Abstract
To analyze, the phenomenon of amitraz-resistance in the citrus red mite, from the angle of population genetics, the degree of development of resistance to amitraz in a mite population obtained by continuous selection with amitraz and its genetic basis were investigated. There were large differences in amitraz susceptibility between the selected population (R) and the unselected population (S) sampled from a citrus grove which has not been treated with amitraz, and maximum value of the ratio of R to S (158-folds) was recorded for the LC50 value. An amitraz-resistant strain selected from an Ehime population, and a susceptible strain from a Hiratsuka population, were crossed and backcrossed. The results of these tests showed that amitraz-resistance of the eggs in P. citri was mainly due to a single, incompletely dominant major gene. The fitness, i.e., viability, age-specific fecundity and reproduction rate, in the amitraz-resistant (RR, RS) and susceptible (SS) strains of the Ehime population were compared under various environmental conditions. There were no significant differences among the strains, except for the results obtained at the temperature of 25°C. Two mixed populations originally composed of resistant (RR) and susceptible (SS) strains with a ratio of 1:1 or 1:9, were reared without selection for amitraz susceptibility for 32 generations at a daily temperature ranging between 20 and 30°C and tested 5 times for their susceptibility to amitraz. The results of these tests showed that the susceptibility to amitraz of the two mixed populations remained stable for 32 generations. From these results, it was concluded that the values of fitness of the resistant mite populations (RR, RS) were almost equal to those of the susceptible population (SS) in an acaricide-free environment.
