Sweetpotato Whitefly

Several species of whiteflies may infest cole crops. Only the sweetpotato whitefly MEAM1, sometimes called silverleaf whitefly, causes economic damage. Proper identification of sweetpotato whitefly is important, because greenhouse whitefly (Trialeurodes vaporariorum) and other whiteflies do not require management in cole crops.

Whiteflies are found mostly on the undersides of leaves. An adult whitefly is 0.06 inch (1.5 mm) long, with a yellow abdomen and thorax, and white wings. Adults fly readily when plants are disturbed.

The tiny, oval eggs of sweetpotato whitefly hatch into translucent, mobile first instars (crawlers) that have short legs and antennae. After the first molt, nymphs lack antennae and legs, and remain fixed to the leaf surface. Older nymphs are oval and translucent, whitish, or yellowish. The last-instar nymph, also called a pupa or red-eye nymph, is the life stage most helpful for distinguishing the species.

Use a hand lens to identify both adults and nymphs of the sweetpotato whitefly:

Sweetpotato whitefly nymphs and pupae have no filaments projecting from the edge of the body, and few or no short filaments on top. In contrast, young greenhouse whitefly nymphs have a fringe of short filaments around their perimeter, and the last-instar nymphs (pupae) have several long, projecting, waxy filaments protruding from their bodies.

Whiteflies damage cole crops by sucking plant sap and excreting copious honeydew that contaminates plant surfaces. Black sooty mold grows on the honeydew, which reduces photosynthesis. Sweetpotato whitefly feeding and excretions slow plant growth and development when they are numerous, which may delay harvest. Sweetpotato whitefly feeding on broccoli also causes bleaching of stems and leaf petioles, called broccoli white stem.

Sweetpotato whitefly is a major problem in California's southern desert and the southern San Joaquin Valley. It is only a sporadic pest in the Central Coast region. Natural enemies can help to control whiteflies, and cultural practices are important in helping to prevent severe infestation. Monitor for sweetpotato whitefly and apply insecticides when warranted. If whiteflies are migrating into the crop, applying insecticide only to field borders may be adequate.

Sweetpotato whitefly is an invasive pest, and no native natural enemies in California currently provide sufficient control. Numerous parasites and predators do attack it, but generally do not keep it below damaging numbers. These natural enemies include parasitic wasps in the Encarsia and Eretmocerus genera, and predators of whitefly nymphs, such as bigeyed bugs, lacewing larvae, and lady beetles.  

Some pathogens also attack sweetpotato whitefly, though naturally occurring infections do not typically provide sufficient control. Pathogens include the entomopathogenic fungi Beauveria bassiana, Isaria fumosorosea (Cordyceps fumosorosea) and Verticillium lecanii.

Use the following cultural methods to control or suppress whitefly problems:

• In regions where sweetpotato whitefly is a regular problem, delay planting or use a host-free period to decrease the severity of whitefly infestation.
  • Numbers peak in late summer and begin to decrease by November. Delaying planting until November can greatly reduce sweetpotato whitefly infestations.
• When possible, plant cole crops at least one-half mile upwind from other key whitefly hosts, such as cotton and melons.
• Maintain good sanitation in winter and spring by removing weeds and other alternate hosts.
• Attempt to produce the crop in the shortest possible time to limit the extent to which whitefly numbers can increase in the crop; proper management of irrigation and nitrogen will assist in this. See the UC ANR production guide for your specific crop for more details.
• Remove and destroy all crop residue and weed hosts of whiteflies as soon as possible (within 5 days) after harvest.

Use biological and cultural controls, as well as sprays of azadirachtin, entomopathogenic fungi, insecticidal soap and narrow-range oils, in an organically certified crop. The entomopathogenic fungi Beauveria bassiana and Isaria fumosorosea (Cordyceps fumosorosea) are commercially available for whitefly control. These products are most effective when used to control nymphs, and when used in combination with other insecticides. Fungicide application can reduce their efficacy.

Insecticidal soaps and oils are not as effective as other insecticides and require frequent applications and excellent coverage.

In areas where sweetpotato whitefly is a problem (San Joaquin Valley and low desert), monitor for whiteflies as follows (note that these periods may vary slightly from year to year):

• From August to October:
  • If a systemic insecticide was not applied at planting, regularly (about twice weekly) check field margins for whiteflies; these areas are usually infested first.
  • If a systemic insecticide was applied at planting, check fields on a weekly basis. Begin checking twice weekly approximately 45 days after the application.
• From November through March, check field margins once weekly.
• Fields are commonly and uniformly infested in the low desert, but whiteflies may be more numerous along field margins during initial infestations.
• Be especially alert for rapid increases in whitefly numbers when nearby hosts are in decline or being harvested. During these critical periods, check cole crop fields twice weekly.

Yellow sticky traps may be useful in detecting initial whitefly migrations into fields. If using these traps, place them out of direct sunlight, with the sticky side facing the crop plants. Monitoring may also involve turning over crop leaves, as whiteflies tend to be present on leaf undersides. This method may be more difficult, as whiteflies quickly disperse when disturbed.

Thresholds are not available for sweetpotato whitefly in cole crops. For organic cole crops or crops that did not receive an at-plant soil application of systemic insecticide, consider applying insecticide only to the field margins if whiteflies are more numerous at the field margins than the centers. This will reduce whiteflies and the costs of application, as well as help preserve natural enemies in the field. Applying insecticide to the field margins may not be necessary in cole crops that received an at-plant application of a systemic insecticide.

Rotate insecticides with different mode-of-action group numbers to manage resistance. This includes all insecticides used in the field for other pests during the current season. Maximize the efficacy of applications by achieving thorough spray coverage. Ground application may provide more complete coverage than aerial.