The crepe myrtle, Lagerstroemia indica, is a highly valued flowering tree known for its long blooming season and attractive bark. Its popularity is often challenged by specific insect pests that diminish its aesthetic appeal and health. Effective management requires accurately identifying the insect and applying targeted treatment strategies. A proactive approach combining immediate removal methods with appropriate chemical or systemic treatments, followed by long-term cultural practices, offers the best solution for keeping these trees vibrant and healthy.
Identifying the Specific Invaders
The most problematic insects are the Crepe Myrtle Bark Scale (CMBS) and the Crepe Myrtle Aphid. CMBS appears as white or gray felt-like encrustations, often found in branch crotches, on the trunk, and on twigs. Crushing these scale insects releases a distinctive pink or reddish fluid, confirming their identity.
Crepe Myrtle Aphids are small, pear-shaped insects that are pale yellowish-green with black spots. They primarily feed on tender new leaves and shoots, clustering on the undersides of the foliage. Both pests excrete honeydew, a sticky, sugary liquid that is a significant secondary sign of infestation.
Honeydew feeds sooty mold, a black fungus that covers the leaves and branches. While sooty mold does not directly harm the plant tissue, it indicates an underlying insect problem and reduces photosynthesis by blocking sunlight. Proper insect identification is necessary to select the correct treatment.
Immediate, Non-Chemical Removal Strategies
For small or initial infestations, immediate, non-chemical methods are the first line of defense. Heavily infested small branches or suckers can be pruned off entirely to reduce the pest population instantly. For CMBS on reachable trunks and larger branches, use a soft-bristled brush or rag dipped in a mild solution of dish soap and water to physically scrub off the scale and egg masses. This manual removal also helps improve the tree’s appearance by removing sooty mold.
Aphids, being soft-bodied, can often be dislodged with a strong jet of water from a garden hose. Focus this high-pressure application on infested areas, especially the undersides of leaves, and repeat it every few days if needed. Horticultural oils or insecticidal soaps can also be sprayed directly onto the pests to smother or desiccate them.
Thorough coverage is important when using these sprays, as they only kill insects they contact. Apply them in the early morning or evening to prevent leaf burn, particularly when temperatures are high. Repeat applications every seven to ten days may be necessary to control successive generations.
Targeted Chemical and Systemic Treatments
When non-chemical methods fail or the infestation is severe, targeted chemical treatments may be necessary. Contact sprays containing active ingredients like bifenthrin provide immediate control for aphids and the mobile crawler stage of scale insects. Apply these sprays in the late evening to minimize the impact on pollinators.
For persistent or widespread CMBS infestations on established trees, systemic insecticides are the most effective solution. These products, often containing dinotefuran or imidacloprid, are applied as a soil drench around the base. The roots absorb the insecticide, distributing it throughout the plant’s vascular tissue for internal protection.
The optimal time for a systemic soil drench is early spring, just as the tree begins to leaf out, allowing the product to move into the canopy before heavy pest activity. Dinotefuran acts quickly but provides shorter control, while imidacloprid can offer protection for up to a year. Always follow the product label precisely to ensure proper dosage and mitigate potential effects on beneficial insects.
Long-Term Management and Prevention
The long-term health of the crepe myrtle depends on cultural practices that reduce its susceptibility to pests. Planting the tree in full sun helps reduce aphid populations and fungal issues like powdery mildew. Adequate air circulation is also beneficial, achieved through proper thinning cuts during dormant season pruning.
Correct watering is essential, as stressed plants are more vulnerable to insects. Deep, infrequent watering encourages a robust root system; apply moisture at the base of the plant rather than on the foliage. Avoid excessive high-nitrogen fertilizers, which produce tender new growth highly attractive to sap-sucking insects.
Pruning should occur in late winter or early spring during the dormant season. Avoid severe topping, sometimes called “crape murder,” which stresses the plant. Regularly monitoring the tree for the first signs of honeydew or scale is important for catching small infestations that are easily managed with non-chemical methods.