High infestations of Bemisia tabaci B biotype were observed in 1999, in a 50 ha soybean field in Balsas region, State of Maranhão, Brazil. The field was sown in May and irrigated with a pivot system, ininterruptedly, during the whole crop season. Whitefly infestation initiated when plants were at the beginning of the vegetative stage, and persisted in variable levels up to plant maturation. Alternate applications of imidacloprid and piridaben were made at ten-day intervals to control insect population. The applications were descontinued between September 10 and 20, during a period of continuous rainfall. After this period, an epizootic occurrence of Verticillium lecanii, infecting whiteflies, was verified in the whole area, reducing the insect population to very low densities. A progressive increase in these populations was observed later on. This reinfestation probably occurred due to the absence of new periods of rainfall, indicating that the humidity supplied by irrigation under pivot was insufficient to promote V. lecanii epizootics in that region since the wind activity is continuous during the crop season.
Insecta; Bemisia argentifolii; silverleaf whitefly; biological control; entomopathogenic fungus