Guess what? It is that time of the year to be on the lookout for the notorious Zimmerman pine moth, Dioryctria zimmermani, larvae or caterpillars that are actively crawling on the bark of trees. During this period, the larvae are highly exposed and susceptible to insecticide-spray applications, after overwintering in bark crevices in silken webs often referred to as hibernacula. Zimmerman pine moth larvae feed on all pines; however, Scotch and Austrian pines are most susceptible. The larvae bore into trees and create masses of pitch at branch whorls on the trunk or on shoots near the terminal leader. The 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 the larvae can cause tops to break off, making the tree unsalable. Young trees are more susceptible to attack by the larvae and are more attractive to adult females for egg laying. This is most likely due to stress from transplanting.
When managing Zimmerman pine moth with insecticides, it is important to use high-volume sprays to thoroughly “soak” the stem and bark, where the larvae are primarily located. Additionally, high-volume spray applications are more appropriate because a thick canopy of pine needles may prevent sprays from reaching the trunk. Pyrethroid-based insecticides such as permethrin (Astro) are recommended and most effective for controlling Zimmerman pine moth.