The impacts of molluscicide pellets on spring and autumn populations of wood mice Apodemus sylvaticus

Methiocarb is a carbamate molluscicide widely used in pelleted form. Wood mice Apodemus sylvaticus are commonly found in arable fields and will eat methiocarb pellets. Two previous field studies on the impacts on arable wood mice of broadcasting methiocarb pellets produced contrasting results, the number of mice being temporarily reduced in one but not the other. The season of application is also likely to influence the degree of impact on populations but the previous trials were both in autumn. The aim of the present study was to determine the effects on wood mice of broadcast applications of methiocarb both autumn and spring and to determine whether depression of brain and serum cholinesterase (ChE) activity; a biomarker of exposure, could be detected in mice alive on treated fields after application. In trials carried out in October 1994 and April 1995, there were decreases in the number of wood mice on arable fields following pesticide application. The October decline (78%) was greater than that in April (33%), even through the activity of the mice appeared to be more heavily centred on the hedgerows; where pellets were not applied, in the autumn. Despite, the reductions in numbers, no depression of ChE activity was detected in mice captured alive after application, suggesting that either exposed animals were not captured or that reactivation of ChE activity was rapid. This study confirmed that broadcast application of methiocarb can reduce wood mouse populations on arable fields. It also demonstrated that such effects can occur irrespective of season and that measurement of ChE activity in mice trapped after application may fail to indicate that exposure of the population has occurred.