They secrete huge amounts of sticky honeydew, resulting in black sooty mold. This in turn attracts ants, which won't damage your tree but are a good indication that you have a sap-sucking insect ...

This honeydew will then eventually turn black as it is colonized by fungi called sooty mold. Crape myrtles are the most popular summer flowering tree in the U.S. Although rarely fatal to the trees ...