Selection for Avermectin Resistance and Resistance Stability in Neoseiulus cucumeris

The predacious mite Neoseiulus cucumeris is an important natural enemy of many pest and spider mite. However, this predator is very susceptible to insecticides such as avermectin, which is commonly used in the orchard. Selected the avemectin resistant strain by using the slide-dip method and the original population were from the population, which reared in laboratory without exposed to any insecticide for a few years. The high resistance of N. cucumeris to avermectin was gained after 54 generations. The LC50 of susceptible strain was only 2.527 0 mgL-1 , however that of resistant strain was up to 163.293 0 mgL-1 .The resistant of resistant strain was 64.62-fold as large as susceptible strain. The models of resistance growth of resistant strain was as follow, y=120.4141/. A resistance recession of resistant strain after 120 generations to avemectin was studied in this paper. The results showed the resistance of resistant strain to avemectin without exposure to avemectin decreased very quickly, and the reduction rate reached 78.46% after 60 generations, and the resistance declined slowly from 60 generations to 120 generations, and the resistant was 10-fold as large as susceptible strain. Furthermore, when the resistance declined to 20% percent of the original resistant level, the resistance re-growth was also studied in this paper. The results suggested that the resistance went up quickly after using avemectin about 24 generations; and the resistant was 43.73-fold of that of susceptible strain. The results suggested that the cultivated resistant strain which applied in the field will reduce the conflict between biological control and chemical control.