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Exclusion Experiments and the Competitive Release of Insects feeding on Collards

Kareiva, Peter
Ecology 1982 v.63 no.3 pp. 696-704
Brassicaceae, Phyllotreta striolata, Pieris rapae, cabbage, collard greens, correlation, food choices, food plants, foraging, gardens, interspecific competition, larvae, phytophagous insects, spatial variation
Interspecific competition among crucifer herbivores was tested by a series of exclusion experiments in collard gardens. Two (of 21 potential) instances of competitive release were observed: Phyllotreta striolata upon the exclusion of Phyllortreta cruciferae, and P. cruciferae upon the exclusion of cabbage worms (Pieris rapae larvae). The exclusion experiments were replicated and repeated at three collard densities and the significant releases of flea beetles occurred only at the densest collard plantings. It is argued that Phyllotreta foraging movements mediate competition in collard—flea beetle associations, restricting competitive release phenomena to dense plantings where beetles have more opportunity to be discriminating in their choice of food plants because they have more opportunity to move among plants. Although exclusion experiments documented a competitive interaction between P. cruciferae and P. striolata, the numbers of these two species per plant were consistently positively correlated. This positive association may be due to parallel responses on the part of both species to variations in plant quality. Several features of the collard—herbivore association made if difficult to detect competition as an organizing force: constraints of beetle foraging, discordant phenologies of herbivore populations, unpredictable colonization of plants by herbivores, and the spatial heterogeneity of herbivore populations. Because these complications confounded competitive patterns in a simple homogeneous garden, in diverse natural plant—insect systems we should expect even greater difficulty in detecting competition among herbivorous insects.