Selection Pressures on Phytophagous Insects Feeding on Small Host Plants

Abstract

moths in the genus Depressaria do seem to be more restricted in their use of plant parts on large plants than on small plants. Second, insect species that feed on small ephemeral plant parts (flowers, meristems, fruits) are more likely to evolve the ability to assess egg-loads on plants than related insect species that feed on more persistent plant parts (leaves, stems, roots). In addition, it seems logical to predict that the smaller the plant the greater the likelihood that egg-load assessment will evolve in insects associated with any plant part. Third, insect species that feed on small plants regularly mature at a smaller size than closely related insect species on larger plants. This hypothesis is likely to hold especially for insects that feed internally on plant tissues. Fourth, insect species that feed on small plants are more likely to feed as grazers, moving among several plants during development, than closely related insects on larger plants. Two corollaries follow: the adoption of grazer-like behavior is more likely among insects in which females regularly oviposit (a) on hosts in dense patches rather than on isolated plants and/or (b) in habitats that support a group of related plant species. The problems for and evolutionary solutions by insects that feed on small host plants are important to the development of the theory of insect-plant interactions because of the sheer extent of habitats that include such plants: grasslands, steppes, tundras, deserts, and forest understories.