Controlling Aphids in Your Garden
Aphids can be an important food source to many beneficial insects. Most plants can tolerate low to moderate numbers of aphids without noticeable damage. On some plants, however, large numbers of aphids can distort foliage and flowers and stunt plant growth. Some species of aphids can also transmit plant diseases when they puncture plant tissues to feed. Aphids excrete “honeydew,” a sweet substance that forms a harmless but sticky coating on leaves. The honeydew is soon colonized by a fungus called “sooty mold,” which is also harmless, but makes leaves look black and dirty. Ants love to feed on honeydew, and to ensure a continuing supply, they protect aphids from their natural enemies. When this happens, aphid management must include ant management.
Tolerate some aphids
Aphids have many natural enemies such as spiders, ladybugs, lacewings, and minute parasitoids (tiny non-stinging wasps) that often keep aphid numbers below damaging levels. So it’s best to tolerate low to moderate numbers of aphids as long as they aren’t causing noticeable plant damage. Beneficial insects rarely appear on the scene until after aphids have begun attacking plants. This “lag-time” can be a day or two or as long as several weeks. As the season progresses, aphid control by these natural enemies improves because more natural enemies are attracted to your garden and more stay to breed. • Aphids commonly found on trees will not infest your garden annuals, and these aphids can help attract natural enemies that will attack pests on other plants.
Aphids are very small insects with soft, pear-shaped bodies. They have long legs and antennae, and most have two tube -like structures called cornicles on their hind end. Adults of some species have wings. Aphids can be many colors and are usually on buds or the undersides of leaves.
Less Toxic Controls
Learn to recognize beneficial insects. Among the most important natural enemies of aphids are the tiny wasp parasitoids that lay their eggs inside the bodies of aphids. These tiny wasps cannot sting people. A parasitized aphid (called a “mummy”) looks puffedup, and its skin hardens and changes color, often to tan, light brown, or black.
Attract beneficial insects to your garden by planting a wide variety of flowering plants. (See fact sheet in this series called “Growing a Healthy Garden to Manage Pests Naturally”). The adult forms of many beneficial insects, including tiny wasps and lacewings, feed on pollen and nectar. Consider buying beneficial insects. Lacewings are more likely to stay in your garden than commercially available ladybugs.
Buy beneficials before aphid numbers are high. If you have an aphid emergency, first use soap or oil sprays to reduce the population. Then, if necessary, release natural enemies. On the other hand, don’t purchase beneficial insects before you have aphids. You will be releasing them into your garden to starve.
Wipe off or prune away colonies of aphids from leaves and buds.
Use a forceful stream of plain water to wash off aphids and honeydew. Do this on a warm, sunny day so that foliage dries off before night.
Use insecticidal soaps to kill aphids on contact and spare beneficials such as lacewings. These products do not leave toxic residues.
Use spray (horticultural) oils to control aphids without leaving toxic residues for natural enemies.
Use slow-release fertilizers. Some aphids reproduce more quickly on plants with high levels of nitrogen in their leaves and buds. Fertilizers such as compost, sewage sludge, or encapsulated materials are better because they slowly release moderate levels of nutrients.
Avoid excessive pruning because it stimulates aphid-attracting growth.
Use a row cover to exclude aphids and other pests but allow air, light, and irrigation water to reach plants.
Control ants by spraying or painting a 4” wide sticky barrier around woody shrubs or trees.
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