Aphids: Safe and Successful Control

David Johnson is former director of grounds maintenance at Children’s Hospital and Medical Center in Seattle. For fifteen years he used integrated pest management techniques to manage their extensive plant collection.

Aphids are probably the most recognized garden insect in existence. More than 4,000 species have been described, and it can almost be said that there is an aphid species for every house and garden plant. Yet, there are a number of strategies that can keep aphid damage at a minimum without resorting to toxic chemicals, which may pose a danger to pets and family members, as well as to the aphids’ natural enemies.

The physical appearance of aphids is variable. They come in a rainbow of colors: black, brown, red, purple, pink, green, and yellow. Some have wings and some do not. Most have naked bodies, but some may excrete a protective cottony substance over their bodies. What they have in common is a soft body about 1/10 of an inch long with a mouth part adapted for extracting plant juices.

Most aphids are selective in the plants they attack, but about 10 percent feed on a variety of plants. Some aphid species do relatively little damage to the host plants, while others may present a serious threat if allowed to multiply unchecked. Aphid populations vary widely throughout the year in response to seasonal factors and the attack of natural predators. For these reasons, it is difficult to generalize about the need for intervention in controlling aphids.

A generalized life cycle begins in the spring with the hatching of overwintered eggs just as the new growth on plants is developing. These aphids are all wingless females, which without fertilization bear succeeding generations of as many as 100 more live, wingless females.

Incredibly, the aphids are actually born pregnant; even before birth, the female’s own daughters are developing within her. This strategy of bearing live young without fertilization accounts for the extreme rapidity with which aphid populations can grow, making control difficult. Young plant shoots can quickly be completely covered with a teeming colony of aphids.

As the colony grows, it may begin to run out of room or food may become limited. When this occurs the aphids adapt by producing winged females which can colonize surrounding plants. Late in the fall, sexual males and females are produced. Some aphids have wings and others do not.

After mating the female lays one or more overwintering eggs. Protected in plant crevices, these eggs withstand inclement weather to hatch in the spring, repeating the cycle.

Aphids can feed on all plant parts, but most prefer young shoots and leaves. They feed by inserting their soda-straw-like mouth into the plant tissue and sucking out the nutrient-rich juice. Aphids become a pest when their feeding begins to affect plant health. A plant may gradually weaken as fewer nutrients become available to it. Aphids can also cause stunting and distortion of plant parts, further weakening the plant.

Most aphids excrete a sweet, sticky substance called honeydew as they feed. This sugary protein mixture can coat the plant and everything under it. Honeydew serves as a food source for ants, bees, and flies. Some of the insects attracted to the honeydew prey on the aphids, but others—ants, for example—attack the beneficial predators. For this reason, controlling ants is an important part of aphid reduction.

Aphids often go unnoticed in the landscape until a tree begins dripping honeydew onto the sidewalk, roof, deck, and parked cars. The health of the plant may not be threatened by large aphid populations, but the honeydew is such a nuisance that controls become necessary.

Were it not for the natural controls that limit aphid population growth, we would soon be overwhelmed by them. Their major predators are green lacewings, ladybird beetles, and syrphid flies. These predators may be observed feasting on aphid colonies. Lacewings are noticeably larger than winged aphids, so you should be able to tell them apart.

Another effective (and fascinating) predator is the braconid wasp, which lays an egg inside the aphid. The egg hatches and the larval wasp eats its way out leaving only the exoskeleton of the aphid. As it emerges, the wasp leaves a small round hole on the aphid’s abdomen.

These holes may be seen with the naked eye and serve as a reliable indicator that the aphids are being controlled naturally. In fact, what may appear at first glance to be a heavy infestation might upon closer examination turn out to be a colony dominated by empty aphid skeletons.

In the majority of landscapes, natural controls do an adequate job of keeping aphid populations in check. Since predator populations lag somewhat in time behind aphid populations, there may be periods in the year, particularly in the spring, when the aphids appear to be out of control, but often the predators can catch up and restore control.

In landscapes that receive a series of pesticide applications, it is often found that aphid problems actually increase. This occurs because aphids can reproduce very rapidly following a pesticide application, whereas the predators respond much more slowly.

Aphids will always be present in the landscape. Eradication is not a realistic option. However, by selecting resistant plant material, maintaining plant vigor, and applying certain controls to reduce population levels, it is a pest we can easily live with. The following aphid control strategies should be adequate to deal with most problems.

Avoid designing pest problems into the landscape. The most effective treatment for aphids or other pests begins in the design stage. The basic rule is simple: don’t use plants that attract pests in areas where the pest can’t be tolerated. Birches are notorious feeding stations for aphids. If you simply must have a birch in your landscape, do not plant it over the deck or next to the driveway. Plant it out back where you won’t notice the aphids or, better yet, substitute a tree that has the same qualities but does not attract aphids.

Plants that receive levels of nutrients, water, and light adequate for the species will usually thrive. Improper levels produce stress on the plants. The presence of excessive aphids may indicate an unhealthy plant. Check first to be sure you are providing the correct growing conditions. Surprisingly, an excess of nitrogen can stress plants—particularly indoors—and result in aphid attack. Use slow-release fertilizer in appropriate amounts to maintain slow, steady growth of houseplants. Small, frequent feedings are pref-erable to widely spaced, more concentrated doses.

The simplest control method is physical removal of aphids. This can be accomplished most easily by a strong spray of water from a garden hose, repeated periodically when the aphids return. Particularly infested plant parts may have to be cut off and discarded.

Another method is to use aphid traps, colored panels covered with a sticky substance, which attract and bind aphids to them. If ants are a problem—they kill beneficial insects—build a sticky barrier between the plants and the pests.

Aphid predators may be introduced into the landscape to supplement existing populations. The most common predator sold is the ladybird beetle. Unfortunately, these beetles have a tendency to disperse when released. Their presence will benefit the general area but probably won’t do much to affect the aphid population in the immediate vicinity.

A better choice for introducing predators is the green lacewing. These are available both as eggs and as larvae. The larvae are more expensive and harder to obtain, but they are more effective at aphid suppression.

Do not expect introduced predators to be able to overcome already damaging levels of aphids. Before introducing any predators, reduce aphid numbers by pinching off severely affected plant parts or hosing off most of the aphids. A single predator release may not be sufficient.

The least-toxic chemical for aphid control is an insecticidal soap. This should be applied carefully, according to label directions, only to the affected plant areas. It will kill aphids on contact, but does not provide any lasting preventative effect, so applications will probably have to be repeated. It is effective in bringing aphid numbers down so that natural predators can regain control.

For a cheaper alternative, try mixing several tablespoons of a simple liquid soap with a gallon of water and squirt with your plant sprayer. On food crops, be sure to rinse thoroughly with water before eating.

On vegetable crops, the appearance of aphids necessitates a quick response. On vegetables with smooth leaves, such as cabbage family plants, a water spray should do the job. Pole beans, which have sticky leaves, may require a soapy water spray.

This is an abridged version of a more extensive fact sheet available in print form from Washington Toxics Coalition for $1.50 plus .50 postage and .13 tax (2.13 total). For more information, visit their web site (http://www.watoxics.org.) Print versions can be ordered from Washington Toxics Coalition either over the Internet or by calling 800-844-SAFE.