Whiteflies Are Not Flies By James A. Bethke, Heather S. Costa and Richard A. Redak

But they can be a big problem. New research shows the best control methods.

No, whiteflies are not flies. Flies like the typical houseflybelong to the order Diptera, which has only two wings. Whiteflies are in theorder Homoptera and have four wings. The order Homoptera also include cicadas,planthoppers, treehoppers, leafhoppers, sharpshooters, spittlebugs, psyllids,scale insects, mealybugs and aphids.

There are over 1,200 species of whiteflies identifiedworldwide; however, only a few of them are classified as pest species. Newlyintroduced species of whitefly can quickly become pests and may adapt to newhost plants and environmental conditions. For example, one year after thesilverleaf whitefly reached greenhouses in California, it replaced thegreenhouse whitefly as the dominant species on poinsettias.

Monitoring for Whiteflies

Any whitefly management programwill require the use of monitoring practices to be effective. Accurate recordsof pest numbers, crop damage and changes in whitefly numbers will allow thedevelopment of treatment thresholds for specific crops and can help to timepesticide applications. Yellow sticky cards are an effective method formonitoring adult whiteflies. One trap per 1,000 sq. ft. of growing area isrecommended. Place traps a few inches above the plant canopy, and move them upas plants grow. Check the traps weekly, and keep a good record of trap counts.Recognizing hotspots and treating them early may reduce the overall need forpesticide applications. Because traps only capture winged adults, it isimportant to also monitor plants. For more information about monitoring, seepage 108.

Production controls

Since whitefly infestations caneasily go unnoticed until they reach high numbers, it is important to minimizepotential infestations by employing all possible control tactics prior tochemical control. Begin the cropping cycle with pest-free plants, and excludethe pest, if possible, with exclusion screens. Many weeds are good hosts forwhiteflies. Therefore it is important to keep the growing area clean and freeof weeds. Biologicals are also a good control method for whiteflies; see thesidebar at the right forinformation about biologicals.

All whitefly stages typicallyoccupy the undersides of leaves of infested plants. The more susceptible stagesare the younger nymphs. The later stages, such as the fourth instar and redeyestage, are more difficult to control. Therefore, the best control is achievedwhen a maximum effort is made to contact the undersides of leaves with repeatedspray applications (about every six days) targeting the earlier, susceptiblestages for a period of about three weeks. Heavier infestations may require moreapplications for a more extended period of time.

An effort should be made to holdwhitefly populations in check early in the cropping cycle. Contactinsecticides, including soaps and oils, may be more effective early in thegrowth cycle when the foliage is less dense and contact with young nymphs ismore likely. Later in the cropping cycle, when dense foliage is present, asystemically acting material may be more efficient in reaching the insects.

Chemical controls

Figures 1-4, pages 58-62, aresummaries of recent whitefly trials conducted at the University ofCalifornia-Riverside. Some of Á the pesticides are registered for use onthe intended target; others are experimental. We do not always use labeled ratesin our trials because these trials are for experimental purposes — toincrease our knowledge about the products and their capabilities. Labelsconstantly change; therefore, it is always the pesticide applicator’sresponsibility to follow all label directions. No endorsement is intended forproducts mentioned, nor is criticism meant for products mistakenly omitted.

There are a relatively largenumber of insecticides available for chemical control of whiteflies, providingmany options for rotation (see Table 1, page 59). In addition to Marathon(imidacloprid), several new chloronicotinoids like acetamiprid (Aventis), Forte(a new formulation of imidacloprid, Bayer), thiamethoxam (Syngenta) andthiacloprid (Bayer), have been registered recently or are in the developmentphase. All of these materials are very effective against whiteflies. Inaddition, a new Novaluron from Crompton/Uniroyal (Pedestal) looks verypromising (see Figures 3-4, pages 60-62).

Whiteflies typically haveseveral generations each year, especially in a greenhouse environment. Theytake about 21-25 days to develop to an adult, depending on temperature. Becausethe nymphs are on the plants for a long time and passing through many molts,whiteflies are susceptible to pesticides that act as IGRs like Distance(pyriproxifen, Valent USA) and Pedistal (novaluron, Crompton/Uni-royal). Foroptimal control on a long-term crop, pesticide class should be rotated everygeneration or two, or about every 4-6 weeks.

Although many effective Ástrategies are available to manage most whitefly pests, the relatively recentintroduction of several new whitefly pest species and whitefly-transmittedpathogens emphasizes the need to constantly be on the alert for the spread ofthese pests or the establishment of new exotic species.

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James A. Bethke, Heather S. Costa and Richard A. Redak

Jim Bethke is a research associate in the Department of Entomology; Heather S. Costa is Floriculture Extensions Specialist; and Richard A. Redak is Professor of Entomology at the University of California, Riverside, Calif. They can be reached by phone at (909) 787-4733 or E-mail at [email protected]

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