Mortality of Birds of Prey Following Field Application of Granular Carbofuran: A Case Study

The death of buzzards (Buteo buteo) found in fodder and sugar beet fields treated with granular carbofuran was investigated. To determine if there was a causal relationship between carbofuran treatment and bird mortality, earthworms of a specific fodder beet field as well as the crop contents of dead buzzards were analyzed for carbofuran contamination. Earthworms of the fodder beet field contained carbofuran concentrations up to 3.2 mg/kg, while remains of earthworms and carbofuran were detected in all buzzard crop contents. The carbofuran concentrations found in the earthworms as well as in the crops of the buzzards corroborate the suspicion that the buzzards found on fodder and sugar beet fields treated with granular carbofuran had died of secondary poisoning with carbofuran via contaminated earthworms. Wildlife die-offs in areas of intensive agricultural activity have often been observed in conjunction with the use of pesticides (Smith 1987). Indeed, most die-offs usually take place over a relatively short time-period and occur in a defined geographic area. Yet, in many instances, die-offs involve only very few animals, making it almost impossible to determine whether the animals had died of natural causes or as a consequence of a human activity, such as chemical crop protection in intensive agriculture. Furthermore, the site where the animals are found need not necessarily be identical with the location where the animals had been exposed to the xenobiotic. Unless the animals had been exposed to a lethal dose of a quickly acting toxicant, the chances are fairly high that the location where the animals are found is not identical to the location of exposure. This especially holds true for birds. In view of this situation, it is understandable that it is difficult to causally relate the use of a specific pesticide on a given field to wildlife mortality occurring simultaneously or post pesticide application in the same field. Furthermore, the high costs involved in cadaver analysis, in conjunction with the small chance of a successful identification of the causative agent, makes investigations of wildlife die-offs involving only very small numbers of animals not a very inviting and promising undertaking. However, in some instances where recurring wildlife mortalities appear to coincide with the use of a highly toxic pesticide, it may be worthwhile to investigate a possible causal relationship between pesticide application and wildlife die-offs. Such a situation was observed in Switzerland, where raptor die-offs, usually buzzards (Buteo buteo) and red and black kites (Milvus milvus and Milvus migrans), have been occurring regularly every spring. Indeed, since 1980 a total of 90 cases, occurring in spring and involving only Buteo buteo, Milvus milvus, and Milvus migrans, were registered by the Swiss Ornithological Institute. Of these 90 birds, 50 were found dead and 40 were reported to be severely ill (Jenni-Eiermann et al. 1995). Although the individual die-offs encompassed only very small numbers of raptors per location (1-3 birds), many of these birds were found either on or in the vicinity of fields that had most recently been sown with fodderor sugar-beet. When seeding fodder-beet, it is general agricultural practice in Switzerland to simultaneously apply carbofuran for seed protection. Carbofuran is a carbamate insecticide-nematicide 1 which is usually applied at a rate of 20-30 kg formulation/ha i.e., 1-1.5 kg active ingredient/ha. Granular formulations of carbofuran have been reported to be highly toxic to wildlife and particularly to birds (Flickinger et al. 1980; Balcomb 1983; Balcomb et al. 1984a, 1984b; Mineau 1993). In general, carbamate insecticides rapidly inhibit cholinesterase's via carbamylation of the enzyme, killing the exposed animal as a result of neuromuscular and central nervous system blockage (Hill 1989). Thus, the question was asked whether the reported raptor deaths were causally related to field applications of carbofuran. Granular 1Carbofuran = 2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarCorrespondence to: D. R. Dietrich (University of Pittsburgh) bamate Effects of Carbofuran on Buzzards 141 Table 1. Carbofuran concentrations determined by GC/NPD a and GC/ MS b of pooled worm samples collected during the field study at the respective sampling dates Carbofuran concentration (mg/kg) Sampling date GC/NPD 7.04.92 <0.02 10.04.92 (Carbofuran applied) 11.04.92 0.8 c 14.04.92 <0.02 15.04.92 <0.02 16.04.92 <0.02 21.04.92 <0.02 28.04.92 0.1 d 4.05.92 0.6 e