Abstract
Stability of hexythiazox resistance in the citrus red mite, Panonychus citri (MCGREGOR) was investigated under hexytiazox-free laboratory and field conditions. Under the laboratory condition, the hexythiazox susceptible (Haibara-S) and resistant (Haibara-R) strains were mixed at the ratios of 50:50, 30:70, 10:90 and 2:98. The hexythiazox resistance reversed rapidly in the populations 50S:50R and 30S:70R, but not sufficiently at the 12th generation in the populations 10S:90R and 2S:98R. In the citrus orchard where the hexythiazox resistance developed after 19 times of successive application with hexythiazox during 7 years, the changes in susceptibility to hexythiazox and the population fluctuation of females were investigated in populations on 6 different trees. The gradual reversion of resistance was observed during 33 months after the final treatment with hexythiazox. Populations of females on the 6 trees fluctuated differently before and shortly after the final treatment, but synchronously about one month after that. It was assumed that the reversion of hexythiazox resistance in the field was ascribed to the hexythiazox-susceptible individuals which remained in the orchard and immigrated from other orchards, the incompletely recessive inheritance of hexythiazox resistance and the reproductive disadvantage of resistant genotype.
