Factors Affecting Abundance and Damage Caused by Cabbage Aphid, Brevicoryne brassicae on Four Brassica Leafy Vegetables: Brassica oleracea var. Acephala, B. chinense, B. napus and B. carinata

Resistance of four brassica leafy vegetables: kale, Chinese cabbage, rape and Ethiopian mustard to cabbage aphid, Brevicoryne brassicae (L.) was studied in cage experiments at the Botswana College of Agriculture, Gaborone, Botswana. A completely randomised design with cultivars as treatments and four replicates per treatment was used. One newly laid nymph was placed on each of the four potted plants of each cultivar and left to breed. Aphid abundance and damage on each cultivar were recorded weekly for six weeks. Direct counts of aphids were made to estimate abundance while the proportions of leaves with honeydew were used to measure intensity of damage. Chlorophyll, water and protein content and leaf thickness were estimated and their effects on aphid abundance and damage intensity were determined. Aphid populations on all cultivars peaked in the third week. Peak populations differed significantly (P < 0.05) between cultivars, with the greatest (828.2 per leaf) on Chinese cabbage and the least (307 per leaf) on Ethiopian mustard. Cabbage aphids caused significantly (P < 0.05) greater reduction of marketable leaves on kale than on the other cultivars. Cultivars with high chlorophyll; high water content and the thickest leaves had lower abundance of cabbage aphid than those with significantly (P< 0.05) lower chlorophyll and water content and thinner leaves; those with thicker leaves, high protein and high water content suffered greatest honeydew damage. Therefore breeding for low protein and water content, high chlo- rophyll content and thin leaves is recommended because the cultivar produced would suffer low honeydew damage.