The trouble with tribbles, in the eponymous Star Trek episode, is that they’re born pregnant. Aphids aren’t quite that fecund, but astronomical numbers of them do sometimes materialize as though out of nowhere in early spring. How do aphids increase their population so rapidly? And is there anything the gardener can do to stop them?
Aphid life cycle
Aphids lay their eggs before winter in Virginia, gluing them to the leaves and stems of host plants. In early spring, nymphs hatch from the eggs and mature into wingless females in 7-10 days (see picture). No males are present at this point. Nor do the wingless females lay eggs. Instead, they give live birth exclusively to more females, without the need for mating – a process termed parthenogenesis. Each female can generate 40-60 offspring that, in turn, give birth to yet more females. Aphids can go through 12 or more generations of parthenogenesis in Virginia. To make matters worse, from time to time a few females develop wings and move to new plants, where they start another infestation.
Eventually, as fall approaches and days shorten, a few males are born. They mate with females, which lay eggs in late fall to continue the cycle again the following spring.
Aphid damage
Aphids have piercing/sucking mouthparts and feed on plants’ sap. The loss of fluid can cause leaves and stems to wilt. This is usually only a problem when a plant is severely infested. Some plants’ leaves may become puckered or twisted in reaction to chemicals in the aphids’ saliva (see picture). Aphids have a particular tendency to feed on new growth, or near flower buds, which can cause stunted growth or reduced flowering.
Another sign of aphid feeding is a black fungus called sooty mold on and beneath plants. Aphids don’t digest all the sugars in the sap they extract from their hosts; they excrete the leftover sugars in a liquid called honeydew. (Fascinatingly, some species of ants stand guard over aphids and harvest the honeydew to feed their colonies.) The sugary honeydew coats leaves and soil beneath the aphids, and sooty mold feeds on the sugars. Usually sooty mold is just an aesthetic problem, but in severe cases it can reduce leaves’ photosynthetic potential.
Finally, aphids can spread plant viruses. Viruses spread by aphids include cauliflower mosaic, cucumber mosaic, lettuce mosaic, plum pox, potato virus Y, and turnip mosaic viruses. Symptoms vary greatly from crop to crop, and even amongst varieties of a particular crop, so consult the links in the text for detailed information. In general, viruses cause stunting, distorted growth, patchy discoloration on leaves, and occasionally necrosis. Note that plant viruses often infect plants other than the one the virus is named for! For example, plum pox virus can infect any stone fruit tree. If you suspect your plants have a viral disease, pull them up immediately, roots and all, and don’t compost them.
Aphids commonly found in Virginia
Nearly all aphids are quite small, 1/16 to 1/8 inch long. Mostly they differ in color and host plant.
White pine aphid Dark, with a white stripe and long legs (see picture). Lay their eggs in straight lines on white pine needles. Have been known to hatch indoors on white pines used as Christmas trees!
Giant bark aphid Large (1/2 inch) and gray. Found on the bark of oak, elm, maple, and other large shade trees.
Rose aphid Green or pink with black legs. Mostly a pest of roses.
Green peach aphid Usually pale green (see picture below) but sometimes dark green or pink. Infest many plants, including asters, roses, English ivy, tulips, and various garden vegetables. Transmit at least a hundred plant viruses.
Wooly alder aphid Dark blue, but covered with waxy white filaments (see picture below left). Mostly affect silver maple, despite the name.
Gall aphids, e.g. Witchhazel leaf (or cone) gall aphid, Spiny witchhazel gall aphid, Elm cockscomb gall aphid: These aphids secrete chemicals that cause their hosts to create an abnormal growth (see picture below right) surrounding the aphid, giving it a secure place to feed and reproduce.
Aphid Control
Prevention is always the best way to minimize pest damage. Sanitation is the first line of defense. Aphid eggs overwinter on host plants, so clean up plant debris at the end of the season, particularly if there was an aphid infestation on those plants. During the growing season, keep weeds under control to deny aphids a habitat.
Natural predators, in particular insects, can help control aphid infestations. Protect these beneficial insects by minimizing insecticide use. As discussed below, sometimes insecticides are needed for severe aphid infestations. If so, reduce the impact on beneficial insects by using insecticides with little residual activity, like insecticidal soaps and botanicals like neem oil and pyrethrins.
Certain plants can also attract beneficial insects. For example, dill and marigolds are favorites of lady beetles and parasitic wasps, both of which feed on aphids. The Clemson and Penn State extensions have lists of insect-attracting plants. Grow these plants around your garden, or interplant them amongst your crops.
Finally, eggs of certain predatory insects can be purchased. If you go this route, be sure to precisely identify the species of aphid in your garden and use a predator that’s known to feed on them. Also be sure to grow plants in or near your garden that will help keep the beneficial insects nearby.
Regular scouting is key to keeping aphid levels low. Check your plants often for signs of aphids, like wilting, aberrant growth, and sooty mold, as described above. Hopefully you can detect aphids before the infestation really gets going.
Physical removal is a low-impact method for removing aphids. If you have sharp eyes and steady hands, pluck aphids off plants and crush or dunk in soapy water. More realistically, knock aphids off plants with a spray of water.
Insecticides may be needed as a last resort if aphid populations are overwhelming. We recommend insecticide sprays with low residual activity, like insecticidal soaps and botanical extracts like neem oil and pyrethrins. These insecticides must directly contact their targets to work. Thus, be sure to spray the entire plant, including under leaves and in junctions between stems and leaves, where aphids often hide. Spray carefully to avoid harming pollinators: spray early in the season before flowers appear, don’t let the spray reach flowering plants, and spray in the evening when pollinators are less likely to be present.
Systemic insecticides like imidacloprid might be useful for shrubs, trees, and other ornamentals. Again, take care to reduce the chance of harming pollinators by treating only when the plants aren’t flowering.
Concluding thoughts
We’re publishing this article in December because winter is the time to prepare for aphids. Clean up plant waste that might bear aphid eggs, sow seeds for plants that attract beneficial insects, stock up on neem oil or insecticidal soap, and get ready to check your ornamentals, seedlings, and over-wintered crops for early signs of aphid infestations. Your plants will live long and prosper.
References and further reading
Aphids University of Kentucky Department of Entomology
Aphids, in-depth Wisconsin Horticulture Division of Extension
Aphids in home yards and gardens University of Minnesota Extension
Aphids in Virginia Virginia Cooperative Extension
Aphid Management Guide (PDF) Texas A&M Extension
Attracting beneficial insects Clemson Cooperative Extension
Attracting beneficial insects Penn State Extension
Featured image (oleander aphids): Malcolm Manners. CC BY 2.0 (cropped)