Abstract:

The resistance to six insecticides in the field populations of Diaeretiella rapae (Hymenoptera: Aphidiidae)，a parasitoid of vegetable aphid collected in Jianxin, Fuzhou City, and Shangjie, Minhou County, Fujian, China, was studied  using dry residual film method. The susceptibility to insecticides in a field population of D. rapae increased significantly after being reared in the field insectarium under insecticide free conditions. Compared to susceptible F₂₁ reared in the insectarium, the resistance ratios were 11.8－31.2 for methamidophos, 9.3－14.7 for fipronil, 9.3－35.7 for avermectin, 7.0－16.6 for fenvalerate and 13.3－30.1 for imidacloprid, respectively in Shangjie populations during June 8, 2002 to November 18, 2004; and 7.5－34.9 for methamidophos, 4.0－14.3 for fipronil, 9.2－38.6 for avermectin, 7.8－18.9 for fenvalerate and 12.1－24.2 for imidacloprid, respectively in Shangjie populations during June 5, 2003 to December 17, 2004. RR in field populations of D. rapae was the highest in May, early June and November, lower in December, and the lowest in the end of June. In particular, RR decreased markedly in about 20 days from the beginning to the end in June. The results indicated that the resistance levels to methamidosphos, fipronil, avermectin, fenvalerate, cypermethrin and imidacloprid in D. rapae were not stable after relaxation from the selection pressure of insecticide, and that there existed significantly seasonal variations of resistance levels to the six insecticides in D. rapae.

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Key words: Diaeretiella rapae, Lipaphis erysimi, insecticide resistance, susceptibility, field population, seasonal dynamics