Least Toxic Aphid Control- May 26, 1999
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County
This is the time of year for aphids. Their piercing-sucking mouthparts allow them to make a good living on the flowers, leaves, stems, and sometimes roots of many host plants in our landscapes and gardens. Aphids are soft-bodied insects that come in many shapes, sizes, and colors. They are small (1/8" or less) and light green. Your first clue is usually curled, stunted leaves on new growth in spring. This is usually accompanied by a sticky liquid called honeydew. A cottony white mass may also be present in some species. Close inspection of the plant and telltale stickiness from the honeydew will usually lead to a correct diagnosis.
While observing, you may see ants walking among the aphids. They are probably collecting the honeydew for food. The honeydew is colorless and sticky because it contains sugars from the plant and is exuded from the anus of the aphid as waste. Some ants will protect aphids and carry them from one plant to another. In this way they cultivate honeydew. You may have noticed in the recent movie, A Bug's Life, the queen ant carried around an aphid like a pet Chihuahua. As memory serves me, at least once she drank the honeydew from the aphid. In time, excess honeydew may build up, fungal spores in the air land on it, and sooty mold can begin growing.
Aphids can reproduce rapidly and use a unique strategy. They produce young by parthenogenesis. Here female aphids lay unfertilized eggs that hatch into female young without fertilization by a male. After a few generations, they produce winged female aphids and they fly to a different food plant. These winged females can also reproduce parthenogenetically, giving birth to winged young. Late in the season, the winged aphids return to the original food plant and some females turn into males. The males and females mate and the females lay eggs that will overwinter. Sometimes, ants will carry the eggs to their nest for the winter and transport them to a food plant the following spring.
Human gardeners may not appreciate the value of aphids as ants do. Even so, we should not be in a great hurry to attempt eradication by chemical means. Though Acephate and Malathion will effectively kill aphids, these insecticides will also kill beneficial predators that provide natural aphid control. Most gardeners are aware that lady beetle, antlion, and lacewing larvae are effective predators of aphids, but there are many others. Many insects including earwigs, assassin bugs, minute pirate bugs, stink bugs, soldier beetles, syrphid fly larvae, aphid flies, and parasitic wasps are natural enemies of aphids. With this assortment of good guys, chemical control methods can be used as a last resort to control aphids.
If a decision is made to take action against aphids, start by using a good blast of water from a high pressure hose nozzle. This may be enough to slow them down until natural enemies can overcome them. Soap sprays can also be effective at controlling aphids. To mix a soap solution, add one tablespoon of dishwashing soap (not detergent) to one gallon of water and apply to a small area of the plant to see if the solution will adversely affect the plant foliage. If the leaves look normal after one or two days, then apply soap solution to the entire affected area. Soap is quite effective against aphids.
If ants are tending the aphids, then it may become necessary to control the ants. The ants also have natural enemies and parasites. Ants can also be controlled by baiting, application of pesticides to the soil or base of plant. This will prevent ants from protecting the aphids while conserving natural enemies.
Other strategies can also be employed. For instance, insect growth regulators (IGRs) are available for aphids. These pesticides target specific insects and prevent them from maturing into reproductive adults. In effect, this is a surgical strike but some aphids may have matured to reproductive age before the application. These pesticides are less harmful to natural enemies or humans than commonly available chemical pesticides. Just a reminder: ALWAYS READ THE LABEL AND FOLLOW THE INSTRUCTIONS WHEN USING ANY PESTICIDE.
By monitoring aphid populations, carefully choosing types and timing of control methods, and encouraging natural enemies, you are practicing integrated pest management (IPM). I know I'm sounding like a bug hugger, but don't worry. Next week's column will not be on harvesting aphid honeydew as a natural sweetener.
The University of Arizona Cooperative Extension has publications and information on aphids and other insect pests. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at firstname.lastname@example.org and be sure to include your address and phone number.
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Last Updated: March 15, 2001
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