Interactions Among Natural Enemies, Herbivores, And Yield In Monocultures And Polycultures Of Corn, Bean, And Squash (costa Rica, Mexico).

The major goal of this study was to investigate the effect of intercropping on the population dynamics and interactions of natural enemies and herbivores, and their effect on plant yields. A minor goal was to do a review of biological control cases. The field studies focused primarily on natural enemies. Specifically, I wanted to know: (1) Are natural enemies more abundant in diverse than in simple systems? (2) What is the relative effect of structural diversity, floral diversity, and herbivore abundance on enemy abundance? (3) Are there differences in predation levels between simple and diverse systems? (4) Do herbivores and/or natural enemies have an impact of yield? To answer these questions, I grew monocultures and polycultures of corn, beans, and squash in Costa Rica and Mexico. Insects were sampled either by sight count or by Malaise traps at regular intervals. Spodoptera eggs and Drosophila larvae were used to determine predation rates in the field. Finally, vegetative and reproductive aspects of the crops were monitored to see how they correlated with changes in insect abundance. In Costa Rica, parasitoid abundance was significantly higher in the triculture than in any other treatment, while predator abundance showed no consistent treatment effect. Overall, herbivore biomass was significantly lower in polycultures than in monocultures. Variation in parasitoid numbers was most closely linked with variation in bean biomass, while variation in predator numbers was most closely linked with variation in herbivore biomass. In Mexico, cowpeas were grown in monocultures at 4 different densities and with and without weeds. Damage to cowpea by a specialist beetle, Ceratoma, increased with increasing cowpea density and was significantly higher in a cowpea-weed intercrop than in weed-free monocultures. In addition, there was a signficant positive correlation between enemy abundance and yield only in the weed-free monocultures. In another season, non-ant enemy abundance per plant was significantly higher in monocultures than polycultures for all 3 crops. At least on corn, removal rates of Drosophila baits were also significantly higher in the polycultures. In polycultures, flower removal had a negative impact on predation rates while addition of herbivores had no effect. For corn, per plant yields were significantly higher in polyculture than in monoculture, although this effect disappeared when herbivore density was artificially elevated. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMIPh.D.Biological SciencesEcologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/127644/2/8324197.pd