|Initial Environmental Assessment: Plant Protection - Series no 13 (NORAD, 1995)|
|Part I: General account|
|3 Possible environ mental impacts|
The purpose of using pesticides is to protect cultivated plants against pests. When properly used, pesticides will result in good conditions for cultivated plants, while the pests are being controlled. However, pesticides do not only affect pests. Other organisms are also affected, either directly or indirectly. When an organism comes into contact with a pesticide, the pesticide will have a direct effect on the organism, depending on the properties and dosage of the pesticide. A pesticide can also have indirect effects, as when it alters the food chains for organisms.
Accumulation of pesticides in the food chains can often be a serious problem. Accumulation can occur when organisms on one level in the food chain eat organisms on a lower level which contain relatively small concentrations of pesticides. When the animal digests such organisms the pesticide is only partially liberated, and is stored in the body of the animal. In the course of time the animal can gain pesticide concentrations in the body which far exceed the amount in each of the organisms it ate. If such animals are eaten by other animals higher up on the food chain, the same thing will happen again. Thus pesticide concentration will become greater higher up in the food chains. This phenomenon is called bioaccumulation, and it means that the species on the higher levels will be affected the most. In some cases this means humans. The impacts of pesticides accumulated in food chains vary depending on which species are being affected. Pesticides will be stored more readily in some organisms than in others, and the toxicity can vary from one organism to the next. Many phosphorous organic pesticides are very poisonous, have long life span and/or have poisonous decomposition products. Some very poisonous, but highly effective pesticides can pass quickly through the food chain. An example are dying insects eaten by rodents, which in turn will be eaten by birds of prey or predators. The effect can spread across enormous distances and affect stocks of birds and mammals which live and breed far away from the sprayed area, for example as with migratory birds and animals. The impacts can vary significantly. If the pesticide is poisonous to the organism, the organism will decline. It will also decline if the pesticide removes the means of subsistence or diminish the living conditions for the organism in other ways. The reverse will happen if the spreading of the pesticide causes the decimation of rival organisms, or if the pesticide actually becomes an additional nutrient source. The latter is the case for organisms which are able to break down pesticide molecules. Such organisms will subsequently increase in numbers.
Impacts on soil organisms: The soil organisms are important for plants and for animals in the higher levels in the food chains. They produce nutritive substances which the plants can utilise, and can also be food for animals on higher levels in the food chains. The interaction between different kinds of soil organisms is so complex that it is difficult to draw any general conclusions about how they are affected by pesticides. It seems that isolated use of pesticides has no long-term effects on soil processes. However, long-term effects may occur after repeated treatments. But it is not certain whether the total impact is positive or negative.
Plant species living in the areas where the pesticide is used can be affected, for example if bees and other pollinating insects decline, or if the plants are affected in a more direct way. One example is when plant populations defined as weeds are exterminated or kept in strict control in and around the areas exposed to herbicide treatment. This will result in a decline in the total seed setting in the area, which in turn will lead to a decline of the species. The species can often be important in interaction with the ecosystem in the nearby areas. Whenever one species declines, another will usually increase. Herbicides may thus alter the composition of plant species in the area. The use of herbicides may result in the decline of a number of plant species.
Insects: The composition of insects is affected by the predominant plants in the area.
Usually the number of insects will decline when the number of plant species declines. Insects are affected not only by the composition of plant species, but also by the fact that many pesticides have been produced with the intention to kill insects. Insects can be affected by coming into direct contact with the insecticide or by eating other affected insects.
Fish: Fish living in downstream areas subjected to pesticide treatment are often affected. This is particularly the case for fish living in brooks, rivers or small lakes which are fed by areas subjected to pesticide treatment. Sudden discharges can also cause problems.
Birds: According to reports, birds' feathers and bodies are rarely directly exposed to pesticides. However, birds may be affected by eating rodents, worms or insects containing pesticides, or by eating pesticide-treated seeds or pesticides granulates. This can have both acute effects as well as long-term impacts, such as reduced reproduction. Birds for whom insects are an important nutrient source, can be affected if the number of insects decline. Both local as well as migratory birds can be affected by pesticides.
Larger animals can also be harmed by pesticides. For example, herbivores can eat pesticide-treated plants and consequently be affected. Predators can be affected by eating plant-eaters containing pesticides.
When using pesticides in large areas, as for example when spraying against locust swarms and tsetse flies, many other groups of animals will inevitably be affected by the pesticide. Such groups are generally described as Non-Target Organisms. Mainly insects and other anthropodes will be affected in such cases, but other kinds of animals, such as birds, can also be at risk. Species with many different functions in the ecosystem can be affected, including plant-eaters, pollinating insects, predators and parasites. In some cases, rare and vulnerable species can be affected.
The choice of pesticide and type of pesticide treatment is generally difficult. It requires not only information about the impact on the organisms which are to be protected or controlled, but also about the following aspects:
· The degree of acute toxicity to humans and animals.
· Properties which may cause the pesticide to be accumulated and concentrated in the food chains (bioaccumulation).
· The extent to which the degradation speed of the pesticide is affected by different climates and different media (such as water, soil, agricultural products etc.).
· The degree of toxicity to the natural enemies of the pest organism.
· The potential risk of developing resistance to the pesticide.
The following mitigative measures can limit the impacts on flora and fauna:
· Use a pesticide which is highly selective, i.e. attacks only selected species.
· Bioaccumulative and highly poisonous pesticides should not be used in areas where there is any risk of water pollution by transport of sediments by soil erosion or heavy rain, or by wind drift. If that is the case, the pesticide can end up in sediments where it can cause the pollution of fish and other edible animals for several decades in the future.
· If there is an ongoing integrated pest management project in the area, pesticides should be used with caution, to prevent any impacts on biological control agents.
Using reduced doses of pesticides can cause some harm on non-target organisms. One should therefore have sufficient information about whether using reduced pesticide doses can control the pest while at the same time reducing the impacts on the non-target organisms. Research on insecticides used on locusts in Africa has shown that reduced pesticide doses can control the locusts, while the negative impacts on other insects in the ecosystem are being reduced. The doses used were reduced to less than half of the doses recommended by FAO and the manufacturer, but good results depended on the type of pesticide used. More recent pesticides tended to be better than old pesticides. The effect of reduced pesticide doses was better in areas with little vegetation than in areas with dense vegetation.