Melon Aphid

The melon aphid, also called cotton 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 0.06 inch (1.5 mm) in length. The melon aphid develops in colonies and prefers the underside of leaves. Unlike other aphids, melon 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.

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

These small, green aphids can be a major problem on young plants where they feed near the tips of runners or in growing points. They cluster in numbers on the underside of growing leaves, distorting and curling the leaves, and produce a large amount of honeydew. The fruits become coated with the sticky secretion, creating an environment favorable for the development of a sooty mold. In addition they vector a number of viruses. In the San Joaquin Valley, this aphid can vector cucumber mosaic, zucchini yellow, and watermelon mosaic viruses, among others. These virus diseases may be more destructive to crops than direct aphid feeding. Several other aphid species cause similar injury, as well as virus transmission. The end result of feeding by this aphid is loss of vigor, stunting, or even death of the plants. Melon aphids will feed on cantaloupe, honeydew melon, casaba, and Persian melons, watermelon, cucumber, and squash.

Silver reflective mulches have successfully been used to repel aphids from plants, thus reducing or delaying virus transmission. In some areas of the state, row covers have also been successfully used. Biological control can have a significant impact on aphid population 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. Releases of this beetle are not effective, however, because it generally does not remain in the field following release. Other general predators, such as lacewing and syrphid larvae, and parasitic wasps, including Lysiphlebus, Aphidius, Diaeretiella, and Aphelinus species, also attack aphids. Biological control is not effective in reducing virus transmission by this aphid.

It is a good practice, where feasible, to remove and bury the few severely infested plants as they appear in spring; this helps prevent rapid spreading of the aphid population.

In desert production areas, exclude aphids by applying row covers (plastic and spun-bonded materials) at planting and gradually removing them at first bloom or earlier if needed. Row covers are not recommended for the San Joaquin Valley.

• Do not remove the entire plastic row cover at one time because a drastic reduction in humidity will shock plants and can lead to collapse. Instead vent the covers and remove them gradually. Covers made of spun-bonded materials do not need venting because hot air is able to escape.
• Remove row covers if the air temperature underneath reaches 104° F before bloom.
• Remove row covers before plants grow high enough to touch hot plastic.

Lay silver reflective plastic mulches at planting. They help plants get off to a healthy start, and are effective until expanded foliage covers the reflective surface. Reflective mulches also need to be removed in the desert areas when summer temperatures are excessive for optimal growth of plants. However, in the Central Valley and cooler areas, mulches have not caused plant damage; in fact, they improve soil moisture and nutrient retention, which may further aid plant productivity.

Preserve habitat for beneficials around the field and keep dust down to encourage parasitism and predation. If populations are high enough to produce large amounts of honeydew, the fruit will need to be washed off. Avoid overfertilizing with nitrogen. Fields infested with melon aphid should be disced or plowed under as soon as harvest is complete.

Biological and cultural controls and sprays of rosemary oil, insecticidal soaps, and certain oils are acceptable for use in an organically grown crop. Rosemary oil is less disruptive of beneficials than soaps and narrow range oils.

Melon aphid is very difficult to control with insecticides. If natural enemies are not destroyed by insecticides applied for other pests, they will help keep melon aphid under control until late in the season.

1. Before planting seed or transplants, set out yellow sticky cards to monitor the movement of aphids and whiteflies.
2. Start checking traps after transplanting or when seedlings emerge.
3. When aphids are observed on traps, begin monitoring crop foliage.

If unusually large numbers of aphids build up in parts of a field early in the season and appear to be retarding growth or causing honeydew buildup on fruit, apply an insecticide to the infested portions of the field. No threshold has been established. Early treatment does not prevent virus introduction; however, treating may help reduce spread of the virus if aphid colonies are present.