Throughout Illinois, it is time to be on the lookout for Zimmerman pine moth, Dioryctria zimmermani, larvae (caterpillars) actively crawling on the bark of trees. The larvae are highly exposed and susceptible to an insecticide spray application, after overwintering in bark crevices in silken webs often referred to as hibernacula. Zimmerman pine moth larvae feed on all pines, particularly Scotch and Austrian. The 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. Excessive tunneling by the larvae can kill terminal leaders. Heavily infested terminals curve downward, resembling a fishhook. Repeated trunk attacks by larvae can cause tops to break off, making the tree unsalable. Young trees are more susceptible to attack from the larvae, and more attractive to adult females for egg-laying--probably due to stress from transplanting.
Management of Zimmerman pine moth primarily involves sanitation and the use of 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. Newly purchased chlorpyrifos will not be labeled for use in residential areas. The optimal time to control this insect is when it is in the caterpillar stage and 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.
Although Zimmerman pine moth larvae are susceptible to natural enemies, including a variety of parasites (parasitoids) and predators, numbers are not sufficient to impact the population and prevent damage.
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.