White, tan, or rust colored resin flowing on the trunks of Austrian, Scots and red pines could indicate an infestation of Zimmerman pine moth (ZPM) caterpillars. Large numbers of these small caterpillars can bore into the trunks of pines, resulting in weakened trees that are susceptible to wind damage. Heavy infestations also impart an unsightly appearance to the trunk and act as a source of plant stress. One or two boring points on an average sized tree is little cause for concern. Heavier infestations, or attacks against small or key landscape trees should be treated through a Total Tree Care approach. This would include cultural, natural and chemical procedures.
An understanding of the ZPM life cycle is critical for proper management. The insect overwinters as a tiny caterpillar in a silken cocoon-like structure located under bark scales. In early spring, the small caterpillars first begin to feed on the bark. Later, they will tunnel into lateral or terminal shoots or directly into the main trunk. In late spring or early summer, caterpillars that may have been feeding on lateral shoots move to the main stem and bore into the trunk, usually in the whorl area.
Resin, flowing from the insect tunnels or pushed out by the caterpillars, accumulates at the mouth of the tunnels on the trunk and scaffold branches. Fresh resin flows will be white to tan, have the consistency of lard, and have a shiny appearance. Resin flows from previous years will be yellow to gray, but will be hard and crystallized and have a dull appearance. Resin flows can persist on trunks for several years. It is important to be able to recognize active infestation sites and not be fooled by older resin flows.
The caterpillars remain inside the main trunk until they complete development. In mid summer, the caterpillars pupate either inside the external resin flows or within their tunnels. The adults emerge as small, gray/brown moths with banded wings. Peak emergence usually occurs about mid to late August. The moths fly at night and are rarely seen.
After mating, the females lay their eggs on the trunk underneath bark scales. After emerging from the eggs, the tiny caterpillars search for sites in which to overwinter, thus beginning a new cycle.
Learn more about Zimmerman Pine Moth management.