In the past we have assumed that sweet potato whiteflies, Bemisia tabaci (Gennadius), were poor fliers with a limited ability to disperse. This assumption was based on the fact that sweet potato whiteflies are extremely small insects (24 -44 μg) and consequently would be subject to desiccation. We also thought small size would limit their capacity to store sufficient energy reserves to sustain flight for long periods of time. Recent experiments have indicated we were wrong on several counts. Data collected in the laboratory using a vertical flight chamber have revealed a number of interesting facts. Of importance is the fact that a portion of the population is capable of sustaining flight for more than 2.5 hours. In a wind-aided situation, this means they can be moved more than 25 miles in a 10 mph wind. Other details of flight behavior are being made clear to us. For example, we found that flight activity is influenced by host plant quality and age of the whitefly. We hope to eventually be able to predict when whiteflies are going to migrate between crops. This will allow growers to make informed decisions concerning planting dates. We are also working in the field to confirm laboratory results.

The sweet potato whitefly is an insect whose economic importance is continually expanding as it becomes a more efficient vector, increases its fecundity and broadens its host range. To keep pace with its ability to develop resistance to existing classes of pesticides, we have undertaken a program to examine the effectiveness of pesticides with novel chemistries and novel modes of action. Several have shown themselves to be very effective against the various life stages of the sweet potato whitefly. Once these are incorporated in to our arsenal of pesticides, we hope to be able to manage resistance by prudently using these new materials.

Populations of the sweet potato whitefly, Bemisia tabaci, have been shown to consist of both migratory and trivial flying morphs. The behavior of these forms as part of the process of short-range migration needed to be examined under field conditions. Insects were marked in a field of cantaloupes using fluorescent dust. During the first growing season traps, used to collect living whiteflies, were placed along 16 equally spaced transects (22.5° apart) radiating out from the field to a distance of up to 0.6 miles. Wind out of the northeast consistently carried migrating whiteflies to traps placed along transects in the southwestern quadrant because cold air drainages dictate wind direction during early morning hours. For this reason, during the second season traps were laid out in a rectangular grid extending 3 miles to the southwest of the marked field. If dispersal was entirely passive or wind directed patterns could be described using a diffusion model. Statistical examination of the data, however, demonstrate that the distribution on all days was patchy. Traps in the immediate vicinity of the marked field caught more whiteflies than the daily median. Large numbers were also collected from around the periphery of the grid. Whiteflies were far less prevalent in the grid's center. As a result, the distribution of captured whiteflies can be described as bimodal. These patterns confirm behavior observed in the laboratory, i.e., a portion of the population are trivial fliers that do not engage in migration and are consequently captured in traps near the field and a portion initially ignore vegetative cues and fly for a period of time before landing in distant traps. This second population comprises the second peak in the model that appeared 1.6 miles from the marked field. On a localized level, 1.6 miles seems to be how far whiteflies move in a day. Earlier studies indicate that whiteflies only fly one day.

A Florida strain of sweet potato whitefly, Bemisia tabaci (Gennadius), was found to have an expanded range which includes several new food crops. To determine why, we examined how it processes plant nutrients. The amino acid and carbohydrate content of phloem sap from poinsettia and pumpkin and of honeydew produced by the Florida strain were analyzed Honeydews produced by a strain from Arizona feeding on both plants were also analyzed Poinsettia phloem sap contained 15 amino acids; 14 of these were in pumpkin phloem sap. Almost all the same amino acids were in the honeydews produced by the two strains on the two hosts. Carbohydrates in phloem sap and honeydew were common transport sugars, like sucrose. Both honeydews contained trahalulose, a disaccharide not previously associated with insects. Both strains processed phloem sap and honeydew from both plants in the same manner, but the Florida strain produced significantly larger quantities of honeydew; it is therefore assumed to process more phloem sap. Since this strain has access to more phloem sap it also has access to more of the amino acids, which are in short supply in the phloem sap of some plants, allowing it to broaden its range.

Although problems associated with the sweet potato whitefly, Bemisia tabaci (Gennadius), are not as dramatic as they were in 1992. they were still significant in 1993. Laboratory research in 1993 focused on defining the cues that result in migratory behavior, specifically host quality. In addition, field studies were conducted to learn more about timing, direction, and distance flown. Our goal is to develop a predictive model that can be used for forecasting whitefly movement. During our behavioral studies, B. tabaci was presented with two cues that lead to disparate behaviors. More than 70% of the whiteflies we tested ended their flights (within three presentations of the cue) when given a choice between settling on a 'host' (550 nm interference filter) and continued flight. Only 6% of the individuals we tested, demonstrated what would be considered to be true migration Both endogenoous and exogenous factors can play an important role in determining when insects will fly. Finally, although the oogenesis-flight syndrome is thought to be a strong component of insect migratory activity, whiteflies do not appear to postpone egg production until after they have engaged in flight. In the field marked whiteflies were also collected in the most distant of these traps. These field results support our hypothesis that most whitefly movement in the fall in the Yuma Valley is in a SW direction (prevailing winds are from the NE). Within a 3 hr time frame whiteflies can travel as far as 2.2 miles from the source field. We examined the effects of female flight distance and eggload. We found significant differences in the eggload of field collected whiteflies versus whiteflies collected in traps at all distances from the source field. There was no relationship between distance flown and eggload. These results may indicate that dispersing individuals are capable of delaying egg laying until a suitable host plant is located