POPULATION-DYNAMICS OF AN INSECT HERBIVORE IN SIMPLE AND DIVERSE HABITATS

Describe and analyzes the demographic response of Epilachna varivestis, a beetle that feeds on Phaseolus vulgaris, to 4 experimental habitats at Ithaca, New York: bean monocultures, bean-mustard mixtures, and 2 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. 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 1st generation offspring. Non-host plants inhibited immigration and hastened emigration. Predaceous coccinellids ate more eggs in vegetationally diverse habitats. Variation in larval/pupal survival could be related to greater larval movement at high densities. Non-host plant interference influenced beetle populations more than mortality from natural enemies. -from Author