The subdued winter and early-spring interest of crape myrtles' (Lagerstroemia indica) peeling gray-, red- or white-barked branches sets the stage for their spectacular late-spring-through-summer display of white, pink, orange, red or purple blossoms. In fall, the tissue-petaled flowers yield to equally dazzling foliage. Crape myrtle's four-season show enhances gardens across U.S. Department of Agriculture plant hardiness zones 7 through 9, until infestations of insects and the sticky gray mess they make ruins the display.
Sap-sucking insects -- aphids, scales, mealybugs and whiteflies -- line up to feed on crape myrtles like the swarm of beaux paying court to Scarlett O'Hara at the Twelve Oaks barbecue. Unfortunately for you, these sloppy eaters cover the table in sugary goo. Called honeydew, it's a sticky waste resulting because they ingest far more sap than they metabolize. As it drips from their feeding sites to drench the plants and surrounding objects, honeydew soon attracts sooty mold fungus. The two combine into a sticky, grayish to charcoal-black coating.
Although aphids are crape myrtles' most common pests, all four invaders pierce tender leaves, buds and stem tissues with sharp, sucking mouths and ingest phloem sap. A plant supporting them in large numbers suffers from loss of moisture and nutrients. Its leaves become yellow and distorted, and its growth may slow. Although sooty mold feeds on honeydew without penetrating the plant's tissues, it may block the sunlight necessary for photosynthesis and weaken your crape myrtle even more.
Before reaching for an insecticide to rid your crape myrtle of sooty mold, give your garden's insect predators a chance to control the situation. Ladybugs, green lacewings, parasitic wasps and a variety of other bugs feed on the invaders. If these good bugs need assistance in managing the bad ones, spraying your plant with a strong blast of water dislodges aphids, mealybugs or whiteflies. A soft toothbrush or scouring pad effectively removes scales. Prune and dispose of lightly infested twigs and leaves.
If the sap stealers resist your cultural-control efforts, consider using insecticidal soap. It suffocates them without leaving long-term residue harmful to their predators. On a cloudy day with the temperature below 90 degrees Fahrenheit, mix 5 1/3 tablespoons, or the label's specified amount, of soap concentrate per 1 gallon of water. Water your crape myrtle well. Wearing protective clothing, socks and shoes, apply the solution with a handheld trigger or backpack sprayer. Coat all the leaves on both sides until the soap drips from the entire shrub. To avoid possible sun damage, rinse with water two or three hours after spraying. Repeat the treatment every one or two weeks as needed. Once the insects and their honeydew are gone, sooty mold starves and weathers away.
- Fine Gardening: Lagerstroemia Indica
- James City County/Williamsburg Master Gardener: Sooty Mold
- Clemson Cooperative Extension: Crepe Myrtle Diseases & Insect Pests
- University of California Statewide Integrated Pest Management Program: Sooty Mold
- University of California Statewide Integrated Pest Management Program: Scales
- University of California Statewide Integrated Pest Management Program: Mealybugs
- University of California Statewide Integrated Pests Management Program: Whiteflies
- Colorado State University Extension: Insect Control -- Soaps and Detergents
- University of California Statewide Integrated Pest Management: Pesticide Information -- Active Ingredient, Soap