Crape myrtle aphid is one of the most common insect problems that occurs on crape myrtles.

The aphids overwinter as eggs, which hatch in April. The aphids have multiple generations throughout the remainder the of the growing season. Additional generations are produced by the aphids giving birth to living young. Aphids mature in about 10 days. Both winged and wingless aphids are born during the growing season. Winged forms fly to start new colonies on other crape myrtles.

Aphids insert their mouthparts into leaves and suck out the sugary sap. As a result of ingesting all this sap, they excrete a sweet, sticky liquid called honeydew. This honeydew can completely coat leaves and other objects below (yes, your cars!). The honeydew can attract ants, flies, wasps or other insects and can form a substrate for the sooty mold fungus which causes a black coating on the tree. The fungus itself does no harm to the tree, but is unattractive.

Frequently, populations can be kept under control by natural predators. However, the proliferation of mosquito sprays has caused a decline in beneficial insects. 

The problem aphids can be treated with pesticide products.  Products that contain pyrethroids, carbamate, or organophosphate are recommended and may cause less harm to non-target insects than other formulations. Always follow label instructions.  In addition, horticultural oils can be applied in the fall after leaf drop to prevent the eggs from hatching and starting the problem again. Sprays of water on the aphids may also help to knock down the population to manageable levels.  

This link includes a table with recommended pesticides: https://content.ces.ncsu.edu/crapemyrtle-aphid-2