Large numbers of adult whiteflies have been reported outdoors in the Champaign-Urbana and Bloomington areas. At this time of year, it is common to have adult banded-winged whiteflies in large numbers near rural areas of Illinois. However, this year many whiteflies are not that species.
Whiteflies are tiny, sap-sucking pests most commonly associated in Illinois with indoor plants. Adult whiteflies get their name from a white powder on their wings and body. They are 1/16-inch long or less, with wide, mothlike wings. Brushing the foliage of infested plants causes many adults to fly up into the air. On the leaves of host plants, adults lay eggs that hatch into oval, somewhat transparent, colorless to yellowish larvae. They transform into oval pupae before the adults emerge. Fully grown larvae and pupae are similar in size to the adults. Both the larvae and pupae are located mainly on leaf undersides, where larvae feed by sucking sap. Heavily infested leaves yellow prematurely and may curl at the edges, with a slightly cupped appearance. High populations over the entire plant result in reduced growth, premature leaf loss, and death of the plant.
Banded-winged whitefly adults are identifiable by wide, gray bands on their wings. In Illinois, these insects feed mainly on velvetleaf, a common weed in field crops; and each velvetleaf plant produces large numbers of banded-winged whiteflies by late summer. As these populations become very large, the velvet-leaf plants have produced seed and are declining, which reduces this whitefly's food supply. It is com-mon to find adult whiteflies on many kinds of plants. Fortunately, the larvae are restricted to velvetleaf and closely related plants, so serious injury to landscapes or greenhouse crops does not result.
This year, however, the whiteflies we are seeing do not have bands on their wings and by general appearance seem to be silverleaf whiteflies. Identification of silverleaf whitefly is based on pupal characteristics, so accurate identification of these adults is not practical. Silverleaf whiteflies are not known to overwinter in Illinois but may migrate from more southern states. They also have a high reproduction potential, making it likely that the currently high number of whiteflies is due to season-long migration and reproduction.
Greenhouse growers should be particularly watchful for movement into poinsettia and other crops. Sticky traps near vents, as well as plant inspection, should determine whether these whiteflies have invaded from outdoor populations. One may need to treat poinsettias for whiteflies earlier than usual to avoid damage. Imidacloprid, sold as Marathon, should be effective. However, treatment this early can make residual protection from whiteflies and other pests continuing through bract formation more tenuous. Outdoor ornamental plants are unlikely to be seriously harmed by whitefly infestation this late in the growing season. Treatment outdoors is probably not warranted.