The effects of four insecticides (dimethoate, pirimicarb, imidacloprid, and spinosad) applied in an ST4 Potter tower at sublethal concentrations (50% of those recommended commercially for aphid control) were studied in the laboratory on aphidiid Aphidius ervi (Haliday) adults, an important parasitoid of the pea aphid Acyrthosiphon pisum (Harris). The most selective treatment on the hymenopteran was imidacloprid, followed in decreasing order by spinosad, pirimicarb, and finally dimethoate, which quickly eliminated the parasitoid and thus its capacity to produce progeny. Three toxicity groups were distinguished in the study. The least damaging treatment to adults of A. ervi was imidacloprid, followed by a group of medium toxicity made up of spinosad and pirimicarb, and lastly dimethoate, which was extremely toxic to A. pisum.