Predation on 2-Spotted Mite, Tetranychus-Urticae Koch (Acarina, Tetranychidae) by Haplothrips-Victoriensis Bagnall (Thysanoptera, Phlaeothripidae) and Stethorus-Nigripes Kapur (Coleoptera, Coccinellidae) on Seed Lucerne Crops in South-Australia

Abstract

Three predators were commonly associated with twospotted mites Tetranychus urticae: the tubular black thrips Haplothrips victoriensis, the black mite-eating ladybird Stethorus nigripes, and a predatory mite Phytoseiulus persirnilis Athias-Henriot (Acarina : Phytoseiidae). Of these, H. victoriensis was the most abundant, especially at low mite densities; S. nlgripes was present when mite numbers were high; P. persirnilis appeared only in cooler months, when mite activity had waned. A beating method for simultaneously sampling towspotted mites and their predators was developed as a quick method of estimating populations on closely planted lucerne. In laboratory feeding studies, larvae of H. victoriensis completed development on either mite eggs or lucerne pollen or both. Development was quickest on lucerne pollen. The mean consumption rate of thrips fed only mite eggs was 40 eggs per day for the 3-day first stadium and 108 per day for the 10.3-day second stadium. Where the ratio of thrips to mobile mites in the field was less than about 1:10, as occurred when chemicals were sprayed, the mite population had the potential to increase rapidly. A damage assessment study indicated that lucerne plants could tolerate moderate mite feeding (up to an average of 45 mites per leaf in the middle part of the plant). It is concluded that predators can exert effective control of mites under most field conditions.