Organo-tin Resistance in Tetranychus urticae (Acari: Tetranychidae) in Australia

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Abstract
Cyhexatin resistance was confirmed in laboratory tests on two strains of Tetranychus urticae Koch from apple and pear orchards in 1981. Another 21 strains from orchards were tested from 1982 to 1985. Cyhexatin resistance was confined mainly to the Goulburn Valley of Victoria (the main pear-growing area in Australia) and the Murrumbidgee Irrigation Area in southern New South Wales, but was widespread in these areas. Three formulations of cyhexatin were used in the laboratory bioassays. A wettable powder formulation gave satisfactory results except with females of more highly resistant strains. The problem was not experienced with an emulsifiable concentrate formulation; therefore, we concluded that levels of resistance in these strains should be based on data obtained with the latter. Levels of cyhexatin resistance did not exceed 15-fold in females of field populations. Laboratory selection with cyhexatin on one of the more resistant strains (ShJ) only marginally increased the level of resistance. Resistance in ShJ did not increase in the field, although 16 cyhexatin sprays were applied in 3 years. In the laboratory, resistance reverted slowly toward susceptibility without miticides (e.g., after ca. 160 generations, resistance to cyhexatin in ShJ fell from 9.1- to 1.9-fold at LC50) and 34.5% of the population still survived in a discriminating concentration. Levels of cyhexatin resistance in larvae were considerably lower than in females. Cyhexatin resistance conferred cross resistance to azocyclotin and fenbutatin oxide.

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