THE EFFECT OF SHORT-SEASON COTTON PRODUCTION SCHEDULES ON BEMISIA-TABACI (GENNADIUS)

Three short season cotton production schedules designed to reduce the number of overwintering pink bollworms, Pectinophora gossypiella (Saun ders), in cotton in the Imperial Valley, CA, were tested in each of tw o years to determine their impact on the phenology of sweetpotato whit efly, Bemisia tabaci (Gennadius), populations. Yellow sticky card trap s and cotton leaf samples were used to monitor adult and immature stag es, respectively. In both years, B. tabaci populations continued to in crease for up to six weeks after the last cotton irrigation dates. Sig nificant reductions in B. tabaci adults and immatures did not occur un til after the cotton plots were treated with defoliants. Adults trappe d on yellow sticky traps (13 x 21 cm) placed horizontally 13 cm from t he soil surface in cotton fields peaked at 5,200 and 12,800 per card p er week in 1986 and 1987, respectively. Red-eyed nymph stage B. tabaci reached densities of nearly ten per one half leaf on sixth and twelft h mainstem node leaves in 1986, and 62 per one quarter leaf on seventh mainstem node leaves in 1987. A schedule that includes irrigation ter mination by 15 August and defoliation by 1 October to meet the mandate d 1 November stalk destruction deadline was too late to prevent the wh itefly population from reaching its peak, and would not significantly reduce whitefly dispersal into sensitive crops planted before 15 Septe mber. To break the population cycle from cotton to these later crops, cotton must be last irrigated by 21 July and defoliated by 20 August, and new vegetable and sugar beet crops must be planted after 15 Septem ber. Since whiteflies continue to emerge from defoliated cotton for th ree weeks after defoliation, this schedule provides a one to two week host crop free period. No yield reduction was noted on cotton planted by 20 March when following this schedule compared to cotton produced w ith last irrigation and defoliation dates up to four weeks later than these dates.