Effects of Plant Density and Diversity on the Population Dynamics of a Specialist Herbivore, the Striped Cucumber Beetle, Acalymma Vittata (Fab)

Abstract

To determine the effects of plant density and diversity on the population dynamics of a specialist herbivore, the striped cucumber beetle (Acalymma vittata [Fab.]), cucumbers (Cucumis sativus L.) were planted in monocultures and in polycultures with corn (Zea mays L.) and broccoli (Brassica oleracea L.).Population densities of A. vittata and its insect predators, as well as cucumber growth, survivorship, and reproduction were monitored in these experimental plots. By controlling total plant density, host plant density, and plant diversity, it was possible to distinguish the effects of these three confounding variables. Densities of A. vittata were 10–30 times greater in monocultures than in polycultures, both per plot and per plant, even when total plant density and host plant density were held constant. The lack of any effect of plant density on per—plot beetle abundances emphasized the importance of diversity per se in influencing beetle populations. These differences in beetle abundance caused by diversity resulted from differences in tenure time and movement patterns among plots, rather than from differences in colonization, reproduction, or predation. In mark—recapture studies, a greater proportion of beetles marked in monocultures than in polycultures was later found, and of those marked in monocultures, a greater number were later found in monocultures than in polycultures. All measures of growth (leaf area, growth rate, and vine length) and reproduction (fruit production and number of flowers) of Cucumis sativus were most strongly affected by diversity, but also were affected by plant density. Both per—plot and per—plant values were greater in monocultures than in polycultures. The number of beetles was strongly correlated with the total amount of plant growth and reproduction in monocultures but not in polycultures. For plots with equal amounts of leaf area, monocultures had an order of magnitude greater number of beetles; thus, differences in host plant quantity did not explain the differences in beetle abundances between monocultures and polycultures. Numbers of beetles were much more strongly related to total plot characteristics than individual plant characteristics. Although defoliation was negligible, beetles significantly decreased cucumber survivorship and longevity by disseminating bacterial wilt disease (Erwinia tracheiphila [E. F. Sm.]).