Aphids are among the most common pests to affect crepe myrtles (Lagerstroemia indica), often appearing during the warm summer months. These small, sap-sucking insects can quickly colonize the newest growth on the tree. The resulting infestation can affect the plant’s appearance and overall health. Addressing an aphid problem on crepe myrtles involves a strategy that moves from identification to immediate physical removal, targeted treatment, and long-term preventative care.
Recognizing Aphid Infestation on Crepe Myrtles
Visual confirmation of an aphid infestation is often made easier by noticing the secondary symptoms they cause. The aphids themselves, typically the crepe myrtle aphid (Tinocallis kahawaluokalani), are small, pale yellowish-green insects, about one-sixteenth to one-eighth of an inch long. They are usually found clustered on the undersides of the leaves, and their feeding can cause new foliage to appear distorted or develop yellow spotting.
A telltale sign of their presence is a clear, sticky residue called “honeydew,” which the aphids excrete after consuming the plant sap. This sugary substance coats the leaves, branches, and often the ground beneath the tree. Soon after the honeydew appears, a black, velvety fungus known as sooty mold will begin to grow on the sticky surface. While sooty mold does not directly infect the tree tissue, it blocks sunlight, reducing the leaf’s ability to photosynthesize.
Immediate Non-Chemical and Physical Removal Methods
Controlling a mild to moderate aphid population begins with physically dislodging them using a strong stream of water. Use a garden hose nozzle set to a focused, high-pressure spray to knock the insects off the leaves. Direct the spray primarily at the undersides of the leaves and the tips of new growth, where aphids congregate.
Perform this hosing early in the day so the foliage has time to dry completely before evening, which helps prevent fungal diseases like powdery mildew. Repeat this physical removal every few days until the aphid population is noticeably reduced. For branches heavily infested and covered in sooty mold, selective pruning can immediately remove the problem area.
Biological control relies on encouraging natural predators, such as lady beetles and lacewings, which are voracious consumers of aphids. Lady beetle larvae, in particular, can consume hundreds of aphids during their development. Avoiding broad-spectrum chemical insecticides preserves these beneficial insects, allowing them to naturally regulate the aphid population.
Choosing the Right Insecticidal Treatment
When physical removal is insufficient, low-toxicity, contact-based products are the next line of defense. Insecticidal soap is a highly effective option that works by breaking down the aphid’s outer protective layer, causing dehydration. Thorough coverage is essential, especially on the leaf undersides, because the product must physically contact the insect to be effective.
Horticultural oils, including Neem oil, also work by smothering the soft-bodied aphids on contact. When applying any oil or soap, do so when temperatures are below 90°F to prevent phytotoxicity (damage or burning to the leaves). Applying in the early morning or late evening ensures the spray dries slowly, increasing its effectiveness.
Systemic insecticides, such as those containing imidacloprid, are an option for severe or persistent infestations, but they require careful application. These chemicals are applied as a soil drench around the base of the tree, absorbed by the roots, and move throughout the plant. Systemic treatments make the plant toxic to sap-sucking insects for an extended period.
Because these chemicals can transfer into the pollen and harm pollinators, application timing is critical. The safest time to apply a systemic product is immediately after the crepe myrtle has finished blooming for the season, or in the very early spring before new growth appears. This timing minimizes pollinator exposure to the insecticide residue.
Cultural Practices for Long-Term Control
Preventing aphid recurrence involves modifying the growing environment to make the crepe myrtle less attractive. High-nitrogen fertilizers encourage a flush of new, soft growth, which is the preferred feeding site for aphids because the sap is rich in amino acids. Instead, select a slow-release fertilizer with a balanced NPK ratio, such as 10-10-10 or 3-1-1.
Fertilize only in the early spring as the leaves emerge, and then again around mid-summer if needed, avoiding excessive feeding. Maintaining a healthy tree also means ensuring proper hydration, as stressed or drought-afflicted plants are more vulnerable to insect attack.
Pruning practices also play a significant role in long-term control by improving the tree’s health and reducing hiding places for pests. In late winter or early spring, remove any branches that are crossing or growing inward toward the center of the tree. This thinning process increases air circulation and light penetration throughout the canopy, which discourages aphids and the development of sooty mold and powdery mildew.