Crape Myrtle: Beauty with Biological Control
Crape myrtle, Lagerstroemia indica L., is an exotic ornamental plant, which was introduced to the U.S. from Asia. Its beautiful summer flowers and interesting growth characteristics make it one of the most popular ornamental plants in the southern U.S. (USDA Hardiness Zones 7-10). Horticulturists and homeowners alike love crape myrtle with its red, pink, white, purple, or lavender summer blooms. Crape myrtle grows under a wide range of site and soil conditions. It grows best in full sun but will do well in partial shade. If this were not enough, crape myrtle is easy to propagate and grow, comes in sizes from dwarf shrubs to small trees, and has very few insect and disease pests.
Powdery mildew is the only disease of any consequence; however, resistant cultivars are available. In the upper South, the Japanese beetle and an exotic weevil, Callirhopalus bipunctalus, can be pests. A metallic green flea beetle, Altica sp., and a brown striped beetle, Colaspis floridana, are occasional pests.
The crapemyrtle aphid, Tinocallis kahawaluokalani (Kirkaldy), is the most important insect pest in the mid- and lower South. Crapemyrtle aphid is host specific to crape myrtle, so it does not feed on other plants. These aphids reach high numbers during mid-summer. When they feed on plant sap, they excrete honeydew copiously, which in turn serves as food for the fungas, sooty mold. Sooty mold colors crape myrtle foliage and wood black, prompting many people to apply insecticides for the aphids. STOP and consider the following before you control the aphids!
Crapemyrtle aphids and their sugary honeydew are food for twenty to thirty species of beneficial insect predators (good bugs that feed on harmful bugs) and countless bees and wasps. The crape myrtle flowers also attract and provide nectar and pollen to many important pollinating bees. Crape myrtle and its aphids attract these beneficial parasites and predators to crape myrtle and other plants in the surrounding area. The large numbers of aphids encourage the beneficials to remain in the area. Because the aphids aren't native to the U.S., most of our native predators prefer the native species over these aphids. The predators leave crape myrtle periodically to search the surrounding vegetation (your yard and garden) for their preferred prey -- your other plant pests -- thereby enhancing natural biological control. However, since the crapemyrtle aphids are so populous, the predators usually cannot control all of them on the crape myrtle. You have to tolerate some crapemyrtle aphids while enjoying this kind of biological pest control.
Crapemyrtle aphids prefer certain cultivars of crape myrtle plants. A good strategy for attracting predators is to plant small groups of four to eight plants of crape myrtle. UF researchers are currently investigating this idea using larger blocks of crape myrtle in pecan orchards. Of course, some predators may be drawn to crape myrtle and away from crop plants, but this potential negative effect is far outweighed by the benefits. Crape myrtle is probably the most important woody landscape plant in the southeastern U.S. for augmenting and sustaining many beneficial insects.
Aphid populations are predictable and peak in most years during July in north Florida and south Georgia (later farther north and sooner farther south). However, aphids in different locations peak at different times, making the aphids much more attractive to predators in a large landscape area. During excessive drought periods, crapemyrtle aphids may be the only food available to many beneficial insects. Cultural practices promoting crape myrtle growth such as irrigation, fertilizer, and light pruning stimulate aphids and maintain them longer on the plant.
Plant some crape myrtles, tolerate the aphids and the sooty mold, enjoy the beauty, and help Nature take its course by enhancing natural biological control of pests. For additional information: Research Report BB93-4.