Some gardeners leave small aphid colonies intact in order to provide a food supply for beneficial insects such as lady beetles, lacewings, and hoverfly larvae.
Illustration By Keith Ward
This article is part of our Organic Pest Control Series, which includes articles on attracting beneficial insects, controlling specific garden pests, and using organic pesticides.
Small sucking insects commonly called “plant lice” are usually aphids. Numerous species inhabit vegetable gardens, where they are a primary food source for important beneficial insects including lady beetles and lacewing larvae. Aphids become a problem when a species finds exactly the plant they desire, as often happens with cabbage aphids on fall crops of broccoli, kale or Brussels sprouts. Peas, beans, potatoes, tomatoes and other crops also can be seriously damaged when aphid colonies grow to a damaging size. Some aphids can transmit viral diseases in cucumber and tomato family crops. Learning how to kill aphids naturally is an important part of organic pest control.
What Are Aphids?
Small, soft-bodied sucking insects, aphids typically have a pear-shaped body one-tenth of an inch or less in length. They may be winged or wingless, and often have colors that blend in with the leaf or stem being damaged. Young, tender growth is often preferred, because aphids feed by sucking plant juices through straw-like mouthparts. Aphids commonly feed in groups, and numerous species are found in gardens around the world.
What Does Aphid Damage Look Like?
Curled, mottled leaves are a sign that aphids (or similar insects) are at work. In time, infested plants become stunted and sickly, and a great deal of chlorophyll may be lost when leaves are devastated by aphid feeding. When aphids infest a young stem it often curls and stops growing. Gray-green cabbage aphids often cluster in the sheltered spot where leaves join the stem, or inside broccoli heads or the outer leaves of Brussels sprouts.
When you see ants moving up and down a plant, aphids are likely to be present. Many aphid species excrete copious amounts of a sweet fluid called honeydew, which ants relish and readily gather up. In fact, some ants “milk” their aphid herds and in exchange help protect the aphid colonies.
One key to the enormous success of aphids is their amazing reproductive capacity, which is higher than that of any other garden insect. It is estimated that a single aphid could have 5 billion descendants between spring and fall! Each female produces 50 to 100 offspring, and there may be over 40 generations annually in mild climates (and even more in greenhouses). Fortunately, a wide variety of predators, parasites, and pathogens keep the aphid population in check, but not always well enough to prevent serious damage.
Natural Aphid Predators
Lady beetles (adults and larvae), lacewings, and hoverfly larvae are major aphid predators. Some gardeners leave small aphid colonies found in spring intact in order to provide a food supply for early generations of these important beneficial insects. In addition, larvae of tiny braconid and chalcid wasps feed and develop within the bodies of aphids, eventually killing them.
Organic Pest Control: How to Kill Aphids
One of the best methods of organic aphid control is hand picking, which often requires pinching the infested leaf or stem from the plant and composting it. Be on the lookout for predators among the aphids. For example, if several lady beetle larvae are present, consider leaving some aphids as food for them. To save an infested stem or leaf, use a toothpick to scrape off the aphids.
When aphids inhabit leafy greens, try blasting them from their perches with a strong jet of water. Because of their weak legs, many of the dislodged aphids will not be able to crawl back up the plants. Repeat every few days until the problem is under control.
To combat expanding colonies, you can treat plants with insecticidal soap. Properly diluted insecticidal soap, or a solution of 1 tablespoon of dishwashing liquid per gallon of water, gives good control of aphids by removing their waxy coating, which causes them to die from desiccation. Be aware that soap sprays can taint the flavor of leafy greens, and can damage leaves when applied in bright sun.
To control aphids in fruit trees or shrubs, you can alternate soap sprays with horticultural oil (be sure to follow label directions). Spot treatments with horticultural oil smother aphids. This remedy is helpful when you encounter numerous scattered colonies with no natural predators present.
More Tips for Controlling Aphids
In MOTHER EARTH NEWS’ 2010 survey about common garden pests, aphids were on the watch list of 50 percent of respondents, many of whom prevent problems by attracting beneficial insects with flowers and herbs. Several readers noted the ability of sweet alyssum and other flowers to attract hoverflies, which eat aphids. “We attract a lot of beneficials by planting carefree flowers in the vegetable garden, including calendula, borage, zinnias, cosmos and nasturtiums,” sad a Midwestern gardener with more than 20 years of experience). Other respondents commented on the importance of having some aphids around to serve as food for lady beetles, hoverflies, lacewings and other well-known beneficial insects.
More information on organic aphid control is available from Agri-Food Canada, University of California, and the University of Kentucky.