Is Biological Pest Control Better Than Chemical Control?
Is Biological Pest Control Better Than Chemical Control?
A pest is an organism that reduces the quality or the yield of a crop. They spread disease, and damage crops, which reduces the yield. Aphids and other insects do a lot of damage, as well as slugs and snails. Viruses, fungi and bacteria are also pests that cause disease, such as, potato blight. Weeds, such as wild oats, grow in the wrong place and cause interspecific competition. This makes the crop compete for space, nutrients, light etc. therefore, it is extremely important to control pests, and there are two methods for doing this. Chemical pest control is the use of insecticides, herbicides and fungicides. Biological pest control is the use of other organisms which are predators or parasites of that pest. A proper definition would be "biological control is the use of natural predators, parasites, fungi; pheromone lures etc to control pests, without the use of chemical pesticides." There are many advantages and disadvantages of both forms of pest control, and I will investigate in this essay which method of pest control is more effective.
There are three types of chemical pesticides used to control pests: contact, systemic and residual. Contact pesticides are sprayed directly onto the crop. They are absorbed by the insect through spiracles (gas exchange pores) along its body. Contact herbicides and fungicides are absorbed directly through the surface. Systemic pesticides are also sprayed onto crops, but they're absorbed by the leaf and transported around the plant. Insects that suck sap, such as aphids, take in the insecticide and are poisoned. Residual pesticides are sprayed onto the soil or the seeds before they are planted. They remain active in the soil and kill fungal pores, insect eggs and larvae, and weed seedlings as they germinate.
The advantages of using chemical control are that there are economic benefits. Many of the chemicals are cost effective. They are relatively inexpensive, such as contact herbicides and fungicides, compared to the benefits of their use. The use of chemicals has also economically benefited farmers, as it lowered the amount of labour spent on combating pests, as the application of chemicals is an extremely quick and efficient process. They are fast acting and are capable of killing existing populations instantaneously. Some insecticides kill insects for long periods of time as they are residual. This makes them persistent, as they affect the insect eggs and larvae, thus reducing the population of insects. The use of chemicals has allowed yields of agronomic crops to increase dramatically.
Chemical pest control should be specific. This means it should only have a lethal effect immediately on the pest involved, but harmless to other organisms, such as humans, its predators and processes such as photosynthesis. Pesticides should be biodegradable so that they break down in the soil into a harmless substance when applied, but are chemically stable so that they have a long shelf-life.
However, there a great number of disadvantages of using chemical pest control. Pesticides can be highly dangerous and poisonous substances which must be handled with great care. The reason they are misused are due to the instructions printed on them not being followed as the language is not understood in foreign countries, or the container becoming dirty and the instructions impossible to read. A recent medical study at Imperial College, London, has suggested that tens of thousands of farm workers die every year around the world from poisoning by pesticides. Some of the dangers of pesticides may not be obvious until they have been in use for several years. Death is an extremely serious and major effect of the use of pesticides which cannot be ignored.
Insecticides have some of the most damaging and unexpected effects. They were meant to be specific, but they affected other non-target organisms such as bees which are valuable in pollinating flowers, including those of fruit crops. They also kill ladybirds, the valuable secondary consumer that eats aphids. In killing secondary consumers and pollinators, insecticides may do more damage than that done by the original pests by upsetting the balance of the ecosystem. Secondary consumers (birds and mammals) are not killed directly by insecticides, but they have less to eat when there are fewer insects. This means their population decreases and also affects tertiary and quaternary consumers. When the number of secondary consumers is reduced, the pests they were keeping in check will increase in number and may cause more damage than the original pests.
There was a problem with the insecticide DDT a few years ago. It was good at killing insects so large quantities of it were used. However, it was not realised at the time that DDT was a very long lasting chemical. It stayed in the soil it was sprayed in for up to ten years and plants grow in the soil absorbed the DDT. When humans ate these crops, the DDT was not excreted but remained in their lipid tissues. Luckily no harm came to the humans who had DDT in their lipid tissues, but it did harm animals. DDT appeared in the lipid of animals throughout all the food webs where primary consumers fed on sprayed crops and where DDT drained from crops into waterways.
As DDT is so soluble in lipids, it builds up in the lipid of each animal as it goes up the food chain. Peregrine falcons became very rare in the UK due to DDT. They could not breed successfully because the levels of DDT had built up so much that the shells around their eggs were abnormally thin and broke easily. This is another defect of chemical control: they were designed to be persistent so that they would kill more insects. However, as they break down slowly, they remain in the bodies of insects a long time and ...
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As DDT is so soluble in lipids, it builds up in the lipid of each animal as it goes up the food chain. Peregrine falcons became very rare in the UK due to DDT. They could not breed successfully because the levels of DDT had built up so much that the shells around their eggs were abnormally thin and broke easily. This is another defect of chemical control: they were designed to be persistent so that they would kill more insects. However, as they break down slowly, they remain in the bodies of insects a long time and if they are eaten by animals further up in the food chain, the insecticide passes into that animal. As you go up each trophic level in the food chain, the poison builds up to a lethal concentration. No matter how carefully the chemical pesticide is applied, it still affects other organisms, like DDT did. It did not affect the intended pest only, but also damaged humans, therefore it got banned. If you use chemical pesticide, any of the pests which try to damage the crop will be killed.
Fungicides do not get into the plant, do not protect new growth and are easily washed off by rain. They have to be sprayed on the crops several times in a growing season. Recently, benomyl has been developed which is absorbed by plants and kills the fungi inside it. In the long term, this makes chemical control more expensive, as the fungicide has to be constantly re-applied. This means more fungicide has to be bought, and it is economically inefficient, as farmers have to spare labour on applying the fungicide repeatedly.
Herbicides have had a great effect on ecosystems throughout the world. Weeds are the food of many primary consumers. There are many instances where not only have the weeds disappeared but so have primary consumers, such as butterflies, which depend on them. This disrupts the ecosystem and food webs are affected by pesticides. When the number of pests decreases due to the use of pesticides, the predator numbers die out as there are not enough pests to sustain them. This means that when the next generation of pests arrive, the populations explode as there are no predators to keep the numbers of pests in check. This causes even more crop damage than there would have been without the use of pesticides. There could also be secondary outbreaks, which does not happen with biological control. When the predators and prey are removed, other organisms which are not normally pests replace them. For example, when insecticides were used to kill insect pests of fruit, the secondary consumers that fed on the pests were reduced in number. These consumers also ate the red spider-mite. Now the red spider-mite has become a major pest and it is resistant to many pesticides. As its natural enemies have disappeared, farmers end up with a greater number of pests than when the pesticide was first used.
When a pesticide is used a great deal, the pest may develop resistance to it. This is what has happened with DDT and mosquitoes. It is an example of natural selection. Mosquitoes that were not affected by DDT were the ones that survived and passed on their resistance to their offspring. Where DDT was used a great deal, most of the mosquitoes are now resistant to it. That was a waste of money as DDT had to be tested (raising development costs), and in the end it was harmful to us and the insects were breeding generations that were immune to the effect of DDT. This is the tendency with most chemical pesticides: they have a tendency to become less effective, and are also harmful for other wildlife, especially humans.
The manufacturers need to recover the costs of developing the pesticide quickly as the pesticide only remains effective for a limited amount of time until the pests develop genetic resistance to them. New pesticides have to be used once pests become resistant to it, which costs more money. Insecticide resistance is reaching a crisis point at this moment in time all over the world. The loss of use of the chemical pesticides means all the money gone into developing, researching and producing the product is wasted. Chemical control is not very effective for very long as the pests develop resistance to them extremely quickly. The surviving pests live to reproduce, and insects can reproduce very quickly, the next generation has resistance to the pesticide.
Pesticides are powerful chemicals. Some pesticides break down in the environment, which damages it. They also get into the water supplies when there is heavy rainfall, which is dangerous for humans. They get washed into the soil and spread throughout the waterways. This chemical pollution may kill many organisms some distance from the crop. DDT was even found in the penguins in the Antarctic. Presumably it had been in the fish they had eaten. It took years to discover the harmful effects of DDT. There is the danger that other harmful effects of pesticides have not yet been discovered, so the amount of chemicals on our food is extremely dangerous.
Biological control is the intentional manipulation of natural enemies by man for the purpose of controlling pests. Biological control involves the use of other living organisms to control the pests. They are usually predators or parasites of the pest. Pests are not usually native to the environment in which they are pests in. They have no predators to control their growth and activities. For biological control to be effective, their predators and parasites need to be traced form their original environment, and then introduced into their new environment. Trials need to be carried out to ensure the predator will only attack the pest and no other organism as it will disrupt population growth curves. The predator must not carry any diseases that it might spread to the crops and it must be able to survive in its new environment. It forms a predator prey relationship if it is successful, where the number of predators increases once the number of prey increase, and the number of predators decreases if the number of prey decrease.
There are three ways to apply biological control: conservative, augmentation, and classical. Classical is when the natural enemy is introduced into the environment. The predator is not native to the environment, so classical biological control is when exotic predators are used to combat native pests. This form of biological control was used on a weed that was out of control in Arizona called puncture vine. It had spines that could puncture tyres and it had a major impact on vehicle traffic in rural Arizona. To overcome this problem, a weevil was imported and it attacked, killed and almost completely exterminated the vine. Other examples of classical biological control are Cottony Cushion Scale, Cereal Leaf Beetle and Alfalfa Weevil.
Augmentation is when the native predator of the pest is raised in large numbers then released to control pest numbers. Examples of predators already present in the area and used for augmentation biological control (mass rearing) are lacewings, lady beetles, wasps and the Colorado potato beetle parasitoid.
Conservative is when the native parasites, predators and diseases of native pests are protected from natural regulation so that they increase and theoretically provide control. There are a few examples of this type of biological control, such as, the Japanese beetle-milky disease, Bacillus thuringiensis (B>T) and nuclear Polyherosis virus (NPV). Biotechnology is also a form of biological control. Usually there is genetic manipulation of the crops so that it is less susceptible to pests, but sometimes there is genetic manipulation of the pest.
I feel that biological control has many advantages over chemical pest control. Biological pest control is useful to us when the application of chemicals is impossible due to the damage it may cause to the environment, or when the terrain is difficult so it is physically impossible to apply it. In the long term, biological control is much cheaper than chemical control. Initially a lot of money has to be invested for the research, importation of predators and mass rearing of them. However, once they have been introduced into the environment, they do not need to be re-introduced. They are a permanent method of control, unlike chemical control, and they are inexpensive over the long term. The natural predators of the pest reproduce and provide food for themselves once they have been introduced into the environment. Biological control poses no inconvenience after the organism has been introduced and established. They can spread on their own after being introduced into their new environment and no additional effort is required. Pests do not become resistant to organisms used during biological control, so it is a permanent solution, which is also cheap and virtually free of charge. It is cheaper and more effective to use biological control as it is inexpensive. It also more imperative that this method of pest control is used because supermarkets are demanding that their produce has to be grown without using chemical sprays, as we do not want man-made chemicals on our food.
This method of control, once initiated is quite efficient and fast acting. The predator-prey relationship takes a few months to develop and the number of pests is reduced tremendously. Although chemical control has an immediate effect on the pest, it soon becomes resistant to it, so biological control is a slower process but a more permanent method of control. It is also non-polluting. There is no toxicity or residual problems with biological control, as there is with chemical control. This makes biological control safer to use than chemical control as there is no danger of contamination of waterways and our food. We will not be affected in any way by biological control as it is environmentally safe. Death of humans is not even a remote possibility with biological control because it is ecologically sound.
Biological pest control is extremely specific as it definitely only affects the pest and no other organism, such as humans and plants. It poses very little threat to non-target organisms; therefore the environmental impact is low. It will not affect any other organisms that are also pests on the same plant. It is environmentally safe and there are no secondary outbreaks (it is host specific). By using biological control the pest cannot develop resistance to predators or parasitoids and there is no co-evolution.
However, there are also a few problems associated with biological control. By the addition of a predator, there is an effect on the natural food chains, just as there is on with chemical control. Farmers may end up with a worse problem with pests than they started with. As was explained before, if the number of pests is reduced, the number of predators also falls (secondary consumers). This has a direct effect on tertiary and quaternary consumers. Therefore, the pest is never completely eliminated; otherwise the predator will compromise its own survival. There are always a few pests left and kept at a low level, so crop yield is still affected when using biological control. In this respect, chemical control is better, as it increases crop yield much more than when using biological control. Biological control is very information intensive - the manager needs to understand the ecosystem that they have created because there will be various effects that have already been discussed.
So far, there are only a few known examples of its successes. In the US, the two-spot ladybird was successfully bred and released in orange groves to eat the aphids on the orange trees. There are also, wasps (encarsia) that lay their eggs in the bodies of young whitefly. The whitefly is killed by the development inside it of a wasp from an egg. Fish have been put into lakes to eat mosquito larvae and prevent the spread of malaria by adult mosquitoes. Myxomatosis (a rabbit disease) was spread deliberately in Britain in order to reduce the vast number of rabbits which ate food crops. A subtle method of biological control is the introduction of sterilised males into the populations of insects, so that the population is reduced as the eggs are infertile.
Biological control is unsuitable for use on stored food as the excretory products of the consumers is mixed with the food. This is extremely unhygienic, and would also have a damaging effect on humans. Biological control is also a very slow process. There are many generations before the pest is controlled (lag effect) and the time of parasitization/infection to death is long. The time of death for pests from predators takes longer than if they are sprayed with chemicals. The pest may cause an infection after it has been attacked by a predator, causing crop damage. There may be physical damage to the crops as the predator attacks the pest.
It takes a lot of time for the predator-prey relationship to establish itself. It takes time to reproduce enough of the predators to be effective against the number of pests. The new equilibrium position remains above the economic threshold, so the product yield is not worth the money put into the years of research put into finding a suitable predator. It takes years and years of research as the predator of the pest has to be located, tested, and then released into the environment. Research costs are high for biological control, and the results may not be produced as the behaviour of the predator cannot be predicted. With biological control there is also the danger of introducing an exotic organism into the environment that affects the desired plant itself as well as the pest.
Biological control is host specific, but other pests occur on crops too. There are not enough predators available for all the target pests we are concerned with. Other pests occur on crops too, and biological control is not effective against those as it is a very specific method. Some of the predators for target pests are also predators of the desired plant, which means the problem cannot be resolved. Chemical pesticides do not affect the plant in any way, just all of the organisms that feed on it, whether they are targets or not. Using chemical pesticides means the yield will increase, as all of the pests are killed immediately.
In conclusion, I feel that biological pest control is better as it is cheaper (in the long term), efficient, less damaging to the ecosystem, friendlier to the environment, the extensive research done is productive as it is a permanent solution to pests. Pests do not become resistant to their predators or parasitoids. However, chemical pest control is a lot faster and more effective as it exterminates the pests completely. These are its only advantages however, as it has a negative effect on the environment. It causes the build up of poison throughout the food chain, making lethal concentrations occur at the top of the food chain. This is called Biomagnification - pesticides accumulate in plants, insects and are concentrated more and more as they move up the food chain. This causes problems with fish and wildlife. Pesticides are also a lot more expensive in the long term, and pests keep becoming resistant to it, so money is lost on pesticides that are not effective any longer, and it has to be constantly re-applied. It requires input and additional effort periodically. I think biological pest control is environmental friendly, and a much safer way of controlling pests. It is also much less expensive and pests do not become resistant to its predators. Biological control is more specific and does not affect other non target organisms, and it is a more permanent way of controlling pests as they cannot develop any resistance. Chemical control has too many dangerous side effects to humans, especially, unlike biological control which does not affect any other organism other than the target one. Chemical control can lead to greater crop damage than if no pesticide had been used due to resurgence of pest populations. When pests are killed the predators are either killed, leave the area for lack of food, or starve. When the next generation of pests arrive, the populations explode, because predators are not present to keep numbers in balance. Biological control, on the other hand, develops a predator-prey relationship to prevent this from happening. There are secondary pest outbreaks associated with chemical control, which is not the case with biological control. Once a pest and its predators have been removed, another pest replaces it. There are also many non-target side effects, such as contamination of water, food, and death of workers. Biological control is better than chemical control because it does not cause death of humans, and does not have a severe impact on the ecosystem.
Bibliography
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Books:
* A new introduction to Biology by Bill Indge, Martin Rowland, Margaret Baker.
* Making Use of Biology. Second edition by Pauline Alderson and Martin Rowland. ISBN: 0-333-62093-3. Pages 150-157.
* Oxford Dictionary of Biology, fourth edition by university press. ISBN: 0-19-280102-3
* Key Science Biology by David Applin. ISBN: 0-7487-1676-9
Specialist Publication:
New Scientist
April 2003
Issue 6
Aisha Hussain Page 1 09/05/2007
