Cotton Aphid

The cotton aphid, also called melon aphid, is a rather small aphid that ranges in color from yellowish green to greenish black. Both winged and wingless forms are produced. The winged individuals are somewhat slender and are not as robust as the wingless form. A mature individual measures about 1/16 inch (1.5 mm) in length. The cotton aphid develops in colonies and prefers the underside of leaves. Unlike other aphids, cotton aphid populations do not diminish with high temperatures; they can also be troublesome late in the season (September and October), particularly in the San Joaquin Valley and in northern California.

Cotton aphid has an extensive host range. Some of the crops it attacks besides pistachio are carrot, cotton, cucurbits, and citrus. Host weeds include milkweed, jimsonweed, pigweed, plantain, and field bindweed.

Cotton aphids can be a major problem in first-year, newly budded trees. They distort and cause curling of growing leaves, and produce a large amount of honeydew. Clusters become coated with sticky honeydew, creating an environment favorable for the development of a sooty mold.

Biological control can have a significant impact on aphids so be sure to evaluate predator and parasite populations when making treatment decisions.

Naturally-occurring populations of the convergent lady beetle, Hippodamia convergens, may provide effective control in early spring. However, releases of this beetle are not effective because it generally does not remain in the orchard following release. Other general predators, such as lacewing and syrphid larvae, and parasitic wasps, including Lysiphlebus, Aphidius, Diaeretiella, and Aphelinus species, also attack aphids.

• It is a good practice, where feasible, to control weedy hosts of cotton aphid.
• Preserve habitat for beneficials around the orchard and keep dust down to encourage parasitism and predation.
• Avoid applying too much nitrogen fertilizer.
• Nearby fields infested with cotton aphid should be disced or plowed under as soon as harvest is complete to prevent movement into newly budded pistachio orchards.

Biological and cultural controls and sprays of rosemary oil, insecticidal soaps, and certain oils are acceptable for use in an organically grown crop.

Monitor newly budded pistachio trees for cotton aphid. In most cases biological controls are sufficient to keep aphids at low levels. If unusually large numbers of aphids build up early in the season and appear to be retarding growth, consider applying an insecticide. No threshold has been established.