Migration and dispersal by the sweet potato whitefly, Bemisia tabaci

Research on short-range dispersal (less than 10 km) by supposedly weakly fl ying insects, e.g., whiteflies, has not enjoyed the attention paid to dispe rsal by strong flying insects that are capable of migrating more than 100 k m, such as some leafhoppers. Possible exceptions are studies concerning dis persal by the sweet potato whitefly Bemisia tabaci. We wanted to determine to what extent whitefly flight is truly weak and if it meets some of the cr iteria historically used to define migration. In a flight chamber the major ity of whiteflies landed quickly. A portion (6%) flew for more than 15 min (some more than 2 h). In doing so whiteflies ignored vegetative cues and fo cused on artificial skylight. This was against a downwardly directed airstr eam exceeding 4.0 cm/s. Attempts to associate wing morphological characteri stics with their flight were mostly successful. The shapes of the wings of whiteflies that flew for some time in the chamber, or for some distance in the field, were different than those that did not. In the held B. tabaci wa s found dispersing more than 5 km. In these experiments whiteflies in a can taloupe field were marked with fluorescent dust. Large portions of the mark ed population landed in close proximity to the field and another large grou p was trapped at 2.2 km. We hypothesized that this conformed to flight beha vior observed in the laboratory, i.e., individuals captured near the field quickly responded to vegetative cues and landed, while others dispersed dow n range, initially ignoring these plant cues. This behavior was thought to be persistent. Additionally, flight in the field was not entirely wind-dire cted. Whiteflies were sometimes captured in areas away from prevailing wind s. These are indicative of strong flight and migration. We found, however, that whiteflies did not possess all the characteristics commonly associated with stronger flyers. Whiteflies do not increase wingbeat frequency to com pensate fbr high wing loading. Whiteflies do not possess an oogenesis-fligh t syndrome. In spite of these findings, whitefly flight cannot be character ized as weak. Whiteflies flew in a flight chamber against a strong airstrea m. They also dispersed in field experiments for a considerable distance. Th ere is also information that whiteflies have a migratory form, in the manne r of some strong flying insects. Whitefly flight seems to meet many criteri a associated with migration in insects. As a final note, although most whit efly flight occurs over short distances, it is no less important biological ly and cannot be ignored when developing pest management programs. (C) 1999 Elsevier Science B.V. All rights reserved.