Pests and potential problems
Crape myrtles are trouble-free small trees with the most common problems being powdery mildew, Cercospora leaf spot, aphids, Japanese beetles and sooty mold. The fungus Erysiphe lagerstroemia causes powdery mildew. Patches of white to grayish powdery growth occur on the surfaces of leaves, flowers and new shoots. Heavily infected flowers may fail to open. Infected parts of the plant are usually distorted and stunted. The disease is most serious in shady, damp locations, especially where plants are crowded and air circulation is poor. Development of the fungus is caused by high humidity at night and dry, mild daytime conditions, and often occurs during the spring and fall.
Very few insects are pests of crape myrtle however, Tinocallis kahawaluokalani, the crape myrtle aphid is one of the most important insect pest of the crape myrtle. This aphid is feeds on the leaves and young twigs of crape myrtle. The crape myrtle aphids feeds only on crape myrtle trees. Japanese beetle (Popillia japonica) is also a pest, feeding on leaves and flowers. In addition to crape myrtle, it will feed on nearly three hundred different plant species.
Crape myrtle aphids are pale yellowish green in color with black spots on the abdomen. They vary in length from 1/16 to 1/8 inch long. They survive the winter as eggs, and the eggs hatch in the spring. During the growing season, females give birth to live young. Since it takes about 10 days to reach maturity, several generations are produced each growing season. Aphids feed by inserting their mouthparts into tender new leaves from which they suck plant sap. Plant sap has a high sugar content. When they feed, the aphids excrete large amounts of a sugary liquid called honeydew. With a large aphid population, the honeydew can completely coat leaves. The honeydew serves as food for the sooty mold fungi, as well as various insects, including ants, wasps and flies. As the aphid feeds, it injects saliva into the leaf. The saliva causes yellow spots to develop on the leaf. Several other predators feed on the crape myrtle aphid. These include ladybird beetles (ladybugs) and their larvae (immature forms), green lacewings and their larvae, hover fly maggots, parasitic wasps and entomophagous (insect feeding) fungi.
Leaf and stem surfaces are covered with a black sooty substance, causing them to appear black and dirty or sooty mold. It indicates that there is an insect problem on the plant. These common molds are caused by fungi that grow on the sugary substance, called honeydew, produced by various insects that suck sap from the plant. Aphids, scales, mealy bugs and whiteflies most commonly cause this problem.
Adult Japanese beetles are about ½ inch in length and coppery-brown in color with metallic green heads. They emerge from the soil and feed from May to August. They lay their eggs in the soil. Grubs hatch from the eggs and feed on grass roots. As the weather cools, the grubs move more deeply into the soil, and over winter. Both adult beetles and their larvae (grubs) can seriously damage plants as a result of their feeding. Adult Japanese beetles eat flowers and skeletonize leaves (eat leaf tissue between the veins, resulting in a lacy skeleton remaining). The grubs feed on the roots of plants, especially on the roots of grasses.
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