Ecological Entomology zyxwvutsr (1980) zyxwvu 5, 205-21 1 zyxwvu Taxonomic isolation and the accumulation of herbivorous insects: a comparison of introduced and native trees EDWARD F. CONNOR, STANLEY H. FAETH, DANIEL SIMBERLOFF and PAUL A. OPLER" Department of Biological Science, Florida State University, and Tall Timbers Research Station, Tallahassee, Florida, and zyxwvuts * Office of Endangered Species, U.S. Fish and Wildlife Service, Washington D.C., U.S.A. ABSTRACT. 1. Evidence from leaf-mining insects on Fagaceous hosts suggests that range expansions of insects onto introduced trees often involve species that feed on native hosts closely related to the introduced host. 2. zyxwvuts An examination of the herbivorous entomofauna of British trees illustrates that the size of the entomofauna is partially determined by the taxonomic isolation of the host tree. Introduction The rapidity and facility with which most phytophagous insects shift host-plants or expand host ranges is largely unknown (Strong, 1979). However, we do know that some herbivorous insects are host-plant specific, others are broadly polyphagous, and that host switches may be very rapid and frequent (Strong, 1979; Futuyma, 1976; Morrow, 1977; Zimmerman, 1960; Opler, 1974; Futuyma & Gould, 1979). Host ranges of phytophagous insects may be restricted by characteristics of the host or the herbivore, involving geography, ecology, and plant chemistry (McClure & Price, 1976; Smiley, 1978). Some herbivores feed on a few host species locally, but throughout their geographic range consume many (Hsiao, 1978). Others restrict feeding by habitat, plant growth form (trees, shrubs, herbs, etc.), plant phenology, or a combination of these factors (Futuyma, 1976; Morrow, 1977; Futuyma & Gould, 1979; Feeny, 1970, Opler, 1978). Host plant selection and use involves the discrimination Correspondence: Dr E. F. Connor, Department of Zoology, Oxford University, South Parks Road, Oxford OX 1 3PS. of proximal chemical cues presented by the plant in the form of either differences in nutritional quality or attractants and repellents (Lipke & Fraenkel, 1956; Schoonhoven, 1972; Thorsteinson, 1960; Auclair, 1965; Hsiao, 1969; Jermy, 1966). Whatever determines the distribution of these specific cues among plants may ultimately determine host ranges of herbivorous insects. Related species of plants often present very similar cues and may be included in the host range of a herbivore more often than are taxonomically and there- fore chemically unrelated plants (Ehrlich & Raven, 1964; Feeny, 1975; Southwood, 1961a, 1972; Holloway & Herbert, 1979). We present evidence to suggest that host-plant switches and range expansions are likely to occur between related host-plants and that accumulation of herbivore species on intro- duced plants is in part determined by the presence of closely related native plants in the locality of introduction. Although some herbivores are very host- specific, the emerging pattern is that many insects thought to be host-specific are really oligophagous, feeding on a few closely related hosts. Leaf-mining insects on oak trees in eastern North America are an example, 0307-6946/80/0800-0205 $02.00 zyxwv 0 1980 Blackwell Scientific Publications 14 205