THE TOXIC EFFECTS OF DIRECT PESTICIDE EXPOSURE FOR A NONTARGET WEED-DWELLING CHRYSOMELID BEETLE (GASTROPHYSA-POLYGONI) IN CEREALS

Direct insecticide deposition on adults of the chrysomelid beetle Gast rophysa polygoni L. was measured by use of fluorescent spray tracer. T he beetles captured 1.3- to 2.5-fold the spray volume of an equivalent area of weed leaf surface. This disproportionate rate of exposure dec lined down through the crop canopy toward the ground. Spray deposition was measured for insects placed on both the upper and the lower surfa ces of Polygonum convolvulus L. leaves within winter wheat crop canopi es. The upper-to-lower-surface deposition ratio was 2.3 irrespective o f the position of the leaf in the crop canopy. The mortality of adult G. polygoni exposed to pesticides on their host plant was estimated, b ased on measurements of spray deposition on insects in different posit ions, the laboratory dose-response relationship for dimethoate, and th e distribution pattern of adults on the host plant measured in the lab oratory. At full recommended field rate (280 g ha(-1)) an expected 40. 7% of the adult population would die as a result of direct exposure to dimethoate. The mortalities on specific positions on the plant ranged between zero and 76.7%. To reduce predicted mortality to a level of 1 0%, it would be necessary to reduce field rate to 50% according to the se calculations. Those individuals surviving the dimethoate treatment had a reduced fertility, whereas the egg viability was unaffected.