Crepe myrtles (Lagerstroemia spp.) are a popular feature in many landscapes, prized for their vibrant summer flowers and attractive bark. Despite their general resilience, these ornamental trees are increasingly threatened by scale insects, particularly the invasive crepe myrtle bark scale (CMBS). The adult insects shelter themselves under a dense, waxy, felt-like covering, making them difficult to eliminate with typical contact insecticides. Treatment requires a strategic, multi-faceted approach.
Recognizing Scale Infestation and Sooty Mold
The presence of crepe myrtle bark scale is often first identified by a black discoloration on the trunk and branches. This darkening is caused by sooty mold, a fungus that grows on a sugary substance called honeydew. Scale insects excrete honeydew as they feed on the tree’s sap.
Closer inspection will reveal the scale insects themselves as small, white, or gray encrustations, typically clustered in the crevices of the bark, at branch crotches, and around pruning wounds. If crushed, the adult females exude a distinctive pink or crimson fluid. While heavy scale infestations rarely kill a mature tree outright, the sooty mold covering the leaves can interfere with photosynthesis, which weakens the plant and significantly reduces the quality and quantity of summer blooms.
Initial Steps: Mechanical Removal and Cultural Control
For smaller trees or light infestations, non-chemical methods offer a simple first step in control. Manually scrubbing the infested bark with a soft-bristled brush and a mild solution of soapy water physically removes many adult female scales and their egg masses. This process also cleans off sooty mold, immediately improving the tree’s appearance and allowing for easier monitoring.
A strong, direct stream of water from a garden hose can also be used to dislodge crawlers and some adult scales from the trunk and limbs. Since the scale tends to congregate on smaller, heavily infested branches, targeted pruning to remove these specific sections can help reduce the overall population. Cultural practices also play a role in limiting pest pressure; maintaining good air circulation around the tree and ensuring it is planted in an area with full sun reduces stress and discourages pest establishment.
Targeted Treatment Using Chemical and Biological Agents
When infestations are widespread or severe, chemical treatments become necessary. One option is the application of horticultural dormant oils, which work by suffocating overwintering nymphs and adults. These oil products should be applied thoroughly in late winter or early spring before the tree breaks bud, ensuring the spray reaches all bark crevices where the insects hide.
Systemic insecticides, particularly those containing active ingredients like dinotefuran or imidacloprid, are the most effective control method. These products are applied as a soil drench around the base of the tree, where the roots absorb the chemical and distribute it throughout the plant’s vascular system. As the scale insects feed on the tree’s sap, they ingest the insecticide, providing long-lasting control that bypasses the insect’s waxy shell. Systemic treatments are best applied in early spring as the tree begins to grow, allowing sufficient time for the chemical to move up into the canopy before scale activity peaks.
Introducing or encouraging natural enemies is a complementary approach. Beneficial insects, such as certain species of lady beetles, including the twice-stabbed lady beetle, are known predators of crepe myrtle bark scale. While these predators may not eliminate a heavy infestation quickly enough to prevent aesthetic damage from sooty mold, they are important for suppressing scale populations over time. Avoiding broad-spectrum insecticides helps protect these beneficial species, allowing them to provide natural biological control.
Seasonal Timing and Long-Term Prevention Strategies
Effective treatment timing is closely linked to the scale’s life cycle, specifically targeting the highly vulnerable “crawler” stage. Crawlers are the mobile, newly hatched nymphs that emerge in the spring, typically from mid-April through May. Topical treatments, such as insecticidal soaps, are most effective when timed to coincide with these emergence periods, before the crawlers settle and develop their protective waxy covering.
Soil-applied systemic insecticides should be applied in early spring, just before the leaves emerge. This ensures the active ingredient is fully translocated throughout the tree when the crawlers begin to feed. Long-term management involves cultural practices that promote tree health. Selecting the appropriate planting site with adequate sunlight and avoiding unnecessary stress from over-irrigation are important preventative measures. While research continues, some crepe myrtle species, such as Lagerstroemia speciosa, have shown reduced susceptibility to this pest, suggesting that choosing resistant cultivars may be an option for future plantings.