The Enemies Hypothesis: Tritrophic Interactions and Vegetational Diversity in Tropical Agroecosystems

Abstract

The effects of vegetational diversity on the abundance of natural enemies of herbivores were studied in a traditional agroecosystem in tropical Mexico. Visitation patterns of parasitic Hymenoptera and densities of predators were compared between monocultures of squash and maize and mixed assemblages of maize, cowpea, and squash. The impact of parasitoid visitation was assessed by monitoring the percent parasitism of a target herbivore in the system: Diaphania hyalinata (L.) (Lepidoptera: Pyralidae), a cucurbit specialist. The "Enemies" hypothesis predicts a positive correlation between plant species richness and natural enemy abundance, which results in the regulation of herbivores at lower levels in diverse vegetation than in pure stands. The specific questions addressed in this study were: (1) are the major groups of natural enemies more abundant in patches of diverse vegetation than in monocultures?; (2) if so, does an increase in either abundance or diversity of natural enemies cause a concomitant increase in herbivore regulation?; (3) are the effects of vegetational diversity uniform over major enemy groups? The activity of hymenopteran parasitoids was affected by cropping pattern. Malaise trap captures of parasitoids in general, and of those species that attack D. hyalinata on squash, were consistently greater in tricultures than in squash monocultures, but were also greater in maize monocultures. Parasitism rates of eggs and larvae were higher on squash interplanted with maize and cowpea. These data partially support the "Enemies" hypothesis in explaining lower herbivore levels in diverse vegetation. However, the effects of diversity, per se, do not seem to be as important as the presence of maize in the system. The responses of natural enemies to vegetational diversity were not uniform. Most predators were not significantly influenced by cropping patterns, and some were more abundant in squash monocultures. The results of this study are discussed in terms of possible routes by which members of the third trophic level are influenced by plants in association.