The objective of this research was to improve IPM in Finnish apple orchards by developing a strain of Amblyseius finlandicus that would tolerate field rates of organophosphorous (OP) insecticides. The research consists of four parts: (1) Developing an improved laboratory rearing method for A. finlandicus, (2) Evaluating variability in tolerance to OP insecticides among field populations, (3) Laboratory selection for OP-resistance, and (4) Laboratory evaluation of the resistant strain.
The laboratory rearing of A. finlandicus was improved by collecting and transferring eggs in cotton tufts (= egg-harvesting) from the base colonies to initiate new colonies. When fed on cattail (Typha latifolia L.) pollen, mites developed faster and had a higher ovipositional rate on a bean (Phaseolus vulgaris L.) leaf substrate than on a waxed paper substrate. Two-spotted spider mites (Tetranychus urticae Koch) were an inferior food source for A. finlandicus.
Five colonies of A. finlandicus were collected from Finnish apple orchards and tested for tolerance to two OPs, dimethoate, and azinphosmethyl. The LC50 value for the most resistant colony (Tuorla) was about five times that of the most susceptible colony (Hiekkaharju) for both pesticides.
The most resistant colony (Tuorla) was selected in the laboratory for resistance to azinphosmethyl and dimethoate. After eight selections with dimethoate and eight subsequent selections with azinphosmethyl, the total increase in resistance to dimethoate was 2.0-fold and to azinphosmethyl 5.4-fold compared to the unselected base colony (Tuorla).
Adult and immature survival and reproduction at field rates (150, 300 and 900 ppm a.i.) of azinphosmethyl were compared. The laboratory-selected colony (SEL16) and reciprocal F1 females (from the cross between SEL16 and the most susceptible field-collected colony (Hiekkaharju)), and their offspring had a higher survivorship, and the females deposited more eggs than the unselected Tuorla base colony at all three concentrations. Comparison of life table attributes between the SEL16 and the unselected base colony on untreated leaf substrates suggested that the resistance is not associated with decreased fitness. A mode of inheritance study indicated that azinphosmethyl resistance is incompletely dominant. The laboratory-selected, azinphosmethyl-resistant strain of A. finlandicus appears sufficiently resistant and fit that small scale field releases are justified.
Copyright by Tuomas Kostiainen 1994
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