In a two-species toxicity test system survival and reproduction of both the predator Hypoaspis aculeifer (Gamasida) and the prey Folsomia fimetaria (Collembola) were studied after 21 days of residual exposure to a soil contamination of the insecticide dimethoate. Additional experiments were run to analyze which species-species and compound-species relationships determine the outcome of this two-species experiment. Number of adult F. fimetaria were reduced by both predation and dimethoate

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... e, whereas mites preyed less efficiently on adults than on juveniles. At 0.357 mg dimethoate/kg soil, numbers of juvenile F. fimetaria were mainly reduced by predation on adults and juveniles. At 0.7 mg/kg, an additional dimethoate effect was found, which was attributed to an effect on the reproduction of F. fimetaria, mainly due to lethality of adults. It was reasoned that lethal effects on juvenile springtails are less important. Adult H. aculeifer was not affected by dimethoate exposure, whereas numbers of juvenile H. aculeifer demonstrated a decline only at the highest concentration of 0.7 mg/kg. It is hypothesized that this latter effect is possibly due to food depletion caused by a decreased availability of prey, rather than to the lethal effects of dimethoate on juvenile mites. Such a secondary effect of a pesticide application could not have been derived from a single-species toxicity experiment and demonstrates the additional value of a two-species toxicity test system.