The resistance to and the effects of synergists on the toxicity of six insecticides in Diaeretiella rapae (M’Intosh) (Hymenoptera: Aphidiidae), a parasitoid of vegetable aphid collected in Jianxin at Fuzhou-City, Fujian, China, were studied. In comparison with susceptible F21 progeny, the resistance ratios in resistant F0 parents were 27.6 for methamidophos, 20.8 for fipronil, 47.5 for avermectin, 3.3 for fenvalerate, 4.5 for cypermethrin, and 74.7 for imidacloprid. Piperonyl butoxide (PB), triphenyl phosphate (TPP), and diethyl maleate (DEM) were chosen to be applied in susceptible F21 progeny, as well as in resistant F11 progeny and F0 parents. Significant synergistic effects on the toxicity of the six insecticides were found by using PB, TPP, and DEM in F0 parents; on methamidophos, avermectin, and imidacloprid by PB, TPP, and DEM in F11 progeny; on fipronil by PB and DEM in F11 progeny; and on fenvalerate and cypermethrin by PB in F11 progeny. PB also showed significant synergism on the six insecticides in susceptible F21 progeny, although the synergism was far less in F21 progeny than those in resistant F0 parents. TPP and DEM showed little or no synergistic effects on the toxicity of the six insecticides in F21 progeny. Compared with TPP and DEM, the highest synergistic ratios of PB for methamidophos, fipronil, avermectin, fenvalerate, cypermethrin, and imidacloprid were observed in F0 parents, and F11 and F21 progeny. The resistance levels to methamidophos, fipronil, avermectin, fenvalerate, and cypermethrin could be inhibited strongly by applying PB in F0 parents. From the results, oxidative degradation is believed to play a critical role in resistance to methamidophos, fipronil, avermectin, fenvalerate, and cypermethrin in D. rapae. To a lesser extent, hydrolytic reactions also were partially involved in the resistance to these five insecticides by using the synergists PB, TPP, and DEM. However, although high synergism of PB, TPP, and DEM on imidacloprid was found, the resistance levels to imidacloprid remained high in the presence of PB, TPP, and DEM. The mediated detoxification of oxidative degradation and hydrolytic reactions was thought to be involved in the resistance to imidacloprid in F0 parents.
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Vol. 97 • No. 6