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Résumé :
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Insect herbivores that have high impacts on their host plants reduce the longevity of their habitats and require dispersal mechanisms. This hypothesis was tested by comparing the life history traits, particularly short- and long-distance dispersal mechanisms, of 5 aphid species in relation to their impact on barley, Hordeum vulgare L. The aphids were bird cherry-oat aphid, Rhopalosiphum padi (L.), and rose-grain aphid Metopolophium dirhodum (Walker), important pests of cereal crops; corn leaf aphid, Rhopalosiphum maidis (Fitch), an occasional pest; and rusty plum aphid, Hysteroneura setariae (Thomas), and Sitobion nr. fragariae, which are not pests. The specific impacts of the 5 species on barley were similar, based on a biomass conversion ratio of a 3.4-mg reduction in plant growth for each milligram gained in aphid biomass. The value of the numerical impacts varied, being highest for large species that produced offspring earliest in their reproductive periods. The density dependence of biomass increase was highest for species with high numerical impacts on the plant. Three measures of short-distance dispersiveness varied substantially among the species, but were not related to their numerical impact. Long-distance dispersiveness, measured as the proportion of winged offspring produced in response to crowding, was related to numerical impact for 4 of the 5 species. The 5 th, Sitobion nr. fragariae produced many winged offspring without crowding and is probably adapted to inherently short-lived habitats. Aphid species 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.
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