PHYTOPHAGOUS THRIPS ARE FACULTATIVE PREDATORS OF 2-SPOTTED SPIDER-MITES (ACARI, TETRANYCHIDAE) ON COTTON IN AUSTRALIA

Predatory feeding on Tetranychus uruticae Koch (Acari: Tetranychidae) populations on cotton by phytophagous thrips, Thrips imaginis Bagnall, T. labaci Lindeman and Fuankliniella schultzei Tryborn (Thysanoptera: Thripidae), was investigated in the held and laboratory. Phytophagous thrips are a common early season pest of cotton in Australia, though their true pest status is undefined. In California, the phytophagous t hrips (Frankliniella occidentalis (Pergande)) is regarded as an opport unistic predator of mite eggs, their consumption of which increases fi tness over a diet of leaf tissue alone. Thrips are among the most abun dant of insects on young cotton. If they consume mite eggs, even at re latively low rates, they could have a significant influence on the pro bability of survival of early season spider mites. Consumption of eggs of T. urticae by thrips was investigated in the laboratory. Second in star F. schultzei consumed more eggs per day (ca. 4 eggs per day) than did second instar T. tabaci or T. imaginis (ca. 1 egg per day). Consu mption by first instar F. schultzei was much lower than for second ins tars. Adult T. tabaci consumed ca. I egg per day whilst adults of F. s chultzei consumed only ca. 0.5 eggs per day, although some individuals of this species did consume substantial numbers of eggs. Larvae of al l thrips species showed a type II functional response to prey density. In the field, adults and larvae of T. tabaci and F. schultzei showed a preference for cotton seedlings that were also infested by spider mi tes. In a glasshouse, larvae of T. tabaci showed a highly significant preference for feeding within mite colonies. In the field, suppression of predators, predominantly T. tabaci and F. schultzei, with a broad spectrum insecticide (dimethoate) contributed to outbreaks of mites oc curring earlier than they would have otherwise. The results show that phytophagous thrips eat mite eggs and that they are potentially import ant predators of spider mites in the field, especially given their abu ndance on young cotton and preference for inhabiting situations in whi ch mite colonies are found.