Garden & Landscape Tips

Aphids, or plant lice, are pear-shaped, soft-body insects, ranging from 1 to 10 mm (0.039 to .39 inch)in length. They have long antennas and legs, but usually are wingless. Sometimes they look woolly and sometimes they have a waxy appearance. They may be green, yellow, brown, red, black or other colors. Although tiny, they can usually be seen on the undersides of leaves, along stems or on flowers. They usually do not move or they move very slowly. Aphids can be plant specific or they may be generalists. They usually rely on the wind to move them about.

Woolly apple aphids
Woolly apple aphids
Photo courtesy Gail Ruhl, Purdue University

Aphids are opportunistic, attacking plants that are stressed in some way. Stressed plants emit a chemical signal, which alerts aphids. Stress may be caused by too much nitrogen fertilizer used to encourage green, tender growth, which is what aphids love. Aphids may attack plants that get too much or not enough water or sun, or are weak because of transplant shock. Aphids also may attack plants that have been severely pruned, which encourages new, tender growth.

An overuse of pesticides contributes to the problem, too, because they take a toll on natural predators, such as lady beetles, which means there’s little to keep aphids in check. Healthy plants have the best chances of fending off an aphid attack and suffering the least damage.

Woolly apple galls
Photo courtesy Gail Ruhl, Purdue University

Low to moderate numbers of aphids do not cause major damage on most plants. If there is a large infestation, you’ll see curling on leaves or flowers,or the plants will turn yellow or become stunted. Another sign is the presence of a sticky substance, called honeydew, on the plants. Ants love honeydew, so the presence of ants usually indicates an aphid problem. Ants’ love of honeydew will prompt them to ward off aphids’ natural predators. Ants may even move aphids out of danger to protect the honeydew production. Sometimes aphids form galls, an abnormal growth of plant tissue. Aphids also may introduce a virus in a plant or promote a disease, such as sooty mold.

A lady beetle larva is a natural control of aphids.
Photo courtesy Purdue University

What to do?

The first line of defense against aphids is to knock them off the plant with a strong spray from the garden hose. Be sure to spray the undersides of leaves, too. Do this every few days until you no longer see aphids on the plant.

The second line of defense is insecticidal soap, a natural product made with plant-based insecticides and soap. However, this insecticide and most others do not discriminate between the aphids and desirable insects. Aphids have natural predators, including the larvae of lady beetles (see photo). These good bugs control aphids naturally, without any effort on your part.

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