Dozens of species of aphids may be found on shade trees and woody ornamental plants in Wisconsin. Aphids are small insects; they are typically less than 1/8 inch long, although some aphids may reach almost 1/4 inch. Aphids come in a variety of colors, from bright orange to green or gray to red. One common group, woolly aphids, produces an abundance of flossy, waxy threads that cover their bodies. Aphids feed on plants by sucking plant sap from the leaves, twigs or stems. When abundant, aphids remove large quantities of sap, reducing growth and vigor of the plant. This injury is most common with stem or trunk infesting aphids. Aphids can also cause leaves to curl. Most aphids excrete large quantities of a sweet, sticky substance called honeydew. At times excessive honeydew dropping from trees can be an extreme nuisance.