Leaf curling and discoloration is actually not the main problem caused by aphids on most ornamental landscapes. In fact, the main problem is more of an aesthetic issue called Sooty Mold. Aphids excrete a sticky, shiny substance called "honeydew" all over the plants they feed upon. Later in the Summer, this honeydew is colonized by sooty mold fungi. The fungi feed on the honeydew and not the plant, so very little damage is actually done to the plant. The sooty mold turns much of the plant soot black. The honeydew from the aphids also tends to coat anything under the host plant. It is not uncommon to see your patio furniture, grill, or deck turn sooty black late in the summer if you have a crepe myrtle with aphid activity overhanging any of these things.