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Current Research
In the Columbia Basin of central Washington, patches of coyote willow, Salix exigua, are attacked by a functionally-diverse assemblage of herbivorous insects, including sap-feeding aphids and leafhoppers, gall-making flies and sawflies, leaf-mining lepidopterans, and externally-feeding lepidopterans and chrysomelid beetles. In some patches, gall-making flies in the genus Rhabdophaga reach outbreak densities (see photo), severely reducing the leaf area of the plants and apparently causing elevated plant mortality.
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We are investigating whether such outbreaks are facilitated by an aphid-ant mutualism. In particular, we hypothesize that aphids perform better on galled plants, resulting in higher ant density on those plants, which in turn results in lower pressure on Rhabdophaga larvae by a suite of parasitoid wasps. Preliminary evidence strongly suggests that outbreaks of Rhabdophaga are promoted by the ant-aphid mutualism. We are also determining if the association between aphids, ants, and galls influences the community structure of herbivorous insects on coyote willows. Specifically, we hypothesize that on plants with many aphids and ants (mainly galled plants), the presence of an ant guard will eliminate most or all externally-feeding insects from the plants, allowing only internal feeders to persist. In contrast, plants with few aphids and ants (mainly ungalled plants) will feature a full complement of herbivorous insect guilds.
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