Mealy Plum Aphid

Wingless mealy plum aphid adults are pale green or whitish green with three dark green longitudinal stripes on their backs. Their bodies are covered with a white mealy wax. The winged form has a dark thorax and transverse bands on the abdomen. Mealy plum aphids are often found inside the slightly curled-up plum leaves. After overwintering in the egg stage near the bases of buds, the aphids hatch during bloom and develop into wingless adults. They will have from three to thirteen generations on plums. Then winged adults appear in June and July as warm weather approaches and they migrate to reed grass or cattails. Wingless aphids that remain on vigorous growth of plum trees throughout the summer will not be capable of laying overwintering eggs in fall. Only the offspring of the winged adults that return to plum trees in fall lay the overwintering eggs.

This aphid builds up in large numbers on the undersurface of leaves in spring and causes leaves to become slightly curled and stunted. High populations can devitalize the tree, retard growth, and reduce sugar content of fruit. Honeydew dropping on fruit can cause fruit cracking.

Several natural enemies are important in the control of aphids in the orchard, but aphid populations often require treatment. The best indicator of populations is orchard history. The best time to treat is during the dormant or delayed dormant period. If aphids are a chronic problem in the orchard, apply a treatment early in dormancy; otherwise, sample during dormancy to determine the need to treat as described below. Spring treatments may also be made. After harvest, a zinc sulfate application will provide zinc to the trees as well as hasten leaf fall. Without the leaves on the tree, the aphid life cycle is disrupted. Zinc sulfate (36%) applied at 10-20 lb/acre can be applied in early to mid-October to help in this process.

There are many natural enemies that feed on leaf curl plum aphid; however, fruit size may still be reduced and curled leaves will not uncurl after aphids are suppressed. The recent introductions of Aphidius transcaspicus has led to substantial levels of parasitism of this aphid. Important predators include: lady beetles, green lacewings, brown lacewings, syrphid flies, and soldier beetles.

Biological control and sprays of narrow range oil or neem oil are organically acceptable methods of controlling this pest.

If aphids are a chronic problem, a treatment in late fall/early dormancy (November 1) is a very effective way to manage these pests and is less likely to create water quality problems caused by pesticide runoff than treatments applied during the rainier season in January and February. If leaves are still on trees at this time, aphids and parasites can be present. Oil treatments are not recommended at this time because they are very damaging to parasite populations and not effective for aphid control.

Dormant monitoring. If the November 1 treatment is not applied, be sure to monitor during dormancy. (For more information, see DORMANT SPUR SAMPLE.) If dormant monitoring indicates treatment is necessary, two applications of oil at bloom can be used in orchards where a dormant/delayed dormant treatment is not required to manage scale problem. Parasites are not active at bloom, and they are not affected by the bloom oil sprays.

Spring monitoring. If aphids have been a problem in the past or if a dormant or delayed dormant application was not applied, monitor leaf curl plum aphid in spring along with mealy plum aphid. Follow the monitoring guidelines in SPRING/SUMMER MONITORING to determine if treatment is necessary.

• Photos of aphids