they were so thick on the trees that they stuck togetherlike cold macaroni . . . the foliage was completely stripped fromall the trees . . . presenting an awful picture of devastation. On aquiet summer night one could actually hear the sound of thou-sands of tiny mandibles shredding foliage in the trees. Pelletsof waste excreted by the larvae rained down from the trees in asteady drizzle.”

A single caterpillar can devour a square meter of foliagein a day. Although the larvae prefer oak leaves, they will alsoconsume birch and ash foliage, and when fully grown, they

even eat pine needles. Deciduous trees can withstand a singledefoliation. Several repeated seasons of defoliation, how-ever, almost always kill them. Why? The loss of leaves elim-inates a tree’s ability to photosynthesize—to produce thefood it needs to grow and survive. Without leaves, plantscan’t make and store the food molecules they need to sur-vive. Defoliation also makes trees more susceptible to fungi,winds, and drought.

Since its escape from captivity, the gypsy moth has spreadthroughout the northeastern states and westward intoMichigan, Wisconsin, Colorado, and California. In recentyears, the spread of the moth to distant forests has been facil-itated by recreational vehicles (RVs), on which the moth fre-quently deposits its eggs.

Fearing further spread, California officials now inspect pri-vate and commercial vehicles, such as trucks and RVs, espe-cially those coming from states with gypsy moth problems, at16 border stations on major highways to check the vehicles foreggs (and other insect pests). California and other states thatstrictly enforce controls on gypsy moths require people movingto the state from gypsy moth–infested states to be certified freeof gypsy moths before they can enter the state. Such controlswere initiated in the wake of the severe outbreaks that occurredin 1980 in California and the northeastern United States. Inthe Northeast, trees were defoliated over a 2.5-million-hectare(5-million-acre) area stretching from Maine to Maryland.According to the U.S. Department of Agriculture, since 1980,the gypsy moth has defoliated close to a million acres or moreof forest each year. In 1981, it defoliated nearly 13 millionacres (5.3 million hectares).

Another exotic or alien species that has had a huge impact ontrees is the fungus that causes Dutch elm disease. Accidentallyintroduced from Europe around 1933, the fungus infects theAmerican elm, a tree that, unlike its Dutch counterpart, is notresistant to this organism.

The American elm is a stately tree that once graced parks,boulevards, college campuses, cities, and suburbs throughoutmuch of the eastern United States. It was considered an idealspecies because it was not only beautiful but also long-lived,fast-growing, and tolerant of compacted soils (common inurban environments) and air pollution. Today, most elms areeither dead or dying of Dutch elm disease. In one of nature’scruelest ironies, the fungus itself does not kill the tree; the treekills itself in trying to fight off the organism. Specifically, elmtrees produce chemicals to ward off the fungus, but these sub-stances clog the vessels carrying water from the roots to thelimbs and leaves. As a result, photosynthesis stops, and thetrees die.

The spores of the fungus are spread by bark beetles andalso from the root of one tree to the roots of adjacent trees,killing off one tree after another along a city street. Because itwas once customary to plant elms in rows along residentialstreets, the fungus spreads rapidly from tree to tree. Attemptsto stop it by killing the beetles with the insecticide DDT onlysucceeded in killing many robins and other songbirds. By1976, despite vigorous efforts to stop the disease, it hadspread from Massachusetts south to Virginia and west toCalifornia. Virtually gone are the magnificent elms in St. Paul

FIGURE 3 Leaf-eating caterpillars of the gypsy moth damage hundreds ofthousands of dollars’ worth of forest and shade trees in the northeastern statesannually. They hatch in April from eggs laid the previous year.

In tegrated Pest Management

U.S. Department of Agriculture

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In tegrated Pest Management

and Minneapolis. The lovely elms of Santa Rosa, CA, once atourist attraction, are now succumbing rapidly, althoughefforts are being made to save the trees whenever possible,there and in other cities.

Alien species are a major problem in the United States,Canada, and many other countries. In fact, more than half ofthe weeds in the United States and many of the most destruc-tive insect pests, such as the cotton boll weevil and theMediterranean fruit fly, are foreigners—species that weresometimes intentionally, sometimes accidentally imported intothe United States.

2 Types of Chemical Pesticides:A Historical Perspective

In early agricultural societies during the years up to WorldWar II, farmers used a variety of chemical pesticides such asarsenic, ashes, and hydrogen cyanide. Although some of thesesubstances were effective in combating pests, many were inef-fective and also highly toxic to people. In 1939, Swiss scien-tist Paul Müller discovered that a synthetic chemical known asDDT was a powerful insecticide and started a revolution inagriculture with far-reaching impacts on agriculture, people,and the environment.

Chlorinated Hydrocarbons

DDT is a member of a class of chemicals called chlorinatedhydrocarbons—organic compounds that contain chlorineatoms. DDT’s insecticidal activity spurred research that ledto the discovery of a string of chlorinated hydrocarbons,such as chlordane, aldrin, lindane, endrin, dieldrin, mirex,heptachlor, and Kepone—all of which have since beenbanned or severely restricted in the United States. They’re allnerve toxins that kill pests by altering the function of theirnervous system.

Before being banned in the United States, DDT was widelyused. It proved to be extraordinarily effective in killing lice thatinfested many soldiers in Europe during World War II. It alsoproved to be effective in controlling the malaria-carrying mos-quito in the tropics, so much so that the incidence of malariaworldwide dropped from as many as 50 million cases a year toalmost zero. In India, for instance, the number of cases ofmalaria fell from 1 million per year in the 1950s to just 50,000in 1961.

DDT also proved to be an effective means of controlling avariety of insect pests. Few people questioned the wisdom ofthe Nobel Prize selection committee when it announced thatMüller would receive the award for physiology and medicinein 1948.

While DDT gained popularity and acclaim, studies showingthe biological impacts of DDT and other chlorinated hydrocar-bons led some scientists and public-policy makers to questionwhether the damage caused by these insecticides was worththeir benefits.

Scientists found that, first and foremost, DDT and allother chlorinated hydrocarbons persist in the environmentfor many years, because bacteria lack enzymes needed to

break them down. Studies suggest that DDT and its harmfulbreakdown products remain for 15 to 25 years (Figure 4).Today, despite its having been banned in 1972, DDT and itsbreakdown product, DDE—which is equally harmful—canstill be found in the mud on the bottoms of American lakesand rivers.

DDT and similar pesticides are fat soluble and therefore tend tobioaccumulate—that is, they concentrate in body tissues, especiallyfat. Because of DDT’s fat solubility, it can remain in fat tissues fordecades. Making matters worse, DDT and other chlorinated hydro-carbons build up in food chains, so the highest-level consumershave levels many hundreds of thousands, sometimes millions, oftimes higher than the environment (Figures 5 and 6). This processis known as biomagnification.

These problems and others, which are described in the sec-tion on the hazards of pesticides later in the chapter, caused afuror the world over. As the evidence grew, it became clear thatthe chlorinated hydrocarbons were indeed too risky to use.Consequently, chemists introduced a new variety of pesticides,the organic phosphates.

Organic Phosphates

Malathion and parathion are the two best known organicphosphates. Like the chlorinated hydrocarbons, organicphosphates are neurotoxins. That is, they kill by damaging orinterrupting the nervous system of insects. Unlike DDT,however, they are much more quickly degraded in the envi-ronment than their predecessors. Unlike DDT and chlori-nated hydrocarbons, they are water soluble and thus less

0

1

Aldrin

Chlordane

Lindane

Dieldrin

DDT

2

3

4

5

6

7

8

9

10

11

12

Yea

rs

FIGURE 4 Average persistence of pesticides in the soil.

196

likely to biomagnify. Because of these properties, organicphosphates were originally thought to be safe substitutes forthe chlorinated hydrocarbons. However, experience soonshowed that even at low levels, organic phosphates are haz-ardous to humans. Low-level exposure, for example, oftenleads to dizziness, vomiting, cramps, headaches, and diffi-culty breathing. Higher levels lead to convulsions and death.Symptoms were most common in farm workers and peopleliving around farms.

Like the chlorinated hydrocarbons, many of the organicphosphates have been banned or have been severely restrictedin the United States.

Carbamates

To create a safer class of chemicals that biodegrade even morerapidly, pesticide manufacturers developed an entirely new lineof chemicals, the carbamates. They are nerve poisons, like thechlorinated hydrocarbons and organic phosphates. Perhaps thebest known is the commercial preparation called Sevin (car-baryl). Carbamates persist in the environment but for only afew days to two weeks at most. As a result, they are referred toas nonpersistent pesticides.

3 How Effective Are Pesticides?Each year pests, including weeds, insects, fungi, bacteria, andbirds, consume or destroy approximately 42% of the annualfood production in the United States. By various estimates, pestsannually destroy or consume 33% of crops in the field and 9%of food at various stages after harvest. Damage is worse in thetropics, where two or three crops are grown on a field in a singleyear and where conditions are ripe for insect growth. If thisdamage could be prevented, it could greatly increase the avail-able food supply. Hence the popularity of chemical pesticides.

But how effective are they?Studies show that chemical pest controls do indeed work, but

not as well as one might suspect. One’s perspective on the issuedepends on whom one talks to. Conventional farmers and pesti-cide manufacturers, for example, convincingly argue that chem-ical pesticides have helped farmers produce much more foodthan would have been possible without their arsenal of chemi-cals. In reality, only part of that increase is due to pesticide use.Other factors, such as irrigation, fertilizers, and genetic im-provements, have made much larger contributions to production.

According to agricultural economists, each dollar investedin pesticides results in about $2 to $4 in improved yields. TheU.S. Office of Technology Assessment estimates that withoutpesticides, American farmers would lose an additional 25% to30% of the annual crop, livestock, and timber production(Figure 7). David Pimentel, a Cornell University expert on in-sect pest control, however, believes these figures are exagger-ated. A complete ban on pesticides, he estimates, would in-crease preharvest losses in the United States from 33% lostwith pesticides to 45% without them.

Trophiclevel 1

Trophiclevel 2

Loss by respirationand excretion

Nonmetabolized pollutingmaterial (such as DDT)

Total weight ofliving material (biomass)

Trophiclevel 3

Trophiclevel 4

FIGURE 5 Biomagnification aka biological mag-nification: the increasing concentration of certaintoxic chemicals, such as DDT, in the food chain. Agiven organism takes in large amounts of con-taminated food. Much of the food may not be con-verted into protoplasm but may be burned up asfuel during respiration or excreted aswaste. However, pollutants such as DDT that aretaken into the body along with the food may re-main inside the cells of the organism. As a result,the concentration of the pollutant increases pro-gressively in the food chain.

GO GREEN!When time comes to buy a house, look into natural pest controls for your

lawn, trees, shrubs, and gardens. If your parents use pesticides around theirhome, help them find alternatives or locate companies that rely on natural pest-control techniques.

In tegrated Pest Management

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In tegrated Pest Management

FIGURE 6 Food web of the marsh ecosystem off Long Island, NY, which had been sprayed with DDT for mosquitocontrol. Note the biomagnification of DDT in parts per million as it moved up the food web. The greatest concentra-tions were found in the fatty tissues of fish-eating birds, such as gulls, cormorants, and mergansers.

Organic debrisMarsh 13 pounds per acre

Bottom 0.3 pounds per acre

Bay shrimp 0.16

Silversides 0.23

Eel 0.28

Fluke 1.28

Clam 0.42

Plankton 0.04

Minnow 0.94

Mosquito 0.30

Minnow 1.24

Redwing blackbird

Kingfisher

3.52–18.5,75.5

Gulls

Cormorant26.4

Merganser22.8

GreenHeron3.57,3.51

Osprey

(egg)13.8

Terns 3.15–5.17,4.75, 6.40

Billfish 2.07

Cricket 0.23Energy flow

Marsh plantsShoots 0.33Roots 2.80

Blowfish 0.17

Mud snail0.26

Water plant0.08

FIGURE 7 Benefits derived from insecticides. Untreatedcotton on the left yielded only 0.25 bale per hectare. Cottonon the right, treated with insecticide, yielded 2.5 bales perhectare.

U.S. Department of Agriculture

198