Adults can both fly short distances within the plant canopy and be carried long distances on air currents. Some migrating adults can stay suspended for hours and thereby carried great distances. Even so, whitefly adults are usually more concentrated close to the ground and close to the source of infestation. Adults emerge from pupae during the morning and become more active as temperature increases. Thus, movement is greatest from mid-morning to mid-afternoon. Adults tend to settle randomly after a long-distance flight, but are able to perceive color at short distances and will preferentially select yellow/green objects.
Adults congregate and oviposition is heaviest on the undersides of younger leaves. Crawler movement is limited and subsequent immature stages are immobile so that development occurs on the undersides of leaves close to the site of oviposition. Continued growth of the plant after oviposition causes successively older stages of the whitefly to be found on successively older leaves. Adult population densities within many susceptible vine or bush crops can be determined by counting the number of adults per fully expanded leaf (by gently turning over a leaf at the third or fourth node from the tip). In cabbage, older leaves should be sampled. Sampling 100 leaves per field (one leaf on each of 100 randomly selected plants) can provide a very good estimate of the average whitefly population density in the field, but fewer samples are usually all that is needed to make a control decision.
The location on the plant of the various stages of the sweetpotato whitefly follows the development of the plant. Eggs and early instar nymphs are found on the young leaves and larger nymphs are usually more numerous on older leaves. For example, large nymphs are more noticeable at the sixth to eighth node from the growing point than on younger leaves in melons and tomatoes. On cabbage, higher concentrations of large nymphs occur on the oldest leaves (frame leaves). Thus, the age of leaves inspected affects the observed number of nymph stages. In general, large nymphs are the easiest of the immature stages to sample because they can be counted with the unaided eye. The advantage of sampling for nymphs is that it provides a better measure of actual whitefly population density in the field. Fourth instar nymphs (pupae) should be sampled to estimate percent parasitism, which may be critical in a biological control program. The disadvantage is that an infestation may become well established prior to the detection of pupae.
Action thresholds are levels of pest populations at which control should be implemented to avoid significant damage to the crop. Action thresholds help producers determine both the need for control actions, such as insecticide applications, and the proper timing of such actions. Unfortunately, there is little data with which to establish thresholds for the "B-strain" (now known as thesilverleaf whitefly) of sweetpotato whitefly on most crops. In cucumber, an average of 30 nymphs per square inch of leaf completely stunts the growth. In
poinsettia, more than two to five immatures per square inch of leaf is considered a damaging level. In Arizona cotton, after satisfactory control measures, pupae were reduced to between one and 14 per leaf and adults to 10 to 12 per leaf.
In southwest Florida whitefly numbers on sticky traps are low in the fall but increase rapidly due to migration out of terminated fall crops. The pattern repeats in springtime except that initial numbers are higher, followed by a decrease as whiteflies settle into the spring crop. As crops are terminated, trap counts increase dramatically as whiteflies migrate out of the fields.
Data collected in 1992 at the Texas Agricultural Experiment Station at Weslaco suggest that an average of one large nymph per square inch of leaf at the sixth leaf node is a potentially damaging level in cantaloupe (resulting in approximately 10 percent yield loss). The action threshold in melons is currently estimated at an average of three adults per leaf. Sampling of adults is simpler and often more acceptable to growers than counting nymphs. Having samples of both adults and nymphs can help determine if the infestation is recent (adults present, but no nymphs) or established (nymphs present). With the limitations in available chemical treatments, the use of action thresholds is essential for efficient whitefly management and economical production of susceptible crops.