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specifications. Other data are assessed on a case-by-case basis. Canada has its
own guidelines for environmental chemistry and fate (Agriculture Canada et
al. 1987) and is currently developing guidelines for testing nontarget plants.
Urban and Cooke (1986) describe how these data are used by the EPA to
generate hazard scenarios. Canada uses a similar procedure. The goal is to
assess the likely risk that the pesticide in question will cause mortality to
exposed wildlife and hence the need for fieldwork that would confirm or
negate this assessment.
Some of the difficulties in accurately predicting the impact of a pesticide on
wildlife on the basis of laboratory-derived results are described here. Emphasis
is placed on factors that might cause the hazards to be underestimated.
The community at risk
The first step in a pesticide evaluation is to gain knowledge of the biological
communities that are potentially at risk in the area of pesticide use. In a large
country with distinct physiographic regions or for countries with incomplete
fauna! surveys, this can be a formidable challenge. Species may change their
food habits in response to an overabundant supply (such as during an insect
outbreak) and, therefore, even an intimate knowledge of the "normal" ecology
of a species may not be sufficient.
The extent to which pesticide use modifies the propensity of wildlife to feed
in treated fields or adjoining areas is another question that has not received
much attention. A given array of species in the general area of pesticide use
does not necessarily mean that those species will be exposed.
Fletcher and Greig-Smith (1988) have proposed methods to quantify the use
of fields by birds. Given the complex nature of ecosystems, evaluation must
be focused on a few indicator species. These must be chosen, not so much for
their inherent physiological susceptibility to pesticide use (this is not usually
known), but for the likelihood that their life habits will lead to maximum
exposure. Unfortunately, the choice of indicator species has often been made
on the grounds of cost, logistics, and overall feasibility rather than on the
grounds of more scientifically desirable criteria. This can give rise to misleading
signals and a false sense of security.
Representativeness of the test species
Those evaluating the safety of pesticides to humans benefit by being able to
study several surrogate species to extrapolate to the single species of interest.
In contrast, wildlife evaluators can look at only a few species to predict effects
on a diverse fauna. In some cases, they can work directly with the species of
interest, but often interspecies extrapolation is necessary. There are 1-3 million
species of vertebrates living in the world today. Testing all nontarget species
for likely pesticide impact is impractical as well as unethical.
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