Effects of foliar and soil insecticide applications on the collembolan community of an early set-aside arable field

Effects of foliar and soil insecticide applications on collembolan density and community structure were investigated in an early set-aside arable field. Insecticides were applied separately and in combination to the soil surface (chlorpyrifos) and vegetation (dimethoate). The treatments were established to investigate effects of above- and below-ground insects on plant succession. Starting in 1997, the insecticides were applied from April to November at 2-week (dimethoate) or monthly intervals (chlorpyrifos). Samples were taken in 2000 prior to and after insecticide application in March and June, respectively. Both insecticides are lethal to Collembola and insecticide applications resulted in a strong decline in the density of total Collembola. Application of chlorpyrifos reduced collembolan density to a greater extent than dimethoate; the effect of the combined application on total collembolan numbers was similar to that of chlorpyrifos only. Collembolan numbers recovered after the insecticide applications in 1999, but in the treated plots populations were still reduced in March 2000 before the re-application of insecticide treatments in that year. The insecticide applications changed the dominance structure of the collembolan community, but had no effects on species composition. The results may be of relevance for the interpretation of studies on plant-insect herbivore interactions using insecticides.