(News article for July 17, 2020)

People sometimes find that the leaves of their crape myrtles have a black substance on them. This is sooty mold, and it grows where insects have secreted a sugary substance called honeydew. When crape myrtle leaves are covered in sooty mold, the crapemyrtle aphid is the likely culprit.

Note that if it’s the trunks instead of the leaves that are covered in sooty mold, it’s probably crapemyrtle bark scale insects rather than aphids that are causing the problem. I’ll address these next week.

Aphids tend to be more problematic when trees have excessive nitrogen, so avoid overfertilizing.

Predatory insects such as lady beetles and green lacewings often help keep aphid populations smaller than they otherwise would be. Try to avoid spraying broad-spectrum insecticides that kill these beneficial insects.

If you have a lot of aphids on your crape myrtle and consider the amount of sooty mold to be unacceptable from an aesthetic standpoint, there are insecticide options. (Be aware that, once insects are killed, sooty mold will still remain on the plant for some time.)

Aphids are soft-bodied insects that are susceptible to insecticides like insecticidal soaps and horticultural oils. The fact that insecticidal soaps (“potassium salts of fatty acids”) and horticultural oils work only on direct contact with insects is both an advantage and a disadvantage. It’s an advantage from the standpoint of protecting beneficial insects: There is no residual on the plant to harm beneficial insects that come along after the product has dried. However, this lack of residual activity also means that the aphids or other pest insects won’t be killed if the insecticide doesn’t contact them directly.

Therefore, when using insecticidal soaps or horticultural oils, good coverage (including of undersides of leaves) is essential. Of course, large plants pose a challenge in this respect.

Be sure to follow label instructions when applying insecticidal soaps or horticultural oils to avoid injuring plants.

Another option for crapemyrtle aphids is to use one of the acephate insecticides that can be mixed into a slurry and painted onto crape myrtle trunks. Acephate moves systemically within the plant, to an extent. Not all acephate products are labeled for use in this manner, so be sure to check the label of any product you plan to buy.

A final option I’ll mention for managing aphids on crape myrtles is to use one of the soil-applied imidacloprid products. There are several of these labeled for use on ornamental trees and shrubs.

Imidacloprid is able to move from roots to stems and leaves. Clearly, being able to apply an insecticide around the base of a plant and have it taken up into the plant is an advantage when trying to manage insects on large trees and shrubs.

Using an insecticide that is applied to the soil and taken up by the roots also has benefits from a beneficial insect standpoint. Pest insects come in contact with the insecticide when they feed on the plant, while exposure of beneficial insects that don’t feed on the plant itself is minimized.

However, there are concerns about bees being exposed to imidacloprid in pollen and nectar. Because of this, some imidacloprid product labels require that the product not be applied before or during flowering. So, if you plan to use an imidacloprid-containing product on trees or shrubs, wait until after they’ve finished blooming. On crape myrtle, this is a limitation from a pest management standpoint, since they don’t bloom until summer, and it takes some time for imidacloprid to be taken up into plants.

Be sure to read and follow label directions when using any insecticide.

Let me know if you have questions.

Contact Mary Helen Ferguson.

Crape myrtle leaves with aphids, shed aphid skins, and sooty mold. (Photo by M.H. Ferguson)

Crape myrtle leaves with aphids, shed aphid skins, and a lady beetle larva. (Photo by M.H. Ferguson)