Sub-lethal Effects of the Insecticides Pirimicarb and The effectiveness of parasitoids as biological control agents can be constrained by insecticide use, not only through direct mortality but also as a result of sublethal effects. Several pest aphids have become resistant to a range of insecticides and a resistant strain of Myzus persicae was used in laboratory experiments to investigate sublethal effects of the insecticides pirimicarb and dimethoate on the parasitoid Diaeretiella rapae. Both insecticides produced sublethal effects on D. rapae when the parasitoid attacked and developed in aphids that had been dipped in insecticide solutions. Dimethoate affected oviposition behaviour; females were apparently repelled by residues on the surface of dipped aphids, thus reducing their attack rate and hence the number of mummies produced. Also, the reproductive performance of parasitoids that had developed in pirimicarb-dipped aphids appeared to be adversely affected, in comparison with parasitoids that emerged from uncontaminated hosts, and this was reflected both in lower mummy production and lower attack rates. Pirimicarb, but not dimethoate, affected the sex ratio of the offspring of D. rapae that had developed in dipped aphids, causing a significant increase in the proportion of males. This only occurred when the male parent had developed in a pirimicarb-dipped aphid, suggesting that the effect involves male sterility or mating behaviour, although they appeared to mate normally. These sublethal effects are potential constraints on the efficiency and effectiveness of D. rapae as a biological control agent of aphid pests, but to assess fully their potential impact further studies need to be done using more realistic extended laboratory and semi-field techniques. none