Typical soft-bodied aphid on pine US Forest Service
Aphids, when in small numbers, do little damage to a tree, however, under favourable conditions the aphid population can grow very rapidly and cause serious damage to the tree during the growing season. Aphids attack trees by sucking the sap out of the leaves. The symptoms are very visible on the leaves in the form of multiple puckered marks, yellowing, and twisting of the leaves which gives the appearance of deformed leaves. As the severity of the aphid infestation increases, leaf drop and twig and branch die back can be seen.
Often during an aphid infestation, leaves appear to be dripping sap from the underside. This is actually an excretion from the aphids and is called honeydew. It often drips onto other leaves, other plants and on to the ground. The honeydew then becomes an attractant to ants which feed on it. In most cases the ants are only symptoms of the honeydew and are not actually attacking or hurting a tree.
Recommended Way to Control Aphids:
Aphids can be controlled using newer and safer insecticides, rather than older more harmful chemicals. For major outbreaks spray the tree with either Bug Buster or Trounce. The spray should be directed at the undersides of the leaves and other areas of visible feeding and insect concentrations. Normally only one or two spray treatments are required to achieve control. For less severe infestations or as a preventative treatment, spray the leaves with Insecticidal Soap in the early summer and as needed. For trees that are too tall to spray, use the Once-A-Year Insecticidal Drench w/Merit