Forest Insect and Disease Newsletter

The value of FTC outbreaks: A forest ecosystem perspective

With the inevitable return of the Forest Tent Caterpillar (FTC) apparently just beginning, you might ask is this good or bad? Most people would not hesitate to say that the nuisance, mess and dramatic visual effect were just bad. But how about viewing FTC outbreaks from the perspective of the other plants and animals in the forest ecosystem? Are outbreaks good, bad or just plain ugly? As you might expect the answer is: it depends.

Some birds and other animals benefit from an outbreak of FTC by using the caterpillars as a food source. It's kind of like having a fast food kitchen in your living room. Many species of birds move north in the spring and early summer to raise their young. This is the time when the FTC caterpillars are present. During the time of mating and raising their young, most birds need a diet high in protein and rely heavily on a diet of insects. Adults birds may eat up to their body weight per day and young birds may eat more than ½ of their weight per day. Populations of cuckoos increase during FTC outbreaks probably as a result of the abundance of caterpillars as food. Some mammals also consume FTC. Black bears are known to break down small trees to get at the caterpillars and eat them.

But what is good for some is bad for others. To some birds, an outbreak is harmful, at least temporarily. Heavy defoliation of trees and shrubs removes the protective cover of tree leaves and changes the microclimate. It's kind of like having the roof torn off your house. Nests that normally are hidden by leaves are now exposed and more visible to predators. Also during the day in a defoliated forest the temperature is higher and the humidity is lower than normal possibly reducing the survival of young birds. Defoliation of oaks reduces the production of acorns affecting animals like squirrels, turkeys and bears that rely on them as a food source in the autumn.

An FTC outbreak obviously affects the trees and the most common effect of defoliation is a growth reduction. In a study of defoliated aspen, researchers found little growth reduction during the first year of heavy defoliation, 90% growth reduction during the second year, 90 percent growth reduction during the third year of heavy defoliation and 15 percent reduction the year after the defoliation ended. So in a typical outbreak you might expect at least two years of "almost no growth". In the past twenty years there have been two outbreaks in Minnesota. If we assume no tree growth of aspen for two years in each outbreak that makes four years of no growth in the past twenty years. That equals 20 percent less wood production.

For most host trees, a growth reduction is the only impact of a normal outbreak. Some tree mortality can also be expected if the outbreak extends for five to seven years or if other factors, such as a drought or construction damage, stress the defoliated trees.

Forest tent caterpillars don't feed on all trees equally. They prefer aspen, birch, basswood, and oaks, but generally don't feed on balsam fir, spruces, pines or red maples. These conifers and red maples often develop in the understory beneath the canopies of aspen, increasing in numbers as the aspen stand matures. In normal years (non-FTC years) these species are shaded by the overstory trees. When FTC defoliate the overstory trees the understory non-defoliated trees receive more sunlight and are able to put on additional height and diameter growth. Also, since the overstory trees have no leaves, evapotranspiration is less. This leaves more moisture in the soil for the fir, spruce, red maples and pines to use.

Anyone who has been in the woods during a FTC outbreak knows not to eat an open faced peanut butter sandwich there. It might sound like rain but pattering is caused by caterpillar frass (excrement) from the caterpillars feeding above. This frass serves as fertilizer just like any other manure. It adds nutrients, especially nitrogen, to the soil. As a result, the understory conifers have more sunlight, moisture and nutrients, put on more growth and take a more dominant position in the stand. Forest tent caterpillars act as agents of change and can speed up the ecological succession of a stand. Recently, researchers from the University of Minnesota-Department of Forestry have studied this relationship. In fact, FTC along with spruce budworm, a defoliator of balsam fir and white spruce play critical roles in the development and appearance of the forests in Minnesota. More information

These are just some of the consequences of a FTC outbreak. Outbreaks are a natural part of the ecosystem and something we have to put up with them if we want to live, play or work in the forest. It can be beneficial as well as harmful. It just depends on your perspective.