Abstract

In 1979, a strain of spotted alfalfa aphid, Therioaphis trifolii (Monell) f. maculata (Buckton), from Tamworth, New South Wales, was resistant to pirimicarb (748- fold), disulfoton (48-fold), dimethoate (42-fold), thiometon (25-fold), demeton-S-methyl (20-fold), monocrotophos (11-fold), and omethoate (9.4-fold) compared with a susceptible reference strain at the LC50S Laboratory bioassays indicated low-order resistance to fen-valerate (2.8-fold) and methamidophos (2.6-fold). The strain was susceptible to several insecticides, including chlorpyrifos (ethyl), methidathion, and endosulfan. These three chemicals effectively controlled the resistant strain in the field. Chlorpyrifos was subsequently recommended for control of resistant populations. The same pattern of resistance was detected in strains ofT. trifolii from several other areas of New South Wales and South Australia by 1981, Victoria by 1983, and Queensland by 1984. Chemical control of T. trifolii decreased substantially from the early 1980s because of the implementation of an integrated pest management program based on resistant alfalfa cultivars, imported biological control agents, cultural practices, and chemical control measures that were compatible with beneficial species. Control problems with insecticides were not reported from 1985 to 1990, when chlorpyrifos failed to control T. trifolii at Keith in South Australia. Laboratory tests indicated low-level resistance to chlorpyrifos.

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