In ant-plant mutualist systems, ants patrol their host plants and search for herbivores. Such patrolling can be inefficient, however, because herbivore activity is spatio-temporally unpredictable. It has been proposed that rapid and efficient systems of communication between ants and plants, such as volatile compounds released following herbivory, both elicit defensive responses and direct workers to sites of herbivore activity. We performed bioassays in which we challenged colonies of two Amazonian plant-ants, Azteca sp. and Pheidole minutula, with extracts of leaf tissue from (1) their respective host-plant species (Tococa bullifera and Maieta guianensis, both Melastomataceae), (2) sympatric ant-plants from the Melastomataceae, and (3) two sympatric but non-myrmecophytic Melastomataceae. We found that ants of both species responded dramatically to host-plant extracts, and that these responses are greater than those to sympatric myrmecophytes. Azteca sp. also responded to non-myrmecophytes with an intensity similar to that of sympatric ant-plants. By contrast, the response of P. minutula to any non-myrmecophytic extracts was limited. These differences may be driven in part by interspecific differences in nesting behaviour; although P. minutula only nests in host plants, Azteca sp. will establish carton satellite nests on nearby plants. We hypothesize that Azteca sp. must therefore recognize and defend a wider array of species than P. minutula. © 2008 The Linnean Society of London.
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