Pest Control Library: Aphids

Articles→Pest Control Library

By National Gardening Association Editors

Aphids are found throughout the United States. These small, soft-bodied insects may be pale green, pink, black, or yellow, depending on the species. Some stages of the life cycle are winged, others wingless. Aphids feed on a wide variety of plants, including most edible and ornamental plants. Clustering on tips of new growth and leaf undersides, they suck plant juices causing leaves to become distorted and yellow.

Aphids like the low light levels and cool conditions of spring and fall. They will attack all plant parts but prefer young, succulent growth. There are many species of aphids, some named after the plants they attack, such as pea aphids and peach aphids. In general, all are small (1/16 to 1/4 inch long) and oval-shaped, and can be black, white, green, or pink. Although most aren't very mobile, some forms have wings. All reproduce quickly, and under the right conditions a small number can bloom into a major infestation in no time. By sucking plant juices from leaves and stems, they weaken the plant. More seriously, they can transmit virus diseases that gradually debilitate and kill some plants.

Aphids secrete a sugary fluid called honeydew that attracts ants and may cause the growth of a sooty black fungus on leaves. In small numbers aphids do little damage, but they reproduce rapidly. They can also spread diseases among plants.

Control of Aphids

The first line of defense is prevention. Check plants regularly. Avoid high-nitrogen fertilizers, which create young, succulent growth which these insects love. Isolate infested plants from others and control the pests aggressively.

Start by rinsing plants with a strong spray of water to reduce the population. If aphids return, spray with insecticidal soap or horticultural oil. When spraying, be sure to cover the undersides of leaves. A variety of natural insect parasites and predators also reduce aphid populations, which is the main reason to not be overly aggressive with sprays.

Another organic approach to dealing with aphids is by attracting their predators such as ladybugs, lacewings, or one of the best, the hover or syrphid fly. Adult hover flies resemble wasps and yellow jackets, but there's no need for you to be alarmed; they can't sting and are perfectly harmless. The wasplike adults feed primarily on flower nectar, but during their week-long stint as crawling larvae, each juvenile can consume hundreds of aphids.

Researchers at Oregon State University tested many plants to determine which attract hover flies. The winner was sweet alyssum (Lobularia maritima), which is easy to grow from seed or nursery plants, and prefers the cool temperatures of spring and fall. Runners-up include cilantro (Coriandrum sativum), buckwheat (Polygonum fagopyrum), common fennel (Foeniculum vulgare), and yarrow (Achillea).

For best results, keep the plants in flower with successive plantings and regular deadheading.