Population Dynamics of an Insect Herbivore in Simple and Diverse Habitats

Abstract

Much of the recent experimental work on the effect of vegetational diversity on arthropod herbivores has tested Root's (1973) resource concentration hypothesis and natural enemies hypothesis. Two limitations of this work are that only a part of any arthropod life cycle has been examined, and that there is no common currency by which to compare the influence of resource concentration and natural enemies. In this paper, I describe and analyze the demographic response of Epilachna varivestis, a beetle that feeds on Phaseolus vulgaris (common bean), to four experimental habitats at Ithaca, New York: bean monocultures, bean—mustard mixtures, and two bean—weed mixtures. Beetle population densities were significantly lower in the weed mixtures, and only slightly lower in the mustard mixture compared to the bean monoculture. Colonization and egg survival were lower in the weed mixtures, confirming both hypotheses. Demographic analysis provided a common currency to compare the relative influence of colonization and egg survival, and showed that variation in colonization accounted for 58% of the variation in first generation offspring, while variation in egg survival accounted for only 1.7% of this variation. Larval/pupal survival was higher in the weed mixtures, which contradicted the predictions of the natural enemies hypothesis, and obliterated the potential effect of egg survival on first generation offspring. Independent experiments and observations suggested that non—host plants inhibited immigration and hastened emigration, that predaceous coccinellids ate more eggs in vegetationally diverse habitats, and that variation in larval/pupal survival could be related to greater larval movement at high densities. In this study, non—host plant interference influenced beetle populations more than mortality from natural enemies. Demographic analysis was essential for determining the causes of beetle response to vegetational diversity.