Whiteflies are being commonly found on flowers and other herbaceous plants in Illinois. They are 1/16-inch-long insects with white, powdery wings. They sit on leaf undersides and fly off of infested foliage when disturbed. They are sucking insects, whose removal of sap can cause leaf distortion.
Three species of whitefly are common in Illinois. Perhaps most common is the bandedwinged whitefly. The adult has two dark bands on each front wing. It is hardy in Illinois, feeding primarily on velvetleaf, also known as buttonweed. This weed is common in agricultural and weedy land areas. Huge numbers of adults migrate to other plants, even flying in large numbers into greenhouses. Bandedwinged whitefly feed as adults on many plant species but does not reproduce heavily on them. Thus, bandedwinged whiteflies usually do not warrant control. An exception is attacks on flowering maple, Abutilon, which is the same genus as velvetleaf.
Banded whitefly nymphs and adult on velvetleaf.
Silverleaf whitefly adults on hibiscus.
The other two common species are greenhouse whitefly and silverleaf whitefly. Silverleaf whitefly used to be known as sweet potato whitefly. They do not have dark bands on their front wings, being totally white. Neither of these species is able to survive Illinois winters. They are common pests in greenhouses and are apparently moved outdoors each spring on bedding plants. These whiteflies can build up in numbers through multiple generations through the growing season to be common on plants in late summer and fall. They reproduce on many plants and can increase enough to cause damage.
Damage appears as wrinkled, curled, cupped, or otherwise distorted leaves. Large amounts of honeydew, excreted from the nymphs and adults, make leaves glossy and sticky. Infestations may be first noticed by sidewalks and objects below infested plants becoming sticky from the honeydew. Black sooty mold grows on the honeydew and is another clue that an infestation is present. Sooty mold can reduce the amount of light reaching the leaf surface, resulting in reduced photosynthesis.
Another determination that greenhouse or silverleaf whiteflies are present is to look for nymphs and pupae. Insects with incomplete development go through egg, nymph, and adult life stages; and complete-development insects have egg, larva, pupa, and adult life stages. Whiteflies, along with a few other insects, are thought to be somewhat transitional between incomplete and complete development. They are considered to have incomplete life cycles, but the last nymphal stage is called a pupa.
Whitefly nymphs and pupae appear as oval, clear to light yellow insects on the leaf undersides. Legs are not apparent. The pupae and older nymphs are about 1/16 of an inch long, 1 to 2 millimeters. They are most common on the older, lower leaves of the plant. Their small size and transparent qualities make them difficult to see. Adults and their eggs are most common on the younger, upper leaves of the plant.
Control of whiteflies is usually not necessary unless nymphs are present. Thorough coverage with sprays of insecticidal soap, summer spray oil, or pyrethroids should be effective. Apply weekly, two or three times. Imidacloprid (Merit) applied to the soil is also effective systemically on whiteflies.