If fall or dormant aphid treatments were not applied and you've had aphids before, it will probably be easy to find them in your orchard about now. Leaf curl plum aphid starts out just after leaf out and should have active parasites working on it by this time.
Look for aphid mummies, enlarged brown aphid bodies which may have parasite exit holes to indicate parasite activity. The parasites frequently control the aphids if they're not disrupted.
Many of these parasites are the result of releases made by the University of California in the late 1990s during the prune pest management alliance project. In spring, populations rapidly build up on new foliage, causing affected spurs to develop tightly curled leaves. In May, the aphids migrate from the orchard to alternate summer host plants. If this is the only aphid you have you may not need to do anything at this time.
Mealy plum aphid can also be active now and if feeding is heavy it can devitalize the tree and contribute to fruit cracking when honeydew accumulates. 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. They can have 3 to 13 generations on prunes in one season. The winged form has a dark thorax and transverse bands on the abdomen.
The 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 trees throughout the summer secrete large amounts of honeydew. Tree growth and fruit sugar content can both be reduced by populations of this aphid.
Monitoring for aphids
Monitor trees at the outside edge of the orchard or in known or potential "aphid hot spots." Potential hot spots for aphid infestation are areas of the orchard that have windbreaks or adjacent areas of natural vegetation. Begin monitoring weekly at petal fall. Aphid populations can build up quickly in the spring. It is important to monitor at least once a week. Spend 10 minutes (about 15 seconds per tree) searching 40 whole trees. Look for the presence or absence of aphids and rate the population as significant or not. If aphids (either species) occupy 10 percent or more of the tree's leaf surface as determined by the visual search, the population is significant and a treatment is required if more than 12 of the 40 trees have a significant population.
If you think your orchard may require control, then visit the IPM webpage  for more details on monitoring and treatment threshold guidelines. Following the IPM guidelines, spring treatments are effective and may be applied if necessary.
There are many natural enemies that feed on leaf curl and mealy plum aphids; however, fruit size may still be reduced and curled leaves will not uncurl even after aphids are suppressed. The recent introductions of Aphidius colemani has led to substantial levels of parasitism of the leaf curl plum aphid. Other important predators include lady beetles, green lacewings, brown lacewings, syrphid flies, and soldier beetles.