Seasonal changes of resistance to fenvalerate and cypermethrin in the larval parasitoid Cotesia plutellae (Hymenoptera: Braconidae)

摘要

The seasonal changes of insecticide resistance and stability in hymenopteran Cotesia plutellae, collected from Jianxin, Fuzhou-City, and Shangjie, Minhou-County, Fujian, China, were assessed by using a dry residual film method. The resistance to two insecticides in the field populations of C. plutellae was not stable under insecticide-free conditions in the insectarium. Compared with susceptible F_(11) progeny of C. plutellae in the insectarium, the resistance ratios (RR) in F_0 parents were 18.4 for fenvalerate and 11.4 for cypermethrin based on LC_(50) at 9 hours, and 32.8 for fenvalerate and 28.5 for cypermethrin based on LC_(50) at 24 hours when the parasitoids were left in contact with the insecticides for 1 hour and mortalities were recorded at 9 and 24 hours, respectively. However, the RR in a field population of C. plutellae were 9.2 for fenvalerate and 12.7 for cypermethrin, if the parasitoids were left in contact with the insecticides for 24 hours. The resistances to the two pyrethroids in other field populations collected from Jianxin and Shangjie from November 2000 and July 2004 were also determined. Significant seasonal variations of resistance to the two insecticides in the field populations of C. plutellae were found. The RR were 3.0— 18.4 for fenvalerate and 4.8—20.6 for cypermethrin in Jianxin populations from November 2000 to April 2002 based on LC_(50) at 9 h, and 2.3-13.6 for fenvalerate and 3.6-16.0 for cypermethrin in Shangjie populations from May 2002 to July 2004 based on LC_(50) at 24 hours. The resistance levels were high in spring and autumn and decreased sharply in summer. In addition, significant recovery from the knocked-down caused by the insecticides was found in the F_0 and field populations of C. plutellae which were resistant to fenvalerate and cypermethrin if the parasitoids were left in contact with the pyrethroids for 1 hour. However, no recovery was found in susceptible F_(11) progeny.