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The relative importance of resource-mediated competition versus apparent competition was studied in two native aphids, Monellia caryella, and Melanocallis caryaefoliae, on pecan, Carya illinoensis. We compared reproduction and body size of the two aphid species when they were caged on pecan leaves as single or mixed species, and when green lacewing larvae (Chrysoperla comanche or Chyrsopa nigricornis) were present or absent. The experiment was repeated two times on mature compound leaves in the field in the autumn of 1997 and summer of 1998 and once in a greenhouse, on young seedlings with simple leaves. Our results suggest that interspecific competition was variable and asymmetric; in both the Summer 98 experiment and the greenhouse experiment, reproduction of M. caryaefoliae was significantly reduced in the mixed species treatments relative to the single species treatment, while M. caryella reproduction was significantly reduced only in the Summer 98 experiment. No evidence of an effect of competition on reproduction was found in the Autumn 97 experiment. M. caryella body size was reduced in one of the mixed species treatments in the Autumn 97 experiment. Body size of both aphid species was reduced in the mixed species treatments of the greenhouse experiment. The introduction of green lacewing larvae reduced the reproduction of aphids in two of the three experiments in comparison to controls. However, a significant interaction between aphid and predator treatment (M. caryella in the greenhouse experiment) was found in only one experiment. Variation in the outcome of competition was more likely to be due to aspects of plant quality, including leaf age and previous aphid feeding.