The role of aggregation in the response of Mexican bean beetles to host-plant density

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Abstract
According to the resource concentration hypothesis, specialized insect herbivores predominantly attack host plants growing in pure, large, and/or dense stands because they are more likely to find and less likely to leave such stands. This study examines movement of a herbivorous beetle, Epilachna varivestis, in an attempt to understand why the beetle's numbers per plant increase with plant density. I studied immigration into, emigration from, and movement within host patches by following movements of individually marked beetles, while experimentally varying host-plant density. In order to study the effect of conspecifics on movement, I varied the number of beetles released on the same plant. The probability of intrapatch movement decreased, and the probability of emigration increased when plant density was reduced. Both immigration rate and intrapatch movement were strongly affected by presence or absence of conspecifics. Beetles were much more likely to stop at plants with a high number of beetles, compared to plants without beetles. On the basis of these results I advance a model that provides a mechanistic explanation of why dense patches should acquire high herbivore loads in the E. varivestis-garden bean system. Movement in Mexican bean beetles is strongly aggregative, and in dense patches frequent intrapatch movement provides ample opportunity for beetle aggregations to build up. In sparse patches, however, intrapatch movement is virtually absent and such aggregations are less likely to arise.