Forest Tent Caterpillar: good, bad or just ugly?
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 was just bad. But how about from the perspective of the other plants and animals in the forest ecosystem. Is it 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. 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 of 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. 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.
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 a little growth reduction during the first year of heavy defoliation, 90% growth reduction during the second year, 90% growth reduction during the third year of heavy defoliation and 15% reduction the year after the defoliation ended. So in a typical outbreak you might expect at least two years of "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% less wood production.
For most host trees, a growth reduction is the only impact in 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,. stress the trees.
Forest tent caterpillars don't feed on all trees equally in the forest. The caterpillars prefer aspen, birch, basswood, ash and oaks, but generally don't feed on balsam fir, spruces, pines or red maples. These trees 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 it's really insect frass (excrement). This frass serves as fertilizer just like any other manure. It adds nutrients, especially nitrogen, to the soil. So 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.
These are just some of the consequences of a FTC outbreak. It is a natural part of the ecosystem and something we have to put up with if we want to live, play or work in the forest. It can be beneficial as well as causing damage. It depends on your perspective.
But an outbreak is still an ugly, messy, nuisance.