Aphid Control

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

What are aphids?

Aphids are small, soft-bodied insects that feed exclusively on plants. The can be nearly colorless to green, yellow, pink and even black. They 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 mouthparts 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 protein aphids must take in a lot more sugar 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.

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

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 as well as in commercial agriculture. Accurate identification of pest species is both difficult and extremely important and therefore has gotten considerable research attention.

Chemical & physical control of aphids

For 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 house plants

Plants growing indoors, both houseplants and greenhouse plants, often are plagued by aphids because they are physically separated from their outdoor natural enemies. Lacking any natural enemies aphid populations can grow rapidly and even kill house plants. Once you find aphids on your house plants, or detect their activity by the sticky honeydew they produce, you should take quick action.

First, rinse plants with a spray of water from the kitchen sink sprayer. 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.

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Jack DeAngelis, Ph.D.

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