ASSOCIATIONAL SUSCEPTIBILITY OF COTTONWOOD TO A BOX ELDER HERBIVORE

The population ecology of Phyllotreta cruciferae Goeze, a flea beetle which is an important pest of cole crops (Brassica oleracea) in central New York was studied in experimental gardens of differing vegetational diversity over a three year period. Adult beetles were more abundant on collards (B. oleracea var. acephala) grown in monocultures than on those grown adjacent to natural vegetation. The emergence of individuals forming the new annual generation was also greater in the pure stands. Predators and parasites appeared to have a negligible influence on the adult beetles in both habitats. Further experiments demonstrated that monocultures were colonized more rapidly and experienced greater feeding damage than stands in which collards had been interplanted with tomatoes and tobacco. Choice experiments in the laboratory showed that chemical stimuli given off by non-host plants (tomato, Lycopersicon esculentum, and ragweed, Ambrosia artemisiifolia) interfered with the host finding and feeding behaviour of P. cruciferae. These results indicate that vegetational diversity can exert a direct influence on populations of phytophagous insects.We conclude that the environmental capacity (Determination in Schwerdtfeger's terminology) of diverse natural communities is lower than that of natural or man-made monocultures. The "associational resistance" resulting from the higher taxonomic and microclimatic complexity of natural vegetation tends to reduce outbreaks of herbivores in diverse communities.