Aphids or plant lice are small, soft bodied, slow moving insects that feed by sucking juice from plants. They can usually be recognized by the pear-shaped body and fairly long antennae. Aphids vary in color -- white, gray, green, brown, red, yellow or black. They are usually found in large numbers (colonies) on the undersides of leaves or on stems. There are both winged and wingless aphids in most species. As the aphids feed they secrete honeydew – a sweet sticky shiny substance seen on leaves. Honeydew consists mainly of excess sap ingested by the insect and passed through the body.
Most species of aphids overwinter in the egg stage. The eggs hatch in the spring to produce a generation of females. These female aphids give birth to living young. Generally the first young aphids are wingless, and when a colony becomes too crowded winged forms may be produced. The winged forms migrate to new host plants and begin colonies. Enormous populations are built up from these overlapping generations all summer long.
Late in the season the aphids migrate back to the original host plant, and a generation consisting of both males and females is produced. These individuals mate and the females lay eggs, which overwinter.
Carefully inspect plants for the beginning of an aphid population buildup. Check for natural enemies such as mummies (gray-brown, bloated, parasitized aphids -- indicating wasp parasites at work), and the alligator-like larvae of lady beetles and lacewings. Yellow sticky boards are also used as a monitoring tool for aphid populations. Aphids are attracted to the yellow color and often are visible on the cards before they are detected on the plant.
Natural enemies can be very important in the control of aphids, especially in gardens not sprayed with broad-spectrum pesticides (organophosphates, carbamates, and pyrethroids) that kill natural enemy species as well as pests. Usually natural enemy populations do not appear in significant numbers until aphids begin to be numerous.
Among the most important natural enemies are various species of parasitic wasps that lay their eggs inside aphids. The skin of the parasitized aphid turns crusty and golden brown, a form called a mummy. The generation time of most parasites is quite short when the weather is warm, so once you begin to see mummies on your plants, the aphid population is likely to be reduced substantially within a week or two.
Many predators also feed on aphids. The most well known are lady beetle, lacewing, and syrphid fly. Naturally occurring predators work best, especially in a small backyard situation. Commercially available lady beetles may give some temporary control when properly handled, although most of them will disperse away from your yard within a few days.
Aphids are very susceptible to fungal diseases when it is humid. Whole colonies of aphids can be killed by these pathogens when conditions are right. Look for dead aphids that have turned reddish or brown; they have a fuzzy, shriveled texture unlike the shiny, bloated, tan-colored mummies that form when aphids are parasitized.
Weather can also impact aphids. Populations of many species are reduced by summer heat in the Central Valley and desert areas, and aphid activity is also limited during the coldest part of the year. However, some aphids may be active year round, especially in the milder, central coastal areas of California.
Before planting vegetables, check surrounding areas for sources of aphids and remove them. Aphids often build up on weeds such as sowthistle and mustards, moving onto crop seedlings after they emerge. Check transplants for aphids and remove them before planting.
Where aphid populations are localized on a few curled leaves or new shoots, the best control may be to prune these areas out and dispose of them. In large trees, some aphids thrive in the dense inner canopy; pruning these areas out can make the habitat less suitable.
In some situations ants tend aphids and feed on the honeydew aphids excrete. At the same time, they protect the aphids from natural enemies. If you see ants crawling up aphid-infested trees or woody plants, put a band of sticky material (Tanglefoot, etc.) around the trunk to prevent ants from getting up. Teflon products, which are too slippery for ants to climb up, have also been used. (Note: Do not apply sticky material directly to the bark of young or thin-barked trees or to trees that have been severely pruned; the material may have phytotoxic effects. Wrap the trunk with fabric tree wrap or duct tape and apply sticky material to the wrap.) Alternatively, ant stakes or baits may be used on the ground to control the ants without affecting the aphids or their natural enemies. Prune out other ant routes such as branches touching buildings, the ground, or other trees.
High levels of nitrogen fertilizer favor aphid reproduction. Never use more nitrogen than necessary. Use less soluble forms of nitrogen and apply it in small portions throughout the season rather than all at once. Or better yet, use a urea-based, time-release formulation (most organic fertilizers can be classified as time-release products as compared to synthetically manufactured fertilizers).
Because many vegetables are primarily susceptible to serious aphid damage during the seedling stage, losses can be reduced by growing seedlings under protective covers in the garden, in a greenhouse, or inside and then transplanting them when they are older and more tolerant of aphid feeding. Protective covers will also prevent transmission of aphid-borne viruses.
Aluminum foil mulches have been successfully used to reduce transmission of aphid-borne viruses in summer squashes, melons, and other susceptible vegetables. They repel invading aphid populations, reducing numbers on seedlings and small plants. Another benefit is that yields of vegetables grown on aluminum foil mulches are usually increased by the greater amount of solar energy reflecting on leaves.
To put an aluminum mulch in your garden, remove all weeds and cover beds with aluminum-coated construction paper, which is available in rolls from Reynolds Aluminum Company. Bury the edges of the paper with soil to hold them down. After the mulch is in place, cut or burn 3- to 4-inch diameter holes and plant several seeds or single transplants in each one. You may furrow irrigate or sprinkle your beds; the mulch is sturdy enough to tolerate sprinkling. In addition to repelling aphids, leafhoppers, and some other insects, the mulch will enhance crop growth and control weeds. When summertime temperatures get high, however, remove mulches to prevent overheating plants. An alternative to aluminum-coated construction paper is to spray clear plastic mulch with silver paint. Reflective plastic mulches are also available in many garden stores.
Another way to reduce aphid populations on sturdy plants is to knock them off with a strong spray of water. Most dislodged aphids will not be able to return to the plant, and their honeydew will be washed off as well. Using water sprays early in the day allows plants to dry off rapidly in the sun and be less susceptible to fungal diseases.
Insecticidal soap, neem oil, and narrow-range oil (e.g., supreme or superior parafinic-type oil) provide temporary control if applied to thoroughly cover infested foliage. To get thorough coverage, spray these materials with a high volume of water and target the underside of leaves as well as the top. Soaps, neem oil, and narrow range oil only kill aphids present on the day they are sprayed, so applications may need to be repeated. Predators and parasites often become abundant only after aphids are numerous, so applying nonpersistent insecticides like soap or oil may provide more effective long-term control. Although these materials do kill natural enemies that are present on the plant and hit by the spray, because they leave no toxic residue, they do not kill natural enemies that migrate in after the spray. These and other insecticides with only contact activity are generally ineffective in preventing damage from aphids such as the woolly apple aphid or the woolly ash aphid that are protected by galls or distorted foliage. Do not use soaps or oils on water-stressed plants or when the temperature exceeds 90°F. These materials may be phytotoxic to some plants, so check labels and test them out on a portion of the foliage several days before applying a full treatment.
Supreme- or superior-type oils will kill overwintering eggs of aphids on fruit trees if applied as a delayed dormant application just as eggs are beginning to hatch in early spring. These treatments will not give complete control of aphids and are probably not justified for aphid control alone. Earlier applications will not control aphids. Common aphid species controlled include the woolly apple aphid, green apple aphid, rosy apple aphid, mealy plum aphid, and black cherry aphid.
Many other insecticides are available to control aphids in the home garden and landscape, including foliar-applied formulations of malathion, permethrin and acephate (nonfood crops only). While these materials may kill higher numbers of aphids than soaps and oils, their use should be limited because they also kill the natural enemies that provide long-term control of aphids and other pests. Repeated applications of these materials may also result in the development of resistance to the material by the aphid. Insecticides such as oils and soaps are also safer to use when children and pets may be present. Formulations combining insecticidal soaps and pyrethrins may provide slightly more knockdown than soaps alone, yet have fewer negative impacts on natural enemies than malathion, permethrin, and acephate, because pyrethrins break down very quickly. Avoid the use of diazinon and chlorpyrifos; urban garden use of these materials has been identified as a source of pollution in California’s creeks and rivers. Carbaryl is not recommended because it is not very effective against aphids. Acephate has systemic activity, which means it moves through leaves, thus it can be effective where aphids are hidden beneath curling foliage. Acephate is not registered for use on food crops in the garden because it can break down to a much more toxic material. The soil-applied systemic pesticide disulfoton is sometimes applied in roses for aphid control, but it is a highly toxic material to people.
Professional applicators can make soil injections of the systemic insecticide imidacloprid, which is quite effective against aphids infesting large street trees and not very harmful to beneficial soil organisms. Because it takes a substantial time for the product to get from the soil to the growing points of trees, applications must be made up to 2 months before problems are expected.
When considering application of pesticides for aphid control, remember that moderate populations of many aphids attacking leaves of fruit trees or ornamental trees and shrubs do not cause long-term damage. Low populations can be tolerated in most situations and aphids will often disappear when natural enemies or hot temperatures arrive. Often a forceful spray of water or water-soap solution, even on large street trees, when applied with appropriate equipment, will provide sufficient control.