Abstract
A milbemycins-organophosphate (OP)-resistant strain was obtained in the laboratory, against which the organophosphate insecticide isoxathion (ISX) and the brown oily mixture of milbemycins (BOM) were conspicuously ineffective. High synergistic effect of ISX and BOM on the adult mites, however, was observed when they were sprayed in mixture. Similar synergistic effect was also obtained with other OP-insecticides, especially O, O-diethylphosphorothioates and O-ethylphenylphosphonothioates in combination with BOM. Intoxication symptoms characteristic of milbemycins but not of OP-insecticides were observed following the treatment with the mixture. Recovery coefficient (LC50 after 3 days/LC50 after one day) of adult mites treated with the ISX-BOM mixture was lower than that with the BOM alone. When one of the preparations was sprayed for 2 consecutive days on the same mite population, spraying of ISX-BOM was more effective than that of the reciprocal BOM-ISX. From these results, it appears that the joint effect may be due to an inhibitory effect of the OP-insecticides on the detoxication process of BOM in the resistant mites.
