Abstract
Control failure of Panonychus ulmi (KOCH) with the application of Phencapton (0, 0-diethyl-5-(2, 5-dichlorophenyl mercapt methyl) dithiophosphate) was observed in the apple orchard of Tohoku National Agricultural Experiment Station, Fujisaki-Machi, Aomori Prefecture, in the seasons of 1959-'60, where this chemical had been introduced in 1957 and the initial application had given good result.
In September 1960, the mite population from the control failure tree (Ph-population) was compared in laboratory with a population (M-population) derived from a garden at Zaifu-cho, Hirosaki city, where no chemical had been used. The concentration-mortality tests were made with Phencapton and Folidol (diethyl-p-nitrophenyl thiophosphate) on the samples from these two populations. The resulted LC50 values with each chemical application to Ph-population were 95.35 and 4.45 times of M-population, respectively. Thus, these two populations were extremely different in susceptibility to each chemical; Ph-populations showed marked resistance to Phencapton and cross resistance to each chemical. The slope of logarithmic concentration-mortarity regression line of Phencapton to Ph-population was steeper than that to M-population, while of Folidol the reverse was observed. This may be attributed to their difference in the mode of acaricidal action.
