Throughout Illinois, it is time to be on the lookout for the larvae of Zimmerman pine moth, Dioryctria zimmermani, 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 called a hibernacula. Zimmerman pine moth larvae feed on all pines, pre ferring 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 may 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, most likely due to transplant stress.
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 showing symptoms of Zimmerman pine moth damage. The insecticide permethrin (Astro) or dimethoate (Cygon) can be used to control the larvae by spraying the bark and foliage in April or mid August. The optimal time to control this insect is when it is in the caterpillar stage and before it enters the bark. Once the caterpillar enters the tree, it is too late. 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.
Zimmerman pine moth larvae is susceptible to natural enemies, including a variety of parasites (parasitoids) and predators; however, the 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 to minimize these problems.