This is the time of year to “really” be on the lookout for the larval or caterpillar stage of Zimmerman pine moth, Dioryctria zimmermani, actively crawling on the bark of trees. The caterpillar stage is highly exposed and susceptible to an insecticide spray application. Zimmerman pine moth larvae feed on a wide range pines; however, they seem to prefer Scotch and Austrian pines. Larvae bore into trees and create masses of pitch at branch whorls on the trunk or on shoots near the terminal leader. Extensive tunneling by the larvae may kill terminal leaders. Heavily infested terminals curve downward, resembling a fishhook. Repeated attacks at the trunk, by the larvae, can cause tops to break off, which may ruin the aesthetic appearance of trees. Young trees are more susceptible to attack and more attractive to adult females for egg laying, most likely due to transplant stress.
It is imperative that exposed caterpillars come into contact with insecticide sprays before they enter the bark. Once they enter the tree, it is too late. The insecticide permethrin (Astro) can be used to control the caterpillar by spraying the bark and foliage. It is recommended to use high-volume sprays to drench the stem and bark because a thick canopy of pine needles may prevent sprays from reaching the trunk.