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

On the basis of data compiled for a variety of pesticides, Tucker and Leitzke
(1979) suggested that, in 95% of the cases, if the acute toxic dose of any given
compound is known for one species of bird, then the acute toxic dose of the
same compound will be within a factor of 10 (plus or minus) for a second
species. However, one must be careful of such generalizations. Reanalysis of
part of the same data base (bird toxicity data for 44 organophosphate insecticides)
determined that the range in acute toxicity values between species was
highly dependant on, and therefore an artefact of, the number of species tested.
The average number of species tested was 3.9 for compounds showing a range
of toxicity less than one order of magnitude; those showing larger variation
were tested on an average of 6.8 species.
It has been argued (Schafer and Brunton 1979) that some species appear to
show an inherent susceptibility or resistance to a wide range of environmental
toxicants. Thus, current US regulatory testing using the Mallard (Arias
platyrhynchos) and Bobwhite Quail (Colinus virginianus) should be revised,
because these species do not give a realistic assessment of the hazards of
pesticides to the largest group of bird species, the passerines or perching birds.
A reanalysis of some of the large data bases on acute toxicity of pesticides to
birds (Schafer and Brunton 1979; Tucker and Leitzke 1979) has shown that
phylogenetically related birds are not necessarily similarly sensitive to any
given pesticide (Mineau 1991). This casts doubt on our ability to predict the
hazard to a species on the basis of that in a closely related species. This type
of extrapolation has been especially common in dealing with rare or endangered
species or with species that are not amenable to direct investigation.
Assessing dietary exposure
Knowing which nontarget species are likely to be exposed during pesticide
applications requires an understanding of exposure routes. Ingestion of contaminated
food has been identified as the most likely route of intoxication for
wild birds or mammals and is still the only route that is commonly assessed
in standardized hazard-assessment procedures. However, the dietary route
may not be the only or even the main route in all cases of wildlife exposure
(Mineau et al. 1990).
The food intake of a small organism is greater than that of a larger one when
expressed asa ratio of body weight. Thus, other things being constant, smaller
species tend to be more vulnerable to acute pesticide intoxication. Also, at
certain times of the year, wildlife species may have higher energy requirements,
and hence higher food intakes, than at other times. For example, a bird
feeding young at the nest may have much higher energy requirements than
normal. Climatic conditions and physical factors, such as nutritional status,
disease, and parasite load also influence the toxicity of pesticides to the
organism directly, whereas, indirectly, they may alter food consumption patterns
causing the organism to ingest more or less of the pesticide.
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