Habitat displacement of North American ladybirds by an introduced species

How introduced species displace native species is often poorly understood. Various studies suggest that several species of native ladybird beetles (Coleoptera: Coccinellidae) have declined in abundance in particular habitats (especially crops) and/or geographic regions following the colonization of North America by the seven-spotted ladybird beetle (Coccinella septempunctata L.). However, the factors responsible for such declines are unclear. Long-term sampling of alfalfa fields in northern Utah revealed that as C. septempunctata established itself and increased in numbers, densities of native species decreased significantly over the period 1992-2001. The decline in numbers of native ladybirds mirrored a decline in numbers of their principal prey in alfalfa, pea aphids. Field experiments with induced aphid outbreaks were conducted to test the hypothesis that dispersing native adults have shifted their foraging efforts away from alfalfa as the introduced species has increased in numbers and depressed prey availability. As predicted by the hypothesis, native ladybirds rapidly reaccumulated in alfalfa when prey became abundant. Their presence was brief, however, as they and especially the more numerous C. septempunctata quickly suppressed experimental aphid outbreaks. The ratio of densities within experimental plots with high aphid numbers to densities in the surrounding field was greater for native ladybirds than for C. septempunctata. Also, densities of native ladybirds varied more closely with aphid densities among fields in late May 2001 than did densities of C. septempunctata. Hence, native ladybirds appear especially responsive to local aphid density. C. septempunctata's impact on aphid density in alfalfa appears to have reduced significantly the degree to which the habitat retains foraging native adults. Thus, introduced species may influence the distribution and abundance of native competitors not only by reducing survivorship in local habitats, but also by altering the dynamics of dispersal and habitat use across the landscape.