EFFECT OF EARLY-SEASON INSECTICIDE USE ON PREDATORS AND OUTBREAKS OF SPIDER-MITES (ACARI, TETRANYCHIDAE) IN COTTON

The Australian cotton industry relies almost exclusively on synthetic insecticides for control of early season pests. These insecticides oft en disrupt predatory insect activity in the field. Potential predators of the twospotted spider mite, Tetranychus urticae Koch, in cotton, i dentified in field and confirmed in laboratory experiments, included a theridiid spider, a phytoseiid mite, a lacewing larva, predatory thri ps, several Coccinellidae and several Hemiptera. These predators were mostly generalists, having previously been reported as predators of ap hids or caterpillars of Helicoverpa spp. The effect of insecticides on T. urticae and its predators was evaluated in three field experiments . Cotton was artificially infested with T. urticae then sprayed five t imes at seven to ten day intervals with either dimethoate (140,0 ai/ha ), thiodicarb (750 g ai/ha and 187.5 g ai/ha), endosulfan (735 g ai/ha and 367.5 g ai/ha), methomyl (169 g ai/ha) or amitraz (400 g ai/ha). Tetranychus urticae populations reached higher densities in dimethoate , thiodicarb and methomyl treated cotton than in untreated cotton. Pop ulation densities of T. urticae in cotton treated with low rates of en dosulfan or thiodicarb were similar to controls, while those in cotton treated with amitraz were lower. All insecticides caused significant reductions in at least one predator group. Significant negative relati onships were found between early season abundance of predators and the mid-season abundance of T. urticae and positive relationships between predators and the lag-period for T. urticae outbreaks to develop. Pre dation is implicated as a key factor influencing the early season surv ival of T. urticae. The implications for developing integrated pest ma nagement strategies in cotton are discussed.