Impact of - IDL-BNC @ IDRC - International Development Research ...

Scope of the problem
Although the effects of pesticide use are varied, direct mortality of wildlife, in
and around treated fields and forests, is one of the most visible signs of a
pesticide's impact. More than 30 pesticides registered in North America and
Europe have been known to result in wild bird or mammal kills, even when
used according to the relatively stringent regulations in those regions. Such
cases of direct mortality usually have two characteristics in common: acutely
toxic insecticides are often, although not solely, responsible; and the pesticides
reach wildlife through a ready pathway.
Wildlife species are exposed to pesticides through many routes. They may
ingest toxic substances in food or through preening and grooming; they can
absorb pesticides through their skin, encountering droplets directly or by
rubbing against foliage and other contaminated surfaces; and they can inhale
vapour and fine droplets. The degree to which each of these routes of exposure
contributes to the total dose depends on the crop being sprayed, the chemical,
the animal species exposed, and other factors.
Cases of secondary poisoning can occur when predators consume prey contaminated
by pesticides. Such predators are few because of their position at
the end of a food chain. The death of a predator may constitute a significant
reduction in the local population of that species. Avian predators are especially
important agents of control for a number of species considered to be pests,
e.g., rodents. Poisoning of this kind has been closely associated with organochlorine
insecticides and other substances that are not readily metabolized
and, therefore, accumulate in tissue. Other currently registered pesticides,
even if readily metabolized, can also cause secondary poisoning under the
right conditions, e.g., when the predator encounters a high concentration of
the pesticide in its prey.
Generally, insecticides are more toxic to animals than herbicides, fungicides,
and most other pesticides, except vertebrate control agents. Two groups of
insecticides, the organophosphates and the carbamates, cause most deaths;
they affect the nervous systems of insects and vertebrates alike. Because wild
animals and birds know nothing of "safe reentry intervals" for treated areas,
they run the risk of being exposed to very high doses simply by being in the
wrong place at the wrong time. The use of a toxic insecticides affects not only
the wildlife present in a treated area, but also species with habits (hunting,
foraging, or migratory) that may cause them to wander into areas where
pesticides are being applied.
The impact of pesticides on plants and invertebrates, whether target or nontarget,
may indirectly affect birds, mammals, and other vertebrate wildlife,
particularly when a "pest" (which may be food or shelter for another species)
is eradicated or drastically controlled. To make pest-control programs compatible
with the preservation of wildlife, the most target-specific pesticides
must be chosen and, ideally, unsprayed areas must be provided as refuges.
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