Experimental Studies of Beetle Diffusion in Simple and Complex Crop Habitats

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Abstract
The use of a diffusion technique revealed considerable differences in omnivorous beetle movement patterns within agroecosystems varying in plant composition, arrangement and quality. Simultaneous inoculations of Coleomegilla maculata (De Geer) (Coleoptera: Coccinellidae) spread farther in corn-bean-squash polycultures than in corn monocultures of equal or decreased plant density during 24 h of dispersal. After 6 h, simple diffusion model predictions gave excellent fits to these data as distance-frequency distributions approached normality. Within 24 h, diffusion constants also showed movement to be faster through polycultures. These variations in non-migratory movement resulted in differing patterns of beetle dispersion between fields. At each census, clumping of Coleomegilla density distributions (.sigma.2/.hivin..mu.) was higher in polycultures. For each polyculture, the hypothesis that corn plants offer more preferred foraging habitats (than bean and squash alternatives) was confirmed within 4 h of release. Additional evidence was offered by 2 yr of independent field data showing significantly higher beetle accumulations in corn monocultures than corn-bean-squash polycultures. Feeding rewards may be the proximal factors responsible for these patterns, as indicated by faster and farther movements in a prey-removal monoculture than in an identical field supplied with a superabundance of aphids and pollen. Predation on aphids was shown to prolong significantly Coleomegilla tenure time on aphid-infested corn in comparison to bean, squash, or aphid-free corn plants.