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Formulation-specific concerns
The extent to which wildlife species are exposed to the active ingredient in a
pesticide formulation is usually unclear. Similarly, it is generally not known
how long the various other elements of a pesticide formulation remain with
the active ingredient.
Predictably, granular formulations of organophosphate and carbamate insecticides
are equal or less toxic than the equivalent technical grade materials (Hill
and Camardese 1984). Unfortunately, granular formulations tend to enhance
the availability of a pesticide to birds, which negates any advantage of reduced
toxicity. Liquid formulations, on the other hand, are typically more toxic than
the pesticide parent material (Hill 1986).
Application rates and expected residue levels
Under operational conditions, considerable variation in application rates of
pesticides can be expected. This is generally not recognized as a problem by
pesticide users, because of the relatively wide margin of safety of pesticides
in terms of both efficacy and safety to the crop. Exact application of a pesticide
according to label instructions would require accurate measuring of the various
components, perfect calibration of the equipment being used, faultless
technique on the part of the applicator, and ideal weather conditions and
terrain. Even under the highly regulated and mechanized conditions of developed
countries, these requirements cannot be met. It is more reasonable to
expect that the rate of delivery of the pesticide follows a broad distribution
about the desired application rate. This is especially true in the developing
world where most spraying is done using by knapsack sprayers and is more
vulnerable to human error.
Other situations also give rise to a higher-than-intended rate of application of
a pesticide. Drift is a problem during both ground and aerial applications. For
hazard assessment, an important aspect is the additive nature of droplet drift
associated with multip'e-swath applications. This can give rise to high pesticide
application levels in downwind fields and beyond (Maybank et al. 1978).
If the route of exposure of wildlife species is primarily through the consumption
of contaminated food, the degree of exposure is only approximately
related to the amount of pesticide delivered to the crop and nearby noncrop
areas. In currently accepted procedures for risk assessment (Urban and Cooke
1986), residue levels on foodstuffs are estimated on the basis of standard
factors that assume that the rate of application and surface area are the only
factors having a bearing on residue levels. However, widespread mortality of
waterfowl caused by the use of the insecticide diazinon (Stone and Gradoni
1985; Frank et al. 1991) illustrates the difficulties associated with accurately
predicting residue levels on plant surfaces (Mineau 1991). Residue on grass
blades after a mechanized application of diazinon at the rate of 1.1 kg active
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