Pesticides and their Effects on Wildlife

Abstract

About 560 active ingredients are currently used as pesticides. Applications of these pesticides are made to agricultural lands and other areas inhabited by wildlife. Unfortunately, many agricultural-use pesticides also entail some measure of risk to organisms other than the pest species. Because testing of pesticides prior to registration cannot evaluate all the potential environmentalpesticide-wildlife/fish interactions, current methods of risk assessment do not always provide sufficient safety to nontarget organisms. This is evidenced by die-offs of fish and wildlife from applications of pesticides at environmentally "safe" rates (Grue et al. 1983; National Research Council of Canada 1975), the linking of population declines of some species with agrochemical use (Fox et al. 1989; Grue et al. 1986Grue et al. , 1989, and observations of survival-threatening behavioral changes in laboratory and field animals exposed to typical field levels of pesticides (Costa and Murphy 1982;White et al. 1979;Galindo et al. 1985). It is important to note, however, that the majority of pesticides, when properly applied, have not caused significant injury to wildlife (Cooper 1991).A brief summary of pesticide effects on wildlife and fish are presented for the common classes of pesticides in use today. Organophosphate and Carbamate InsecticidesOrganophosphate pesticides (OP) and carbamates (CB) are among the most heavily used agricultural pesticides in the United States. Both OP and CB insecticides act principally by inhibiting the enzyme acetylcholinesterase (ACHE) (Casarett and Doulls 1986). AChE normally cleaves the neurotransmitter, acetylcholine, from activated neural and myoneural junctions, thus restoring the nerve or muscle to the resting state. Inhibition of AChE by OPs or CBs results in excessive buildup of acetylcholine at the nerve junctions. This buildup disrupts nerve impulse transmission and can cause death by blocking neurotransmission in the respiratory center of the brain or at myoneural junctions of the respiratory muscles (O'Brien 1967; Casarett and DouUs 1986). Sublethal signs of intoxication in wild animals include ataxia, tremors, salivation, lacrimation, loss of fighting reflex, tetany, diarrhea, piloerection (or fluffed feathers), and polydipsia.Many OPs bind permanently to ACHE. Several weeks may be required to synthesize new ACHE and re-establish normal levels of the enzyme in the animal. (Robinson and Beiergrohslein 1980;Fleming 1981). Therefore, additional exposures to AChE inhibitors (OPs or CBs) during this recovery period can diminish the ACHE stores to hazardous levels. In contrast, little cumulative toxic action can occur with CBs because the chemical reaction of the CB with AChE is completely and rapidly reversible (Casarett and DouUs 1986) and the parent compounds rapidly degraded (Tucker and Crabtree 1970;Busby et al. 1987). Multiple applications of dimethoate, chlorpyrifos, methomyl, carbaryl and methyl parathion have been shown to reduce the survival of birds inhabiting edge habitats adjacen...