I have seen these outbreaks a few times over the years. Sometimes there is little damage to the trees and there are times they are all over with vast amounts of trees being defoliated. According to the Minnesota DNR the forest tent caterpillar population has been on the rise in some northern and west-central Minnesota counties since 2007, but may have reached a peak in 2013 when 1.1 million acres of aspen, oak, basswood, birch and other hardwood trees were defoliated.
Minnesota’s native forest tent caterpillars, sometimes called army worms, have outbreaks every 10 to 16 years. “Based on historic trends, we would have expected the population to reach a peak this year or next year, but have found few egg masses in recent surveys of last year’s infested areas,” said Mike Albers, DNR forest health specialist. “We also found up to 90 percent of forest tent caterpillar cocoons in our sample plots were killed by a native fly, which is more than we expected to be killed at this point in the outbreak.”
It’s still too early to tell if the forest tent caterpillar population is declining. If population increase as normally expected, areas in Minnesota that saw large number of caterpillars last year could have even larger numbers this year, and portions of the state that had fewer caterpillars could also expect to see more. However, based on the results of the egg mass survey this spring, it’s difficult to predict what will happen with forest tent caterpillar populations and the impact they will have this summer.
More information about forest tent caterpillar indentification, impact, management or local predictions can be found at http://www.mndnr.gov/treecare/forest_health/ftc.