ODOR-MEDIATED RESPONSES OF PHYTOPHAGOUS MITES TO CONSPECIFIC AND HETEROSPECIFIC COMPETITORS

Plants under herbivore attack produce volatiles, thus attracting natur al enemies of the herbivores. However, in doing so, the plant becomes more conspicuous to other herbivores. Herbivores may use the odours as a cue to refrain from visiting plants that are already infested, ther eby avoiding competition for food. or, alternatively, to visit plants with defences weakened by earlier attacks. We investigated the respons e of one species of herbivore (the spider mite Tetranychus urticae) to odours emanating from cucumber plants infested by conspecific or hete rospecific (the western flower thrips, Frankliniella occidentalis) her bivores. Olfactometer experiments in the laboratory showed that spider mites have a slight, but significant, preference for plants infested with conspecifics, but strongly avoid plants with thrips. These result s were substantiated with greenhouse experiments. We released spider m ites on the soil in the centre of a circle of six plants, half of whic h were infested with either conspecifics or heterospecifics (thrips), whereas the other half were uninfested. It was found that 60-70% of th e mites were recaptured on the plants within 5 h after release. Result s of these experiments were in agreement with results of the olfactome ter experiments. (1) significantly fewer spider mites were found on pl ants infested with thrips than on uninfested plants and (2) more mites were found on plants with conspecifics than on clean plants (although this difference was not significant). From a functional point of view it makes sense that spider mites prefer clean plants over thrips-infe sted plants, since thrips are not only competitors, but are also known as intraguild predators of spider mites. Possible reasons for the sli ght attraction of spider mites to plants infested with conspecifics ar e discussed.