PHYLLOPLANE FUNGI - AN EXTRINSIC FACTOR OF TETRANYCHID POPULATION-GROWTH

In a study on leaf-inhabiting tetranychid mites (Tetranychus urticae K och-TSSM and Panonychus ulmi (Koch)-ERM) we investigated the effects o f an extrinsic factor on the mites environment, namely phylloplane fun gi. In a research orchard four trees were selected and treated with an aerosol application of a phylloplane fungus (Alternaria alternata) in a tap-water emulsion. Applications were made immediately after each s ampling, with the exception of the last sample date. Two tap water con trols for each treated tree were also sampled: a nearest neighbour (<3 m from the treated trees) and a distant neighbour (>30m from the treat ed trees with other apple trees in between). Due to possible migration from the treated trees to near neighbours, the distant control best r eflected normal orchard conditions. Eight samples were taken throughou t the 1994 growing season; however, appreciable mite populations were only observed on the last four sample dates. On the treated trees, the ERM maintained a steady low population (less than ten per leaf) where as the TSSM showed a population outbreak (up to 44 mites per leaf). Co nversely, on the distant trees, the TSSM maintained a low population ( less than ten per leaf) while the ERM showed an outbreak (up to 33 per leaf). Observing on a leaf by leaf basis, when tetranychids were pres ent on a leaf, either one species dominated or the other, suggesting m utual competitive exclusion, the outcome of which was reversed to favo ur TSSM on trees that received an application of fungus. We concluded that the application of additional or supplemental amounts of A. alter nata to apple leaves enhanced the population growth of TSSM compared t o that of ERM. Possible mechanisms are discussed.