Pitfall trap catches of and aphid predation by Pterostichus melanarius and Pterostichus madidus in insecticide treated and untreated potatoes

Aphids were counted in situ and carabid beetle populations assessed by pitfall trapping between 1983–1985 in potato fields in Scotland treated with demeton‐S‐methyl (DSM) or untreated. Macrosiphum euphorbiae (Thomas) (Homoptera: Aphididae) was the most abundant aphid. Nineteen species of carabid beetles were trapped but the fauna was dominated by Pterostichus melanarius (Ill.) and Pterostichus madidus (Fabr.) (Coleoptera: Carabidae). Pitfall trap catches of these carabids were variable but generally lower in sprayed than unsprayed plots for a few days after DSM application and higher a week or two later. Of 1800 P. melanarius and 910 P. madidus dissected, 14.4 per cent and 30.5 per cent respectively, contained aphid remains. The proportion that had eaten aphids often was higher in sprayed than unsprayed plots immediately after treatment. In additional experiments, aphids treated with DSM fell from plants; four to five per cent of the theoretically applied dose of DSM reached the ground in a closed‐canopy potato crop; and no mortality occurred when Pterostichus spp. were exposed to DSM in a field bioassay. It is proposed that observed patterns in trap catch of Pterostichus spp. around times of pesticide application may be due in part to aphids falling to the ground after treatment. Beetles that eat these fallen aphids may be less hungry, less active and less likely to be trapped. Later increases in catches may be attributed to hungry, more active beetles, a consequence of prey removal by the insecticide. Although this hypothesis requires further research, it is clear that the effects of insecticide treatment on carabid populations cannot be measured by pitfall trapping alone.