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Wednesday, Aug. 15, 2007 01:07 pm

When aphids attack

Usually, the pesky little critters only inflict cosmetic damage

Untitled Document I park my car under the canopy of a large bur oak tree. For the past several weeks my car has been covered in a sticky substance: honeydew. Aphids are sucking insects that feed on plants by thrusting their long beaks into plant tissue and withdrawing sap from the plant. Aphids excrete large amounts of sugary honeydew, leaving the plant and everything it falls on — patio furniture, sidewalks, automobiles — sticky.
Aphids — slow-moving, pear-shaped insects that range from 1/16 to 1/8 inch long — may be green, black, brown, red, pink, or another color, depending on the sap of the host plant. They have long, thin legs, long antennae, and a pair of tubelike structures on the end of the abdomen. Various sizes of wingless aphids may be found in dense colonies on stems, the undersides of leaves, and flowers. Heavily infested leaves of plants may be distorted — twisted or curled — or turn yellow as the aphids remove the sap, and a fungus called sooty mold often grows on honeydew deposits, blackening the stems and leaves. Most aphid attacks inflict only temporary aesthetic damage. An established plant, growing vigorously, can usually tolerate a small population of aphids; newly transplanted or otherwise stressed plants are more vulnerable to damage. The life cycle of the aphid is complex. These tiny insects are masters of reproduction. Females give birth to live young without fertilization. The young nymphs start feeding immediately and mature in seven to 10 days, after which they are ready to produce live young of their own. Each female is capable of producing 40 to 60 offspring, generally female. Several generations can result in a population explosion. Early-season generations are generally wingless; later generations may have wings. Winged aphids are able to migrate to new host plants. In the fall, aphids switch from asexual reproduction to sexual. Gardeners have several options for dealing with aphids. The first is to let nature take its course and encourage natural predators such as lady beetles, lacewing, aphis lions, and parasitic wasps to dispose of them. Late in the season, aphids can usually be controlled effectively by such predators. You can also knock aphids off plants with a steady, forceful stream of water. When a population becomes too large on a new or stressed plant, consider spraying the foliage with insecticidal soap, malathion, acephate, or imidacloprid, but be sure to check first for natural predators that may already be taking care of the aphid population. Although the aphids are probably not stressing my oak tree, I will not miss the chore of washing my car window each morning. For more information, go to the University of Illinois Integrated Pest Management Web site’s page on aphids: insects/aphids/index.html.
Jennifer Fishburn is a horticulture educator with the University of Illinois Extension Sangamon Menard Unit. Contact her at 
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