Description
Whiteflies are tiny white 4-winged insects that are closely related to aphids, scales, and mealybugs. Adults are easily observed fluttering about when disturbed. Eggs are tiny, spindle-shaped, and laid vertically on undersides of leaves. Eggs may be single or arranged in a crescent. First stage nymphs are “crawlers” that move a short distance and settle. Three immobile nymphal stages follow; during this period the whitefly loses its legs and antennae and is called a scale. Scales are oval, flattened, and translucent The final part of the immobile period is the pupal stage. Pupae of the two common whitefly pests are most reliably distinguished.
Economic Effects
Whiteflies infest a wide variety of plant species, including the ornamentals poinsettias, begonias, coleus, fuchsias, primulas, salvia, and verbena, and vegetables such as cucurbits, beans, tomatoes and cole crops. Whiteflies suck sap from plants and excrete a sticky exudate called honeydew, which can support the growth of a sooty mold fungus. Whitefly feeding may result in stunting, wilting, and/or yellowing of plants, defoliation, reduced yields, even plant death. They reproduce rapidly in a favorable environment in the absence of natural predators and biocontrols.
Identifying
| The pupal stage of greenhouse whitefly (GHWF) and silverleaf whitefly (SWF) are found on leaf undersides. GHWF pupae are shaped like a disk or cake, and have a fringe of short hairs on the rim. SWF pupae are rounded or shaped like a dome, and are not fringed with hairs on the edges. Viewed from above, GHWF pupae usually show longer hairs protruding from the pupal body, but this may vary and is not a defining characteristic. GHWF are white; SWF, yellowish. GHWF adults are larger, and their wings are held fairly parallel to the surface; the SWF adult folds its wings at a 45° angle, tightly to its body. A hand lens of at least 10x magnification is needed for positive identification of the species. |  |
Crop Management
In order to minimize introduction of whiteflies into the crop environment, sanitation and prevention are essential. Reduce or eliminate broadleaf weeds outside the greenhouse, which serve as whitefly hosts. Screening vents properly helps to exclude whiteflies and other pests, and also helps to keep beneficial insects indoors. Don’t wear white or yellow clothing, as whiteflies are attracted to these colors. A rest period for greenhouses when no crop is grown is used to eliminate whiteflies before starting a new crop. Don’t forget to consider “pet plants” or propagation material as possible whitefly hosts. Vacuuming plants early in the day before whiteflies are active may be effective to reduce numbers of adults. Inspect all incoming stock and treat if needed.
Pesticide compatibility:Chemical controls may help to reduce whitefly numbers before introducing beneficials, but use materials that are least toxic such as insecticidal soap, horticultural oil. Neem products, or insect growth regulators (IGR’s). Dr. Hoddle observed compatilibity of Eretmocerus with IGR’s to be highest with buprofezin and ranked compatibility as follows: buprofezin>fenoxycarb>pymetrozine>pyriproxifen. Hoddle and Van Driesche report that kinoprene use reduces effectiveness of parasitic wasps.
Pest Monitoring
Use yellow sticky cards to monitor numbers of adults. Use a minimum of 1 trap per 1000 sq. ft at the height of the crop canopy and count numbers weekly or biweekly. Look for adults near plant tops or ends of branches. Check leaf undersides for eggs, scales, and adults. Adults and eggs are more often found on younger leaves; scales on older leaves. Scales are difficult to see because they are translucent unless parasitized. If eggs and scales are found, return to observe hatch of crawlers from those plants. Crawlers are susceptible to chemical controls; eggs and scales are not.
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