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Feature ArticleAugust 29, 2001

Gypsy Moth and Forest Management

By Susan Burks, Metro Region Forest Health Specialist

It's the trapping season. Not for beaver, but for the gypsy moth. You have probably seen the orange, tan or green triangular shaped boxes hanging from trees and fence posts around the state. These are used to detect new introductions of the pests and to define the boundaries of any suspected populations. So far, Minnesota is lucky because the moth has not yet become established in the state. However, Wisconsin is in the throws of gypsy moth suppression as populations build to defoliating levels in some parts of the state. As those populations build, they will spread and MN will get to deal with the moth soon. But not just yet.

We will likely have 15 years or more before we have defoliating populations of gypsy moth. That means, we have time to manage our forests to minimize the effects of defoliation. People with wooded lots ought to start now to improve the chances of sustaining susceptible forest resources in the face of the upcoming invasion. Formal guidelines are not yet ready for print, but will be soon. In the meantime, here are some things you can do.

First, determine if your forests are likely to be defoliated by the gypsy moth. Stands with less than 20% oak or aspen are not likely to be defoliated. Those with 20-50% oak or aspen will see some defoliation, but it is not likely to be severe. Those with more than 50% oak will likely be defoliated at some point. Stands with an aspen understory and an oak overstory will likely see a more rapid build up of gypsy moths and thus more defoliation.

Second, determine if your forests are likely to see much tree mortality as a result of defoliation. Although aspen is a preferred host, their leaf chemistry fosters very short-lived population cycles. That means gypsy moth populations crash before aspen trees are overly stressed and significant tree mortality among aspens as a result of defoliation is rare. However, we aren't sure how forest tent caterpillars and gypsy moths will interact. If major outbreaks of the two pests closely follow each other, we may see higher rates of mortality than seen elsewhere. Otherwise gypsy moth management is thought to be a fairly minor concern in aspen management.

Oaks are another story. Tree mortality is a function of the number of years of consecutive defoliation and tree vigor prior to defoliation. Vigorously growing trees are less likely to die as a result of significant defoliation. Factors that affect tree vigor will affect tree survival. So new transplants and very old trees are more vulnerable. Damaged or weakened trees are more vulnerable. Crowded trees are more vulnerable. Young to middle-aged, well-spaced stands are not likely to see much tree mortality.

Third, determine what your land use goals are and if losing some oaks will hamper your goals. When trees die, other trees move into that space or utilize those light and water resources. So losing some trees is not necessarily bad. Some wildlife also like dead and dying trees. But others like deer and turkey are dependent on oak for food and shelter. If the composition of your stand shifts away from oak, can you still accomplish your land use goals?

If you have large acreage, it is best if you get the help of a professional. But once you've answered the questions above, you're ready to outline what management practices will work for you.

Diversifying your stands to reduce the oak/aspen component will help limit the amount of defoliation that occurs. That isn't possible in all stands, but in many stands practices that remove some trees (those gypsy moths like) and leave others (those they don't) will gradually shift the species composition to a mix that is less favorable to the gypsy moth. The result will be less defoliation when the moth gets here. What you leave, needs to represent what you would like to see in the future both in terms of the trees and the wildlife that use them. In those cases where you have few management options, a complete conversion to pine or prairie may make more sense. Careful thought about what you'd like to have in twenty years will help determine what course you take.

Increasing the vigor of your stand will help preserve the trees you have. Thinning is a relatively easy way to increase tree vigor in a dense stand, because it increases the light and water available to the remaining trees. Removing dead and dying trees (except those needed for wildlife). Remove trees crowding those you want to save. Leave healthy specimens of tree species, like ash and walnut that gypsy moths don't like. Talk to a professional about how to protect residual trees during thinning operations and get a copy of the booklet on Best Management Practices from your local DNR office. Pruning specimen trees in an urban setting increases tree vigor. And, in both situations, avoid wounds and equipment/ construction damage.

When the moth becomes established in MN, it will become a problem. People will take positions of all sides of the management debate and discussions will be fierce. Politicians and the media will jump into the free-for-all. But if you start now, you can manage your resources to minimize the impact on your lands. Then you can sit back and enjoy the entertainment.