FACTORS INFLUENCING SPATIAL AND TEMPORAL VARIATION IN ABUNDANCE OF THE LARGE MILKWEED BUG (HEMIPTERA, LYGAEIDAE)

Populations of the large milkweed bug, Oncopeltus fasciatus (Dallas), were censused in patches of Asclepias syriaca L. in eastern Iowa from 1979 to 1982. Bugs were more abundant in patches with large numbers of fruiting plants, but fruit availability explained only 36% of the variance observed. A second contributing factor was proximity of the field sites to major rivers. A possible explanation is that O. fasciatus follows water courses during migration. Bugs were numerous in 1979 and 1982, but were almost completely absent in 1980 and 1981. This absence was not explained by a reduction in fruit abundance. The lack of bugs in 1980 was probably due to severe heat and drought in areas to the south, such as Missouri, which are likely sources for insects migrating to Iowa. Recovery of populations of O. fasciatus in Iowa did not take place until 1982. Thus it is likely that environmental factors along the migratory route influenced population numbers at the destination.