Now is the time to be on the lookout for the larval stage (caterpillar) of Zimmerman pine moth, Dioryctria zimmermani, which 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; however, they prefer Scotch and Austrian pines. The larvae bore into trees and create masses of pitch at branch whorls on the trunk or on shoots near the terminal leader. Excessive tunneling by the larvae can kill terminal leaders. Heavily infested terminals curve downward, resembling a fishhook. Repeated trunk attacks by the larva can cause tops to break off. 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).

The caterpillars must come into contact with the spray before they enter the bark. Once they enter the tree, it is too late. The insecticides permethrin (Astro) and dimethoate (Cygon) can be used to control the caterpillar by spraying the bark and foliage. Dimethoate is no longer labeled for use in landscapes but can be used on Christmas tree farms. 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.