Aphid Numbers and Damage Soar with the Heat
Pear-Shaped Insects Infest Leaves, Causing Distortion, Discoloration
Jul 12, 2009
"Almost every plant has one or more species of aphid that will feed on it,’’ said Howard Russell, an entomologist at Michigan State University.
Aphids may feed on a single type of plant while others feed on a wide range of plants. When temperatures rise, so do aphid numbers.
Know the Enemy
Aphids are small, soft-bodied insects with long, slender mouth parts that they use to pierce stems, leaves and other plant parts and suck out fluids. There are 1,400 species of aphids and their close relatives in the U.S. and Canada. The small, slow-moving pear-shaped insects come in a variety of colors, including red, green, black and yellow. They are prolific breeders and produce many generations a year.
“When the weather is warm, many species of aphids can develop from newborn nymph to reproducing adult in seven to eight days,’’ Russell said. “Because each adult aphid can produce up to 80 offspring in a matter of a week, aphid populations can increase quickly.’’
Warning Signs
Heavy aphid feeding causes curling, yellowing and distortion of leaves and stunting of shoots. They excrete a by-product called honeydew, which may cause leaves and branch stems to glisten, said Jill O’Donnell, a conifer expert at MSU.
“Some of the common conifer-feeding aphids produce lots of honeydew, which leads to black sooty mold,’’ she said.
Ants can indicate aphid populations. Ants, dependent on honeydew aphids produce, protect them from predators, O’Donnell explained.
Another sign of aphids may be the appearance of galls, an unusual swelling of plant tissue due to feeding. It is also possible for aphids to transmit plant diseases.
Controlling Aphids Naturally
Aphids can be controlled by hitting them with a blast of water from the hose or through natural predators such as lady beetles.
Lady beetle larvae become more abundant in large response to their aphid prey, Russell said.
“Usually aphids are kept under control by a host of natural enemies’’ including small parasitic wasps that deposit their eggs inside the aphids, said MSU Entomologist Dave Smitley.
Chemical Collateral Damage
Well-meaning gardeners who treat aphid-infested plants with insecticides may do more harm than good, Smitley said.
“If an insecticide was used about four to six weeks earlier, it may have eliminated most of the natural enemies,’’ Smitley said. “Because aphids reproduce rapidly, populations may grow fast after the natural enemies are gone.’’
Soapy Solution
Insecticidal soap is recommended for aphid control. A one percent solution of insecticidal soap usually does the trick, but Smitley cautions gardeners not to exceed a product's recommended dilution rate “because the soap can injure plant leaves, especially flowers.’’
“Most host plants can tolerate low to moderate numbers of leaf-feeding aphids,’’ Russell said. “Although aphids seldom kill a mature plant, the damage and unsightly honeydew they generate sometimes warrant control measures.’’
