Abstract
To assess the role of host plant density in the life of a specialized herbivore, natural populations of the large milkweed bug, Oncopeltus fasciatus, were monitored in 5 paired study plots of high and low milkweed density (Asclepias syriaca). At all sites the plot with a higher density of milkweed pods supported more O. fasciatus per pod. At some sites, O. fasciatus load (cumulative biomass of bugs/pod) correlated positively with pod density within plots, and milkweed below a certain density escaped O. fasciatus exploitation. However, at a site that recevied an exceptionally large influx of colonizing bugs, almost all pods were heavily fed upon, regardless of their dispersion. This herbivore clearly demonstrates the resource—concentration effect. The effect orginates with the oviposition patterns of colonizing ♂ ♂ and its strengthened by differential mortality of nymphs searching for suitable feeding spots. Since these behaviors are common in other insects herbivores, the resource—concentration effect may be an important organizing factor in many natural, as well as cultivated plant—athropod associations.
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