Effects of Nonhost Plant on an Insect Herbivore in Diverse Habitats

Vegetation texture (i.e., plant density, species diversity, and structural complexity) may influence the abundance of a herbivore by affecting its movement, altering the suitability of host plants, and by changing the herbivore's vulnerability to attack by natural enemies. We investigated these effects on the Mexican bean beetle (Epilachna varivestis) on snap bean (Phaseolus vulgaris) growing in high—and low—density monocultures and intercropped with short (0.5 m) or tall (1.2 m) corn plants (Zea mays). Beetle populations consistently reached higher densities on beans in monocultures than on beans intercropped with tall corn. Neither bean plant density nor plant species diversity alone significantly affected beetle abundance. Instead, the presence of tall corn plants contributed most to lower bean beetle density in the tall corn—bean intercrop, primarily by reducing adult colonization. However, the tall corn also increased the suitability of intercropped bean plants to the beetles because adults preferred to feed on and larvae developed faster on beans in the tall corn—bean habitat. Host plant effects were independent of the influence of microclimate and risk of predation. Abundance of natural enemies of the bean beetle was low and overall, predator density and bean beetle larval survival did not differ significantly among habitats. However, it is difficult to predict the effect or mortality factors, such as natural enemies, on bean beetle populations because larval mortality over time was affected by habitat type (i.e., there was a significant interaction between habitat type and time). Our results were partially consistent with the resource concentration hypothesis. Whereas Mexican bean beetles colonized bean monocultures more readily than they did tall corn—bean diculture, the presence of more suitable host plants in the diculture acted to increase, rather than decrease, bean beetle density in the tall corn—bean habitat. However, higher adult feeding preference and faster larval development on beans in the tall corn—bean habitat did not offset lower adult colonization in this habitat.