Aphids on butterfly milkweed (Asclepias tuberosa) fruit
Aphids on honeyvine milkweed
A heavy infestation of aphids on a grape vine (Vitis sp.)
Aphids in various stages of maturity and white cast skins
Honeydew from aphids on a tulip tree (Liriodendron tulipifera) leaf
Aphids are a very large and diverse group of insects that plague the garden especially during the beginning and end of the growing season. At least 4,000 aphid species have been identified; variations in color, size, and appearance make it impossible to generalize.
Symptoms and Diagnosis
Aphid damage may appear as pale yellow spots on leaves, leaves that are curled, puckered, or stunted, and/or distorted blossoms. Look for aphids at the beginning of the growing season. They should be especially abundant on stems and buds where nitrogen concentrations are higher. Aphids multiply more rapidly with high nitrogen levels. Another sign to look for is a sooty black layer on the leaves. Because aphids suck more plant sap than they can use, they exude honeydew onto leaf surfaces. Envied by honeybees and ants, this honeydew substance is very sweet and molds quickly.
Life Cycle
Females lay eggs toward the end of the growing season in the bark or bud scales of their favorite plant. When the eggs hatch the following spring, the nymphs are all female and are called "stem mothers". These females will give live birth to daughters without mating. Toward the end of the growing season, males are produced, mating occurs, and the eggs will again overwinter in the plant material.
Integrated Pest Management Strategies
1. Through good horticultural practices, aphids can be minimized. One step is to control the amount of nitrogen added to the garden. By using slowrelease fertilizers such as ammonium or urea-based fertilizers, compost, decomposed manure, fish emulsion, or liquid seaweed, you slow the rate at which the aphids can reproduce. Another step is to prune moderately in winter and early spring, saving the heavy pruning for midgrowing season. This prevents the aphids from destroying fresh growth in early spring. The last step is to encourage a continuum of flowers that will attract the aphids’ natural predators— ladybugs and praying mantis.
2. Try spraying the aphids off the plants with plain water, or remove them by hand with a cloth.
3. Insecticidal soap can be used, but in moderation, as it will kill beneficial insects as well as the aphids.
4. Insecticidal baits that contain boric acid can be used to control the ants, which have a symbiotic relationship with the aphids. The ants protect the aphids and get the honeydew that they ooze in return. Use caution; boric acid is toxic to people and animals. Ant barriers placed at the base of infested plants is another way to discourage ants and allow predators to attack the aphid population.
5. Silica Aerogel/Pyrethrin is a product that will dissolve the cuticle layer on the insect causing it to dehydrate and die.
6. Use chemical insecticides. Pesticides registered for use include carbaryl (Sevin), disulfoton (Disyston), malathion, bifenthrin, imidacloprid, permethrin, petroleum oil, and pyrethrins.
Organic Strategies
Strategies 1 and 2 are strictly organic approaches. For an organic approach to Strategies 3, 5, and 6, consult the Organic Materials Review Institute (OMRI™) for appropriate insecticidal soap, pyrethrin, and pyrethrum products.