There are several types of aphids that feed on conifers. Feeding damage can produce needle discoloration (yellow or white), needle deformation, premature needle drop, stunted growth, or branch death. In addition, aphids secrete sticky honeydew that falls on objects beneath the feeding sites. An unsightly black sooty mold often grows on the honeydew residues.
Species of the genus Cinara are large, long-legged, spider-like aphids that feed on the bark of twigs and main stems of trees. Host plants include Douglas fir, junipers, arborvitae, and various pines. The white pine aphid (Cinara strobi) is regularly noted on eastern white pine in Iowa. Cinara aphids are often tended by ants, which feed on the honeydew and protect the ants from natural enemies.
The spotted pine aphid (Eulachnus agilis) is a smaller green aphid with several black spots on its body. Common hosts include Scots, red, and Austrian pines. This insect is very active and only feeds on pine needles.
The pine bark aphid (Pineus strobi) is actually an aphid relative called an adelgid.
These insects are covered with a white, cottony wax and may be found feeding on the bark of the trunk and larger branches, on the bark of twigs, or at the base of the needles. Trunks of heavily infested host trees (eastern white, Scots, and Austrian pines) look white (as if painted) because of the fuzzy covering on the insects.
Aphid infestations on conifers rarely require treatment because their populations are kept in check by natural controls (natural enemies, host plant resistance, and weather). Remedial action for aphids would be justified if: 1) aphid populations are high; 2) significant host plant damage is anticipated; and 3) the timing is appropriate. Determining whether control options are needed begins with frequent and thorough inspection of the affected host plants. When aphid populations are increasing and natural enemies are not feeding on the aphids, remedial controls are justified, listed in order of preference and increased environmental impact.
The least toxic, lowest impact control method is to use a forceful water spray to dislodge aphids from host trees. Thoroughly spray infested plants parts (e.g. foliage, stems) with sufficient pressure to knock the insects off the host, but not injure the host plant. You must repeat this type of application because there is no residual effect. Another low impact control option is insecticidal soap, which is available from garden centers, hardware stores, and department stores under various trade names. Look for a product that lists as the active ingredient potassium or sodium salts of fatty acids. Horticultural oil is also an effective treatment material against aphids. Traditional insecticides used in the home landscape for aphid control on conifers include Isotox, Malathion, and Sevin.
This article originally appeared in the 5/16/2003 issue.