Aphids have a complex life cycle that sometimes requires two or more different plant species to complete.Aphids usually overwinter as eggs hidden in bark crevices or near buds. Within a week of hatching, aphids, without mating, give birth to live young (nymphs). Aphids produce many young, each of which is capable of reproduction in about a week. As a result, aphid populations can build to enormous levels in a short period of time. Periodically, winged aphids develop and fly to other locations to avoid crowding or to mate. Aphid populations can vary greatly in size through time. A large population one year does not necessarily mean there will be an equally large or larger population the following season.

Aphids suck plant juices through a fine, needle-like stylet and feed on many plant parts. Damage from feeding is quite variable, ranging from no apparent damage to off-color foliage, twisted and curled leaves, gall formation, poor plant growth, and branch dieback. Most healthy, mature trees and shrubs are able to tolerate aphid feeding, even when there are many feeding individuals. However, large aphid populations can significantly weaken recently transplanted or stressed plants.

While aphids often go unnoticed, their waste, called honeydew, is more conspicuous. Honeydew is often described as a "clear, sticky liquid raining from trees." It coats bark, leaves, and objects beneath the plant. Honeydew is easily seen on car windshields and lawn furniture, and can be difficult to remove (see Yard and Garden Brief Removing Sap and Honeydew from Cars, H454R). An unsightly black fungus called sooty mold sometimes grows on honeydew. Sooty mold does not damage the plant, but heavily infected leaves can have a lowered photosynthetic rate. Sooty mold is never treated with chemicals; eliminating honeydew is the best way to control it (see Yard and Garden Brief Sooty Mold, P440S). Honeydew also attracts sweet-feeding ants, that will eat the honeydew and sometimes protect the aphids to maintain the food source. The presence of ants on woody plants can indicate the presence of aphids.

There are several options for aphid management. The first option for aphid management is tolerance, since aphid populations are naturally regulated in many ways. Natural enemies, such as lady beetles, green lacewings, damsel bugs, syrphid flies, parasitic wasps, and certain fungi can all lower aphid populations. Additional natural control is provided by the weather, such as heavy rainfalls. In the absence of these natural controls, aphid numbers can build rapidly. One reason that aphid populations can become very large is that they reproduce much more quickly than their natural enemies, and can quickly rebound from harsh weather. Remember that leaving aphids alone generally causes little damage to the unstressed plant, and has the added benefit of giving natural enemies the chance to exert natural control.

High pressure water spray from a garden hose is also a good option for aphid management on smaller plants. The water spray acts much like a heavy rain, knocking the aphids off the plant.

Insecticides are effective in reducing aphid numbers. It is best to use the least toxic products first. Horticultural oil can be applied in late winter to suffocate aphid eggs, or during the spring and summer to kill nymphs and adults. You can also use insecticidal soap (e.g., Safer's), or the fungus Beauveria bassiana (e.g., Naturalis-O). The fungal spores germinate on the body wall of the aphid, grow through it to the inside, and kill the aphid. The insecticide neem (e.g., BioNeem), made from an extract of the neem tree, is also effective against aphids. These products are examples of 'biorational insecticides' because they kill pests with minimal impact on natural enemies.

Aphids can also be managed with insecticides such as acephate (e.g., Orthene), malathion, diazinon, permethrin, or chlorpyrifos (e.g., Dursban). Keep in mind that these insecticides kill the natural enemies as well as aphids, and can make aphid problems worse. Avoid spraying broad-spectrum insecticides when possible to preserve natural enemies, and to maintain a good long-term strategy for aphid management.

CAUTION: Read all label directions very carefully before purchasing and again before using an insecticide. Information on the label should be used as the final authority.