Long-term effects of insecticides on Diaeretiella rapae (M'Intosh), a parasite of the cabbage aphid

Five insecticides were tested in the laboratory for their selective activities between the cabbage aphid, Brevicoryne brassicae (L.), and its associated parasitoid, Diaeretiella rapae (M'Intosh). Based on the concentration/mortality regressions, the synthetic pyrethroids tralomethrin and cypermethrin were the most active insecticides against both the cabbage aphid and the parasitoids within the mummified aphids, whereas the carbamate methomyl was the least active. Selectivity ratios, at LC50, for B. brassicae vs. D. rapae in the mummified aphids showed that all insecticides tested were more active against B. brassicae than against the parasitoids. All insecticides used at the recommended field rates reduced the emergence of adult parasitoids from mummified aphids by 55-97% of that of control. Survival and longevity of emerged D. rapae were significantly reduced by all insecticides and rates used. Consequently, the latent effects of these insecticides applied at and below the recommended field rates were estimated on the parasitoids through two successive generations. The recommended rates used for both methomyl and cypermethrin reduced fecundity of D. rapae recorded at the F1-generation by 75% of that of control. The emergence of F1-progeny D. rapae was not significantly affected in any treatment, whereas the survival and longevity of adult parasitoids were reduced by most treatments. At the F2-generation, the fecundity of parasitoid females was still lower than in the untreated control. Emergence and survival of adult parasitoids seemed to be slightly affected, whereas the adult longevity was highly affected in all treatments. Methomyl, tralomethrin and cypermethrin might be preferred because they had minimal adverse effects on emergence of parasitoids from the mummified aphids and their subsequent survival, longevity and fecundity, when applied at rates little below the recommended field rates.