Timing of food plant availability: Effect on survival and oviposition of the bean leaf beetle (Coleoptera: Chrysomelidae)

Abstract

The objective of these experiments was to estimate the effects of large-scale adoption of delayed soybean planting on population dynamics of the bean leaf beetle, Cerotoma trifurcata (Forster), in the Midwest. If growers throughout a region (e.g. thousands of contiguous hectares) planted soybean at the end of the optimal planting period (i.e., late May), beetles would be forced to remain on alternate food plants until soybean emerged. To simulate this consequence of delayed soybean planting, overwintered adult C. trifurcata were caged on foliage of selected plant species for 5-28 d. Plants tested were alfalfa ('Blazer' and 'Vernal'), oats ('Starter'), wheat ('Guard'), maize ('Garst 8532' and 'Renze 6338'), and soybean ('Corsoy 79') as a control. Beetles that survived were transferred to soybean ('Corsoy 79') foliage for the remainder of their lives. In all treatments, female lifespans and oviposition rates were reduced significantly relative to the soybean control. Females caged on grasses had significantly shorter lifespans and lower oviposition rates than those caged on alfalfa. Lifespan and oviposition declined in proportion to number of days without soybean. Survivors from alfalfa regained relatively high oviposition rates once transferred to soybean. In contrast, beetles caged on grasses for 14 d or longer never oviposited even after being transferred to soybean. Results suggest that large-scale adoption of delay soybean planting would reduce beetle survival and reproduction, with the magnitude of reductions being greatest in regions without alfalfa.