Although aphids seldom kill a plant, the damage and unsightly mold growth they cause sometimes warrant control. Aphids cause curling, yellowing, and distortion of leaves and stunting of shoots. In urban environments, aphids can produce copious amounts of “honeydew” excretion, which often turns black with the growth of a sooty mold fungus. Sooty mold is usually just unsightly, coating leaves with a black residue, but can kill a small tree that is already under stress.
Check plants frequently for aphids, including the undersides of leaves. Look for curled green leaves and/or wilted buds. Many species of aphids cause the greatest damage when temperatures are 65 to 80 degrees Fahrenheit. The presence of ants often indicate aphids, because ants act as “farmers,” protecting the aphids, in order to harvest their honeydew excretions.