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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
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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.
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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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