DISPERSAL OF 5 APHIDS (HOMOPTERA, APHIDIDAE) IN RELATION TO THEIR IMPACT ON HORDEUM-VULGARE

Insect herbivores that have high impacts on their host plants reduce t he longevity of their habitats and require dispersal mechanisms. This hypothesis was tested by comparing the life history traits, particular ly short- and long-distance dispersal mechanisms, of 5 aphid species i n relation to their impact on barley, Hordeum vulgare L. The aphids we re bird cherry-oat aphid, Rhopalosiphum padi (L.), and rose-grain aphi d Metopolophium dirhodum (Walker), important pests of cereal crops; co rn leaf aphid, Rhopalosiphum maidis (Fitch), an occasional pest; and r usty plum aphid, Hysteroneura setariae (Thomas), and Sitobion nr. frag ariae, which are not pests. The specific impacts of the 5 species on b arley were similar, based on a biomass conversion ratio of a 3.4-mg re duction in plant growth for each milligram gained in aphid biomass. Th e value of the numerical impacts varied, being highest for large speci es that produced offspring earliest in their reproductive periods. The density dependence of biomass increase was highest for species with h igh numerical impacts on the plant. Three measures of short-distance d ispersiveness varied substantially among the species, but were not rel ated to their numerical impact. Long-distance dispersiveness, measured as the proportion of winged offspring produced in response to crowdin g, was related to numerical impact for 4 of the 5 species. The 5th, Si tobion nr. fragariae produced many winged offspring without crowding a nd is probably adapted to inherently short-lived habitats. Aphid speci es that rapidly affect the quality of their host plants have evolved a level of long-distance dispersiveness proportional to their impact on their host plants.