Aphid Control

- Aphids do a lot more damage than you think! -

Summary: Aphids, sometimes called greenfly, are small insects that suck plant juices from stems and leaves and can severely stunt or even kill their host plants. Early detection and control with insecticidal soap are the keys to managing aphids in greenhouses, house plants, gardens, and landscapes.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

Identification of aphids

Aphids are small, soft-bodied insects that feed exclusively on plants. These small insects can be nearly colorless to green, yellow, pink and even black in color. They can form dense colonies on leaves and stems (right) sometimes completely obscuring the plant surface. Most aphids are wingless but a few individuals in a colony may have wings.

Aphids occur on nearly all plants, their populations can build quickly and their effects can be severe. Aphids feed by inserting tube-like mouth-parts into the plant's vascular system and withdrawing plant sap. Plant sap is a complex mixture of water, sugar and a small amount of protein.

To get enough of the protein that they need aphids must take in a lot more sugar (plant sap) than they can use -- excess sugar is expelled through the body as a concentrated sugar solution called honeydew. This honeydew may drip from the plant and attract sugar-loving insects like ants and wasps.

aphids on a stem, notice single aphid above and a colony below; photo by Elizabeth DeAngelis

While aphids feed on plant sap, they may inject their host plant with toxins that deform plant tissue. This is why plants are often twisted and deformed above where aphids are feeding. Aphids can be found on both above-ground stems, buds and leaves and below-ground roots and rhizomes.

common misspellings and misnomers for aphids: plantlice, plant lice, greenfly, green fly, green bug, aphis

Natural aphid control in gardens

Aphids have a wide range of natural enemies. Predators like syrphid fly larvae, lady beetle adults and larvae, and lacewings as well as parasitic wasps, and others, are important in aphid control. The typical cycle is for an aphid colony to build rapidly in spring followed by a rapid decline in summer after the colony is discovered by a predator. Sometimes more than one type of natural enemy will "work" a colony at the same time.

Aphids are important pests in gardens, greenhouses as well as in commercial agriculture. Accurate identification of pest species is both difficult and extremely important a natural control program and therefore has gotten considerable research attention. The following is an excellent in-depth resource: Aphids on the World's Crops: An Identification and Information Guide, 2nd Edition (2000)by R. L. Blackman, V. F. Eastop. (more information).

Because aphids are somewhat host plant specific these pests rarely spread from one type of plant to a completely unrelated species. It is unlikely that widely different crops will be affected by the same aphid species. The more usual situation is that certain crops will consistently have aphid infestations while others seem to remain relatively free of these pests.

Chemical & physical control of aphids

For small greenhouses and home gardeners the best chemical control is insecticidal soap. The guidelines for controlling aphids are the same as those for spider mites, using strong overhead watering and insecticidal soap. See Using Insecticidal Soap and Spider Mite Control for details about this method.

The best physical control is the use of row covers (see Related Articles below) early in the season where they are practical. Row covers are made of a thin, strong, non-woven fabric that allows light, water and air to get in but excludes flying pests like aphids, leaf-feeding beetles and thrips. They work great in greenhouses and over garden rows. Place row covers, loosely, early in the season before aphids and other pests get started.

Aphid control on indoor plants

Plants growing indoors, both house plants and greenhouse-grown plants, often are plagued by aphids because they are physically separated from their natural enemies which normally keep their numbers in check. Lacking any natural enemies aphid populations can grow rapidly and even kill plants. Once you find aphids, or detect their activity by the sticky honeydew they produce, you should take quick action.

First, rinse plants with a spray of water. Large plants can be taken outdoors, in summer, and rinsed with a garden hose. Once thoroughly rinsed to remove as many aphids as possible, spray thoroughly with a 1-2% solution of insecticidal soap (see links above and below). Use genuine insecticidal soap not household soaps as these can burn plants. After 30 minutes, rinse plants again to remove soap residues. You may need to repeat this procedure several times a year. See Aphids and Spider Mites On House Plants for more information.

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