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Pesticides andBiomagnification

In agricultural ecosystems, thousands ormillions of individuals of a single cropspecies are planted in a monoculture.

Pests

To maximize growth of the desiredcrop, we try to eliminate organismsthat we consider pests.

Pests are organisms that might compete withor damage crop species. Agricultural pests areplants and animals that reduce crop yields.

Pests

For example, weeds are plant pests,mosquitoes are insect pests, and miceare rodent pests.

The term pest is usedonly in reference tohuman wishes. Thereare no pests in nature.

Pests

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All organisms are simply producers andconsumers within food webs.

By controlling pests, crops grow in thenear-absence of their natural consumersand competitors.

Insects are also considered serious pestsin the forestry industry. When farmers plant a monoculture, they

often create the ideal environment for pests.

For example, when a population of Coloradopotato beetles finds a field of potatoes, theybegin feeding and reproducing rapidly.

Uncontrolled, the population couldskyrocket and devastate the entire crop.

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Pesticides

One of the most common ways to controlor eliminate pests is to use poisons thatkill pests - pesticides.

Pesticides

There are many different kinds ofpesticides…

Herbicides are used to kill plants…Pesticides

Other pesticides includeinsecticides, rodenticidesand fungicides.

Humans even usemolluscicides and piscicidesto kill snails and fish,respectively.

Once applied, pesticides vary greatly inhow long they persist or remain active inthe environment.

Some long-livedpesticides persist formany years. Otherpesticides are short-lived, lasting only amatter of daysbefore they degrade.

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Characteristics of Pesticides

In general, pesticidesobtained from naturalsources are less persistentthan synthetic pesticides.

However, modernsynthetic pesticides areless persistent than thosedeveloped 30 or moreyears ago.

Characteristics of Pesticides

Pesticides vary widely in the numberof species they are able to control.

Broad-spectrum pesticides are toxicto a range of species, whereasnarrow-spectrum pesticides aretoxic to a limited number ofspecies.

Characteristics of Pesticides

For example, DDT(dichlorodiphenyltrichloroethane), a once widelyused insecticide, is toxicto most insect species.

Characteristics of Pesticides

Bt, a modern pesticidederived from bacteria(Bacillus thuringiensis) ishighly toxic only tocaterpillars, beetle larvaeand fly larvae. It is nottoxic to most beneficialinsects.

How Pesticides Work

Pesticides work by causing physical orbiological harm to the pest organism.

How Pesticides Work

Diatomaceous earth, for example, iscomposed of the fossilized remains of atype of algae called diatoms.

This abrasive powder scratches the waxyouter coating of small organisms, such asinsects, causing them to dehydrate.

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Other pesticides may interfere with biologicalprocesses, such as photosynthesis, or causedamage to vital organs.

Some pesticides are delivered by contact.In this case, the target pest must betouched by the pesticide.

How Pesticides Work

Alternatively, somepesticides are indirectlyapplied. For example,herbicides can besprayed on the soil andtaken up through theroots of the weed.

Insecticides can be sprayed on a plantand later consumed by an insect.

Issues With Pesticides

Pesticides have helped farmers reducecrop damage from pests and increasefood production.

Pesticides have also helped controlpopulations of biting insects, such asmosquitoes, that spread diseases.

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Issues With Pesticides

While such benefits canresult in more food andbetter health for some,pesticide use has a numberof environmental costs.

Pesticides are often applied throughaerosols or sprays onto fields, forestsand gardens.

A serious drawback of this mode of delivery isthat some of the pesticide never reaches thetarget species because it is carried away bythe air or lands on the soil.

Issues With Pesticides

These pesticides then becomepotential sources of pollutionto the soil, air and water.

Non-Target Species

Pesticides may also harm non-targetspecies - species they were notintended to kill.

Because broad-spectrum pesticides controlmany different pests, they may kill non-damaging and potentially beneficial organisms.

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For example, a broad-spectruminsecticide may kill species of predatoryinsects that might normally feed on pests.

Killing beneficial organisms creates asituation in which farmers become moredependent on pesticides.

Non-Target Species

When natural pest controlsare killed (i.e. predatoryinsects) farmers mustreplace them by usingmore pesticides.

Non-Target Species

Improper use of pesticidescan also kill non-targetspecies. For example,spraying an insecticide atthe wrong time of year maykill honeybees, which areessential for pollinating fruitcrops.

Non-Target Species

As a result, less fruit will be produced.

One of the most serious sideeffects of pesticide use is theirtendency to accumulate inindividual organisms.

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Biomagnification

This happens because some pesticidesare not broken down or eliminated withother body wastes.

If an individual continues toeat food contaminated withpesticide, it will accumulatein the body.

Biomagnification

If the pesticide is long-lived, then theconcentration of pesticide in the individualswill increase to levels much higher than inthe environment.

Biomagnification

Pesticides that bioaccumulate do sobecause they cannot be easilyexcreted from the body.

This is because they are not soluble inwater, but are soluble in fats and oils.

This process is called bioaccumulation.

Biomagnification

All individuals are part of a food chain. As aresult, toxins stored in the fats and oils of anorganism at one trophic level are passed on tothe organisms at the next trophic level.

Biomagnification

The higher up the foodchain, the moreconcentrated thepesticides become.

This process is calledbiomagnification (orbioamplification)

Biomagnification

If a pesticide biomagnifies in a food chain,it may reach toxic concentrations.

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Many species of predatory birds, includingosprey, bald eagles, and peregrine falcons,declined in numbers because DDTbioaccumulated in their bodies.

DDT interfered with calcium metabolismand the female bird’s ability to producestrong egg shells.

Biomagnification

Other fat-soluble toxins,such as mercury andpolychlorinated biphenyls(PCBs) also bioamplify inthe food web.

Arctic ecosystems are particularly vulnerableto biomagnification of toxic substances.

Biomagnification

Many long-lived top consumers such aswhales, polar bears, walrus, and fish livein the arctic.

In addition, Inuit that live in theseenvironments rely on these same speciesas their traditional food supply.

This results in a potentiallydangerous exposure ofhumans to toxins in their food.This is one danger associatedwith consuming food from thetop of the food web.

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Pesticide Resistance

When pesticides are used for longperiods of time, some pest species maybecome resistant to the pesticide.

Pesticide Resistance

This means that the pesticide is no longerable to control the pest.

Individuals that exhibit the greatestresistance are more likely to survive anapplication of pesticide than those withlittle or no resistance.

The individuals thatsurvive will reproduce andpass on their resistance totheir offspring. After many generations, the population can

become highly resistant to a particular pesticide.

Weeds and insect pests are likely to developresistance because they reproduce frequentlyand produce many seeds or offspring.

When resistance develops in a pest, thefarmer needs to apply a greater concentrationof pesticide to have the same effect, or switchto a different pesticide.

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Reducing Pesticide Dependence

There is little doubt thatpesticides havedramatically increasedglobal food production.By reducing competitionand other pests, cropsgrow faster and havehigher yields.

Reducing Pesticide Dependence

However, the benefits ofusing pesticides must beweighed against therisks of pollution, harmto non-target species,biomagnification andpesticide-resistance.

Reducing Pesticide Dependence

One alternative type of agriculture, organicfarming, uses no synthetic pesticides orfertilizers.

Organic farmers sometimes have to acceptcrop losses due to naturally occurring pests.

Reducing Pesticide Dependence

However, these losses may be offset bythe higher price growers get for theirorganic products as well as savings fromnot purchasing synthetic chemicals.

Reducing Pesticide Dependence

Organic farmers rely on a range ofecologically sustainable techniques:

1. Biological control– predatory insects, mites and disease-

causing micro-organisms feed on andinfect prey species

– e.g. parasitic wasps, ladybugs

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2. Altered timing– better timing of planting and harvesting can

avoid peak pest populations

Reducing Pesticide Dependence

3. Crop rotation & mixed planting– when farmers do not grow

monocultures in the same locationyear after year, pest populationsdo not have the sameopportunities to establish andprosper

4. Baiting pest– pheromone baits can be used to confuse

some mating insects

While organic farming minimizesenvironmental impacts, it could result in lowercrop yields. This may not be acceptable.

Reducing Pesticide Dependence

In such cases, an intermediate approachcalled integrated pest management (IPM)is often employed.

IPM takes advantage of all types ofmanagement methods. The goal is tomaximize efficiency and minimize costsand harm to the environment.

Reducing Pesticide Dependence

IPM farmers use many of the techniquesemployed by organic farmers, but usesynthetic pesticides and fertilizers whennecessary.