Laboratory Evaluation of a Laboratory-selected Organophosphate-resistant Strain of Amblyseius finlandicus (Acari: Phytoseiidae) for Possible Use in Finnish Apple Orchards

Abstract

The survival of adults and immatures, and reproduction at rates of 150, 300 and 900 ppm a.i. of azinphosmethyl were compared among four types of Amblyseius finlandicus females and their offspring; a laboratory-selected colony (SEL16) resistant to azinphosmethyl, an unselected base colony (Tuorla), reciprocal F1 females from crosses between SEL16 and a very susceptible colony (Hiekkaharju). The SEL16 and reciprocal F1 females had a higher survivorship than the unselected Tuorla base colony after 2 and 4 days on leaves treated with the three concentrations of insecticide. Azinphosmethyl decreased oviposition by the surviving females of all types, but the decrease was smaller in the SEL16 strain than in the base colony. The survivorship of immatures of SEL16 on fresh azinphosmethyl residues after 6 days was low at all three concentrations, but no survivors were found in the base colony at any of the concentrations. Females deposited significantly more eggs on the untreated areas of the leaves. The importance of this behavior for biological control is discussed. There were no significant differences in developmental rate, ovipositional rate, sex ratio and ovipositional period between the SEL16 and unselected base colony on untreated leaf substrates, suggesting that resistance was not associated with decreased fitness. A mode of inheritance study indicated that azinphosmethyl resistance was incompletely dominant. This was also suggested by the results of bioassays, where the reciprocal F1 females performed nearly as well as the SEL16 strain on leaves treated with field rates of azinphosmethyl.