Pesticide Toxicity

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Background information

Estimating fairly the acute toxicity of pesticides to birds is not easy. This assessment depends for the most part on two laboratory-derived measures of lethality. First, the median lethal dose (LD50), a statistically derived single oral dose of a compound which will cause 50% mortality of the test population. Second, the median lethal concentration (LC50) which similarly derives the concentration of a substance in the diet which is expected to lead to 50% mortality of the test population. For a number of reasons explained elsewhere (see Mineau et al. 2001 in reference list) we prefer to use the median lethal dose.

When carrying out comparative risk assessments for pesticides, it is essential to use the most unbiased estimates of toxicity possible. Pesticides are customarily tested against a handful of bird species; yet, there are an estimated 10,000 species in the world; over 800 species occur in Canada and the U.S. alone and hundreds are expected to be exposed to pesticides. Often, analysts have used the lowest value available from all species tested in order to compare the toxicity of different pesticides. This approach, however, introduces a systematic bias related to the amount of test data available and reported for each pesticide.

Alternatively, a common test species alone is reported such as the Mallard Duck or Northern Bobwhite. However, this ignores data that may be available for other species or our knowledge of the relative sensitivity of those common test species relative to other bird species. These methods also ignore the fact that the common test species tend to be on the large side whereas we know small birds tend to be more sensitive to pesticides by virtue of their size alone (Mineau et al. 1996).

The approach followed here is outlined in detail in Mineau et al. (2001). It consists of constructing a mathematical distribution of all available data for each pesticide. When data are insufficient to construct such a distribution, an extrapolation factor which is based on the known relative susceptibility of the test species is used instead. The listed reference values are calculated to be the best estimates of the LD50 for a bird species at the 5% tail of the distribution of sensitivity to a given pesticide regardless of its body size. In other words, we expect 95% of bird species to have a median lethal dose value (LD50) higher than the value tabulated. In our opinion, these values represent the fairest value with which to compare the relative toxicity of pesticides to birds.

How to read the table

For each pesticide (identified by common chemical name and Chemical Abstracts Service (CAS) Registration Number) is listed the number of species tested. The higher this number, the most confidence can be placed on the resulting toxicity reference value. If the data were insufficient to fit to a distribution, the number 1 or 2 appears indicating that an extrapolation factor was applied either to a single species or to the geometric mean (the de-transformed mean of the log values) of 2 species. When several factors were possible, they were picked from table 1 so as to result in the smallest estimation error possible. Note that these values are estimates only - even though these are the best estimates currently available for the toxicity of this pesticide to birds at large.

You cannot find some pesticides in the table

By using an available single species LD50 and dividing this value by the species-specific factors provided in table 1, you can derive an acute toxicity reference value directly comparable to the values given here. Once again, we urge caution. A single toxicity value is not much on which to base the toxicity of the pesticide in question to birds at large.

Toxicity versus risk

A toxic pesticide need not represent a risk to birds if exposure to that pesticide is limited or negligible. However, a high acute toxicity makes it much more likely that a pesticide will be implicated in kill incidents (Mineau et al. 1999). Models were built to successfully predict the probability of bird mortality based on the acute reference values given here and physico-chemical properties of the pesticide (Mineau 2002). As a rule of thumb, any toxicity reference value over 100 is unlikely to ever represent an acute risk to birds. Because extrapolation factors were often applied to limit values (e.g. to an acute LD50 > 2,000 mg/kg), comparison of pesticides with toxicity reference values much over 100 are not warranted. In other words, a pesticide with a reference value of 1000 is not necessarily safer to birds than one with a value of 200. Both can be assumed to be equally safe in an acute sense. However, none of the values given here reflect anything other than acute toxicity. The chronic or reproductive toxicity of pesticides to birds, or their indirect effect on birds through their food is not considered here.

References and further reading

Mineau, P., B.T. Collins and A. Baril. 1996. On the use of scaling factors to improve interspecies extrapolation of acute toxicity in birds. Regulatory Toxicology and Pharmacology 24:24-29.

Mineau, P., M.R. Fletcher, L.C. Glazer, N.J. Thomas, C. Brassard, L.K. Wilson, J.E. Elliott, L.A. Lyon, C.J. Henny, T. Bollinger, and S.L. Porter. 1999. Poisoning of raptors with organophosphorous pesticides with emphasis on Canada, U.S. and U.K. J. Raptor Research 33(1):1-37.

Mineau, P., A. Baril, B.T. Collins , J. Duffe, G. Joerman, R. Luttik. 2001. Reference values for comparing the acute toxicity of pesticides to birds. Reviews of Environmental Contamination and Toxicology 170:13-74.

Mineau, P. 2002. Estimating the probability of bird mortality from pesticide sprays on the basis of the field study record. Environmental Toxicology and Chemistry 24(7):1497-1506.

Mineau, P. 2003. Avian Species. In Encyclopedia of Agrochemicals, J.R. Plimmer, D.W. Gammon and N.N. Ragsdale [eds.]. Wiley Interscience, vol 1., pp. 129-156.

Mineau, P. 2004. Birds and pesticides: Are pesticide regulatory decisions consistent with the protection afforded migratory bird species under the Migratory Bird Treaty Act ? The William and Mary Environmental Law and Policy Review 28(2): 313-338.

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