Toxicity of pesticides to birds; the enzymic factor.

Introduction The toxicity of pesticides to birds has long been a matter of concern to conservationists, and is a factor taken into consideration by regulatory authorities when making environmental risk assessments for new chemicals. Experience has shown that toxicity data obtained using rats, mice and rabbits do not provide reliable guidance as to avian toxicity. Where the toxicity of a pesticide is found to be substantially and consistently different between birds and mammals it is of interest to establish the toxicokinetic and/or toxicodynamic factors which are responsible. The first part of this paper will consider some metabolic differences between birds and mammals which are related to the selective toxicity of certain pesticides. A number of pesticides have been shown to cause the deaths of birds even when used in an approved fashion in agriculture and horticulture. Thus, persistent organochlorine insecticides have been implicated in the deaths of grain-eating and predatory birds [ 11, and non-persistent insecticides of high acute toxicity such as carbophenothion and aldicarb have caused deaths of Anser geese and gulls respectively [2, 31. In the llnited Kingdom, the Annual Report of the Wildlife Incident Scheme (M.A.F.F.) draws attention to a considerable number of bird casualties on agricultural land attributable to a range of non-persistent pesticides (usually insecticides) each year. Until now, attention has been focused upon the lethal effects of single chemicals. However, birds are exposed to combinations of chemicals under field conditions and there has been growing concern about possible toxic effects which may be the consequence of interactions between pesticides. The second section of this paper will deal with the toxicological consequenes of the induction or inhibition of enzymes involved in the metabolism of pesticides. It will focus on situations where the modulation of the activity of an enzyme by one pesticide can increase the sensitivity of birds to other pesticides.