EFFECT OF SOIL SULFUR LEVELS ON FEEDING PREFERENCE OF BREVICORYNE-BRASSICAE ON BRUSSELS-SPROUTS

Two cultivars of Brussels sprouts, Brassica oleracea var. gem mifera w ere grown in pots treated with nutrient solutions containing five diff erent levels of soil sulfur. Three replicates, each with 10 pots, were infested with the alate form of the aphid, Brevicoryne brassicae. Ano ther three replicates were not infested with aphids. Total apterae aph ids were counted on each leaf of the infested plants four weeks later. At the same time all plants were assayed for total glucosinolates and free thiocyanates. The greatest number of aphids per leaf on infested plants was found for the highest levels of sulfur applied. The highes t concentrations of total glucosinolates were found in the leaves of u ninfested plants that received the highest levels of sulfur. The lowes t level of total glucosinolates was found in the leaves of infested pl ants that received the highest amount of sulfur, and these leaves had the highest level of free thiocyanates. The results suggest that aphid s were attracted in greatest numbers to leaves that had the highest sy nthesis of glucosinolates and that their presence resulted in reduced glucosinolate levels by a combination of a depression in metabolic rat e and a stimulation in myrosinase activity.