Aggregation patterns ofBemisia tabaci in response to insecticide applications

We measured the effect of insecticide applications on the distribution patterns of the sweet potato whitefly,Bemisia tabaci (Gennadius), in fields of cantaloupe,Cucumis melo L., in Yuma, Arizona. Whitefly infestations were measured by counts of adults, eggs, first to mid/fourth instars, and late-fourth instars (‘red-eyed’) nymphs. Adults were sampled from the entire leaf, and immature stages were counted in a 1-cm² area of a leaf. The indicesb (Power Law), β (Patchiness regression) and I _d (Morisita) indicated that all life stages were aggregated, but results for the three indices were not similar for determining the relative aggregation levels between treatments. In general, indices β andI _d indicated higher aggregation in insecticide-treated fields, whereasb had mixed results. The Morisita index was sensitive to a few unusually high means among a series of low densities in the treatment plots, what could be attributed to refuges due to failure in the insecticide applications. Despite the usefulness of the Power Law and the Patchiness regression for describing the relationship between spatial or temporal variability and mean densities, we suggest thatI _d is more appropriate for expression of spatial distribution because it is based on a precise definition of aggregation.