SEASONAL CROPPING PATTERN EFFECTS ON ABUNDANCE OF BEMISIA-TABACI (HOMOPTERA, ALEYRODIDAE) AND INCIDENCE OF LETTUCE INFECTIOUS YELLOWS VIRUS

The sweetpotato whitefly, Bemisia tabaci (Gennadius), was trapped thro ughout the southern desert agricultural region of California during tw o consecutive growing seasons. Trap data revealed changes in whitefly population densities that provide insight into the epidemiology of let tuce infectious yellows virus (LIYV) in fall melon and lettuce. Whitef ly abundance increased rapidly from July to September in cotton. Durin g that period, there were significant correlations between number of c otton fields in a region and number of whiteflies trapped in that regi on. Beginning in August and September, whitefly densities increased in melon, and the proportion of viruliferous whiteflies increased in cot ton and melon. After the defoliation of cotton was initiated in Septem ber, whiteflies migrated to melons, which not only sen ed as their hos t but also as a reservoir for LIYV. In October and November high numbe rs of viruliferous whiteflies were found in melon and lettuce. As melo ns were harvested and the fields dried, viruliferous whiteflies migrat ed to newly emerged lettuce.