I have been seeing large aphid populations on crape myrtles. Aphids are soft-bodied, slow-moving insects that reproduce rapidly. Their color can vary from green to brown to red to black. They may be winged or wingless, but wingless forms are most common in Oklahoma. They have piercing-sucking mouthparts and feed by sucking sap from plant tissues.
The life cycle of a typical aphid species may produce several wingless generations in the spring, followed by a generation of winged forms. The winged forms can fly to other plants where many more wingless summer generations can be produced. Aphid populations can increase rapidly in extremely short periods of time. During warm weather some species can complete a generation in less than two weeks.
Many aphids prefer to feed on young, succulent growth. Some feed in sheltered locations, such as inside leaves that they have caused to curl or become distorted. Aphids attack trees and shrubs of all kinds, but do not usually seriously injure them. New plant growth may become distorted or stunted before predators and parasites decimate the aphid population. Aphids and other plant sap sucking insects excrete large amounts of honeydew, a sticky substance often seen on leaves, pavement, automobiles and other surfaces below the infested foliage. Honeydew consists mainly of excess sugars ingested by the insects and passed through the body. Ants are often attracted to the sugary honeydew and occasionally tend the aphids much like people tend cattle: some ants even carry the aphids to new plant parts to establish more colonies.
Sometimes a black fungus called sooty mold will grow on the honeydew deposited on foliage below aphid colonies. This fungus can detract from the plant's appearance and reduce the amount of light reaching the leaves, thus reducing photosynthesis.
Aphids are present on most plants, generally at noninjurious levels. The honeydew excreted by aphids may be useful for identifying aphid populations. Aphids are often controlled by natural forces, such as driving rains or low or high temperatures. A good recommendation for the control of aphids is to wash the plant with a strong stream of water.
Natural controls, including natural enemies — ladybird beetles, lacewings, syrphid flies, and parasitic wasps — usually bring aphid populations under control shortly after they become noticeable.
Residents should search the aphid colonies for these natural enemies. High numbers of these beneficial insects usually indicate that aphid problems are being controlled without intervention. Do not apply chemical insecticides on plants to control aphids, since these will also kill off the beneficial insects and the aphid population will come back with a vengeance.
Ray Ridlen is an agriculture/horticulture educator for the Oklahoma County Extension Service. His column addresses frequently asked horticulture questions. For more information, call 713-1125.