Increases in vegetational diversity have been associated with reductions in numbers of herbivorous insect pests in many agricultural settings. One manifestation of increased vegetational diversity is the inclusion of weedy vegetation around crop plots. Plots of broccoli in a agroecosystem field study were surrounded by either (i) bare ground, or (ii) weedy margins, and numbers of both apterous and alate green peach aphids, Myzus persicae (Sulzer), on broccoli were recorded. Cages designed to exclude aphid predators and parasitoids were placed on broccoli plants in both types of treatment plots. Broccoli plots surrounded by bare ground had aphid densities four times as high as broccoli plots surrounded by weeds. Furthermore, alate aphid densities in plots surrounded by bare ground were five times those in weedy plots. This result coupled with the results of the exclusion cage experiment indicate that slate colonization may play an important role in the efficacy of weedy margins as a means of reducing aphid pests on broccoli.
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