A bit about aphids: Protect your garden

By Joan Allen - UConn Home & Garden Education Center
Featured Article - posted Thu., Jun. 20, 2013
Photo by Deb Tyser, UConn Soil Nutrient Analysis Lab. - Contributed Photo

Anyone who grows plants of any kind, whether it’s houseplants, trees, perennials, herbs or vegetables and fruits, probably has a little bit of familiarity with aphids. There are more than 4,000 aphid species, and more than 250 of them are important plant pests. Some aphids feed only on one specific host plant, while others are happy to dine on hundreds of different plants.

Aphids have tiny, pear-shaped bodies with long, delicate antennae and legs. Most range in size from 1/16 to 1/8 inch in length. Colors include the common yellow to green shades and more striking colors including reds, browns and black. Aphids that feed on various host plants may take on the color of the sap of the plant they are feeding on, so some species will be found in an assortment of colors. A pair of tube-like cornicles is located on the upper rear area of the abdomen. The color and length of the cornicles can aid in identification. A few aphids produce a whitish, waxy exudates that forms a coating on their backs, giving them a fuzzy or waxy appearance. These are called woolly aphids. For most garden aphid problems, the identification of the aphid species is not necessary to select effective control measures when needed.

Aphids feed on plants by piercing cells and ingesting sap through a hollow stylet-type mouth part. Plant sap is high in sugars, and as the sap passes through the aphid’s digestive tract, some of the sugars are removed and used by the aphid. Quite a bit of sugar remains in the sap, and this is excreted as a liquid known as honeydew. Honeydew is attractive food for some ants. Ants may be seen where there is an aphid population, and they will protect the ants from predators. Where aphids are numerous, honeydew can accumulate on any surface below them, including leaves, cars, patios, etc., and create a nuisance. It is sticky and may be colonized by sooty mold fungi that give the surface a black coating.

But do aphids live on sugar alone? No. All animals also need amino acids, the building blocks of proteins. A bacterial resident inside special cells within aphids converts glutamate, a metabolic waste product, into amino acids that can be used by the aphid.

What symptoms do aphids cause on their host plants? Most healthy plants can tolerate a small to medium amount of aphid feeding. When conditions allow a large population to build up, plant injury can occur. Symptoms caused by aphid feeding include yellowing, wilting, stunting and deformed growth. Some aphids inject a toxin into the host plant during feeding and this can lead to deformity. Curled leaves that result from this provide shelter to the aphids that congregate inside.

An important impact of aphids is that some species vector damaging plant viruses. In some cases, control of the aphid vector is one effective component of controlling the virus. In other cases, the characteristics of how the virus is harbored and transmitted by the aphid make vector control ineffective. For this reason, it is important to identify the virus and vector when this is suspected.

To protect your plants from aphids, monitor them early in the season so you will notice if populations begin to build up. Aphids are most active during the cooler weather of spring, early summer and fall. They can be found on stems, bark, leaves, flowers and even roots. Many prefer the undersides of leaves, so be sure to turn the leaves over and check for them there. When monitoring, also look for signs of parasites and predators of aphids. The naturally-occurring beneficial insects are often effective at keeping the aphid population in check. Predators include lady beetles and their larvae, and lacewing larvae. Parasites include species of wasps and flies. A parasitic wasp lays an egg inside the aphid. The larva consumes the insides of the aphid then emerges through a small hole that it makes in the aphids body. When monitoring, look for brown, swollen “mummies” of aphids with the tiny exit holes on their abdomens. Aphids have been noted kicking at the wasp to prevent egg-laying!

Large populations of aphids can be dislodged from the host plant using a strong spray from a garden hose. Most of the aphids will be injured or disoriented enough that they will not be able to find their way back to the plant. In some cases, a chemical product may be warranted. Insecticidal soaps and horticultural oils are the safest for beneficial insects. Broad spectrum chemical insecticides are more likely to kill the parasites and predators that should be encouraged and preserved. Whenever using pesticides, make sure the product is labeled for both the pest and the plant type that it is being applied for.

For more information on aphids and other garden and landscape pests, contact the UConn Home & Garden Education Center toll-free at 877-486-6271 or by e-mail at ladybug@uconn.edu.

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