Forest Insect and Disease Newsletter

Forest tent caterpillar population is building in 2013

The forest tent caterpillar (FTC), Malacosoma disstria, is a native defoliator of hardwood trees. These caterpillars feed primarily on aspen, birch, basswood and oaks. Defoliation will normally be obvious by early June and finished by mid– to late–June. Moths will fly in late June and early July. Outbreaks are three to eight years in duration and, in any given location, defoliation is usually noticeable for two to four consecutive years.

Acres defoliated by forest tent caterpillar 2005-2012

Potential Forest tent caterpillars activity for 2013

Northwide populations peak every ten to sixteen years. And looking at the charts, it looks like these populations could be building towards a 2014 or 2015 peak level of defoliation. During a recent peak year, 2001, over 7.5 million acres of hardwoods were defoliated by FTC.

During outbreak peaks, forest tent caterpillars can number from one to four million caterpillars per acre. They create a nuisance to people living or vacationing in forested areas from late May to mid-June. Luckily there is only one generation per year so that defoliation only occurs once a year.


Forest tent caterpillars outbreak map 2001

In the forest, defoliation from FTC usually causes little damage to aspen tree health and vigor. Most hardwoods develop a second set of leaves by mid-July. Oaks and birches that are also suffering drought stress, root damage or are over-mature become vulnerable to additional "secondary" insects which can kill them in one to three years. FTC play critical roles in structuring aspen mixed-wood forests. A recent study by the University of Minnesota showed that the repeated defoliation of aspen mixed-wood forests by forest tent caterpillars is linked to forest dynamics, how forests grow in Minnesota.

Acres defoliated by foreset tent caterpillars, 1985 to 2012