For the central and northern portions of Illinois, this is the time to take notice that the caterpillar stage of Zimmerman pine moth, Dioryctria zimmermani, is actively crawling around on the bark of pine trees. During this period, the caterpillars are highly exposed and susceptible to insecticide-spray applications. It is important to treat at this time before the caterpillars tunnel into pine trees and start the overwintering process. Zimmerman pine moth may feed on all pines; however, they tend to prefer Scotch and Austrian pines. The caterpillar tunnels into pine trees, creating masses of pitch at branch whorls on the trunk or on shoots near the terminal leader. These pitch masses resemble large galls. Excessive tunneling by the caterpillar can kill terminal leaders, thus reducing aesthetic appeal. Heavily infested terminals curve downward, resembling a “fishhook.” Successive attacks by the caterpillar can cause tops to break off, again making the tree unmarketable. Young trees tend to be more susceptible to attack by Zimmerman pine moth and are more attractive to adult females for egg laying. This may be due to stress from transplanting.
The key to managing Zimmerman pine moth with insecticides is to use a high-volume spray to thoroughly soak or saturate the stem and bark, where the caterpillars are located. High-volume spray applications are more likely to penetrate the thick canopy of pine needles, which may prevent sprays from reaching the trunk. Pyrethroid-based insecticides, including permethrin (Astro), are recommended and most effec-tive for controlling Zimmerman pine moth.