Forest Pests: Insects, Diseases & Other Damage Agents

Importance. - Scale insects are common pests of ashes and are distributed throughout the United States. These insects suck the sap from plants, weakening them and making them susceptible to other pests. Scales often kill branches and occasionally individual trees.

Identifying the Insects. - Scale insects appear as protrusions on the branches and twigs. Mature oystershell scales are 3 mm long, brownish to purplish gray, narrow, and rounded toward the rear with concentric bands. White peach scales are 1.5 mm wide, circular, and white to gray or yellowish. European fruit lecaniums are 4.7 mm wide, circular, reddish to dark brown, and often covered with white powder.

Identifying the Injury. - Crown dieback is the principal symptom of severe infestations. Honeydew may be produced, which supports a black, sooty mold that turns leaves and branches black. Heavily infested plants produce pale foliage and few new shoots and exhibit twig and branch dieback.

Biology. - Females lay eggs under waxy domes. The newly hatched nymphs, called crawlers, crawl to new sites on the bark and begin feeding. The oystershell scale overwinters in the egg stage. Two generations occur in Maryland, where crawlers are present in May and June and again in mid-July. The white peach scale has four generations per year in Florida and three in Maryland. The crawlers are present in May, July, late August, and early September. The European fruit lecanium produces one generation per year.

Control. - Contact insecticides are effective when timed with the appearance of crawlers. Dormant oil sprays will kill overwintering scales.