Resistance to whitefly (Bemisia tabaci) in cotton (Gossypium hirsutum) in the Sudan

Over the past 10–15 years, the cotton whitefly (Bemisia tabaci Genn.) has caused severe damage to cotton in many tropical countries. Effects of three leaf-hair densities (LHD) (high, moderate and low) and three leaf shapes (LS) (normal, okra and super okra) on whitefly numbers were examined in the Sudan in 1982. Resistance was conferred by low LHD and okra or super okra LS, with significant reductions in numbers of whitefly adults per leaf and adults, scales and pupae per unit area. Adults per leaf were lowest on the most resistant line, ORS-13 (exotic Gossypium hirsutum cv., low LHD/okra LS) throughout the season. Pupal parasitism was significantly higher on this line than on a hairy plant of normal leaf shape. Honeydew causing stickiness of lint was reduced to acceptable levels on resistant lines. Leaf shape influenced the microclimate, in that relative humidity was lower and temperature was slightly higher in the canopy of Sudac-K than in Barac (67)B. Fewer whiteflies on Sudac-K may be due to a more adverse microclimate for whitefly and improved host-searching conditions for adult parasites. A breeding programme in the Sudan should be maintained for selection of field-resistant varieties.