Now is the time to be on the lookout for Zimmerman pine moth, Dioryctria zimmermani (Grote), larvae (caterpillars) that are actively crawling on the bark of trees. The larvae are highly exposed and susceptible to an insecticide spray application. Zimmerman pine moth larvae feed on all pines, especially Scotch and Austrian. Larvae bore into trees and create masses of pitch at branch whorls on the trunk or on shoots near the terminal leader. These pitch masses resemble galls. The larvae can kill terminal leaders. Heavily infested terminals curve downward resembling a fishhook. Repeated attacks by larvae can cause tops to break off, making the tree unsalable.
Adults are gray with a 1 to 1-1/2 inch wingspan. The forewings are gray and mottled with a zigzag line pattern of darker gray. The adults are active at night from mid-July to mid-August and can live from 3 days to 2 weeks. Female moths lay between 20 to 30 eggs underneath bark in the whorl region of trees. These eggs hatch in late July to early September into pinkish green larvae with brown heads. The body is covered with small black dots. The larvae are approximately 3/4 inch long when fully grown. They are generally located inside shoots or in pitch masses. Young larvae are active on the outside of the tree in mid-August. These larvae overwinter in bark crevices in silken webs. They emerge from these hibernacula in early April and crawl around on the bark before boring into the shoots and stem. The pupae are brown and found in shoots and pitch masses from mid-July to late August.
Management of Zimmerman pine moth involves sanitation and the use of chemical insecticides. On Christmas-tree plantations, scout regularly by visually inspecting trees for the larvae and then later for pitch masses on the main stem or terminal leader. Prune out damaged wood and injured shoots or remove trees that are showing visible symptoms of Zimmerman pine moth damage. The insecticide chlorpyrifos (Dursban) or dimethoate (Cygon) can be used to control the larvae by spraying the bark and foliage in April or mid-August. The best time to control this insect is in the caterpillar stage before it enters the bark. High-volume sprays should be used to drench the stem and bark, as a thick canopy of pine needles may prevent sprays from reaching the trunk.
Planting resistant varieties of Scotch pine such as the short-needled varieties from Greece, Turkey, and west and south Eurasia may be a long-term alternative option to minimize problems with Zimmerman pine moth.