Selective and persistent toxicity of insecticide mixtures to the diamondback moth,Plutella xylostella and its parasite,Oomyzus sokolowskii

Selective and persistent toxicities of five mixtures and four single insecticides to the diamondcack moth, Plutella xylostella,and its parasite,Oomyzus sokolowskii, were evaluated in the laboratory and field. Of the nine insecticides tested, Bt+avermectin showed the highest selectivity, followed by spinosad, fipronil, chlorfenapyr, nicotine+oleate+cypermethrin, avermectin+phoxim and avermectin+imidacloprid. These seven insecticides were ideal ones of controlling P. xylostella. Avermectin+chlorpyrifos showed little selectivity and thus should not be recommended in the control of P. xylostella when O.sokolowskii and other natural enemies are relatively abundant. Chlorpyrifos was not effective against P. xylostella but has very high toxic to the parasite, and thus should not be recommended in the control of this pest and any other pests when natural enemies are relatively abundant. The results of field trials showed that the eight of abovementioned insecticides showed short persistent of toxicity against P. xylostella. Chlorpyrifos showed high initial and long persistence toxicity against O. sokolowskii; Avermectin+chlorpyrifos showed high initial toxicity against O. sokolowskii but its toxicity rapidly declined with time; the other six insecticides showed not only low initial toxicity but also short persistent toxicity against the parasite.