Bill Barnacle, Silviculturist and Susan Burks, Forest Health Specialist
Gypsy moths are exotic, defoliating insects whose feeding can contribute to significant tree mortality. Currently, the moths are well established in eastern Wisconsin and are moving westward. The moth is not yet established in Minnesota largely because of the eradication efforts of the Minnesota Department of Agriculture. However, it is just a matter of time, perhaps five to ten years, before the moth becomes permanently established in Minnesota.
Landowners can minimize the impact of gypsy moth through active forest management. Because trees are long lived and slow growing, forest management practices are most effective when applied well in advance of gypsy moth defoliation. Forest managers are advised to begin now.
Once the moth becomes established, the most important factor affecting a forest's susceptibility to defoliation is the proportion of the forest made up of tree species that gypsy moth caterpillars prefer to eat (see table). Stands dominated by preferred species are defoliated at higher rates, more often, and for longer periods of time than stands composed of avoided species.
Site conditions and individual tree vigor play a role in how much tree mortality occurs after defoliation. Only a portion of those trees defoliated is at risk of mortality. A stand of trees that has a high proportion of preferred species and trees under stress or of low vigor, are at risk of significant mortality. Vulnerable sites are characterized by frequent droughts, slow growth and low amounts of foliage in the crowns. Ridge tops, and sites with shallow or overly dry sandy soils are examples of vulnerable sites.
The greatest single indicator of the likelihood of tree mortality is a tree's physical condition at the time of defoliation. A tree with a full crown and only a few dead branches has a good chance of surviving defoliation. A tree with a small crown and 50 percent or more dead branches has a poor chance of survival.
There is a similar correlation with a tree's crown class or its position in the canopy. A dominant tree that gets lots of sunlight has a good chance of survival. A suppressed tree that gets little direct sunlight has a poor chance of survival.
When gypsy moths become established in Minnesota and defoliate large areas, the repeated defoliation and tree mortality will likely shift susceptible stand composition away from oaks and other preferred species toward non-preferred species. On rich, good sites, species such as red maple, sugar maple and green ash may replace lost oaks. On drier, nutrient-poor sites, where seed sources occur, red and white pines may replace the oaks. In northern Minnesota, balsam fir will likely increase in number.
There are two primary strategies in forest management that can help mitigate future damage due to gypsy moth defoliation. The first strategy involves reducing the likelihood of defoliation by reducing the percent of preferred host species through selective thinning. While reducing the component of preferred hosts won't prevent defoliation, it can lessen the severity and shorten insect outbreaks. The second strategy aims to reduce tree mortality following gypsy moth defoliation by increasing tree and stand vigor. This can also be done through thinning. Remove damaged and suppressed trees and encourage non-preferred species where appropriate. Maintain host stand diversity and healthy preferred host species as an important component of the stand.