| The "Enemies" hypothesis, that a lower density of herbivores in diverse vegetation is due to the augmentation of natural enemies, was tested in squash monocultures and maize (corn)-bean-squash tricultures in southeastern Mexico and in California. The main squash herbivore in the tropical agroecosystem was Diaphania hyalinata (Lepidoptera: Pyralidae). I monitored the density of this pest in experimental plots in farmers' fields through three growing seasons; when abundant, its densities were significantly higher in pure stands of squash. I assessed the action of natural enemies indirectly, by monitoring their abundance patterns, and directly, by rearing parasitoids from field-collected eggs and larvae. Parasitoids in general, and also the subset of species that attack D. hyalinata, were more abundant in polycultures. These findings support the "Enemies" hypothesis on a theoretical basis; they also call attention to the need for practical research on traditional farming methods before they are replaced through the casual importation of approaches developed in the temperate zone and perhaps not well suited to the local ecological and socio-economic conditions.; In California, I monitored the population densities of the minute pirate bug, Orius tristicolor (Hemiptera: Anthocoridae), and its prey, the western flower thrips, Frankliniella occidentalis (Thysanoptera: Thripidae) on squash in monocultures and tricultures with corn and cowpea intercropped. The predator exhibited a more rapid rate of colonization in triculture plots and the thrips densities on squash were lower and fell earlier in the season in these systems. I demonstrated the capacity for thrips control by Orius on squash in outdoor predator inclusion/exclusion cage experiments.; The exact mechanisms by which natural enemies were augmented in crop mixtures remains a matter for further study though structurally related complexity seems to have been more important as a determinant than either plant diversity per se or food resource availability. |
