Forest Insect and Disease Newsletter

How forest tent caterpillar populations build outbreaks

We always knew that the really large FTC outbreaks in the northern forests were synchronized with simultaneous outbreaks in NW Ontario, Wisconsin and Michigan. But, we didn't realize how the FTC populations near Mille Lacs and along the west-central counties contributed to the big picture until our aerial survey started mapping individual stands in 1995. Since then we've been able to follow the progress of two FTC outbreaks as they are building up. See maps below.

Forest tent caterpillars outbreak map 1998 to 2001

Forest tent caterpillars outbreak: 1998 to 2001

Here's what we are finding:

  • There are always some populations of FTC active in every year in the west-central counties, usually in oak and basswood stands. Outbreaks usually don't last very long in these stands.
  • There are two locations in central MN where populations build-up and spread out in advance of the northwide outbreaks. They are (1) northern Kandiyohi County and (2) along the SE shores of Mille Lacs Lake and in the Rum River Forest in Mille Lacs County.
  • Northwide outbreaks seem to spill out of the districts in NW Ontario and SE Manitoba into the northern counties of MN, somewhat like waves of an invasion that causes defoliation in aspen and birch forests.
  • Several years prior to a northwide outbreak, the east central population builds up and intensifies locally. The west-central populations leap-frog north from Kandiyohi into Pope, then Ottertail, Becker and Polk Counties.
  • Just about that time, the northwide outbreak begins and within a couple of years, MN forests experience two to seven million acres of FTC defoliation. Let's hope for the minimum this time.
  • Then the entire population collapses right before our eyes and all we are left with is a few billion friendly flies and some tiny FTC populations down in Pope and Kandiyohi Counties still causing defoliation.
Forest tent caterpillar outbreak: 2007 to 2012

Forest tent caterpillar outbreak: 2007 to 2012

It will be interesting to see what the next two or three FTC seasons bring us and if populations are building up in Canada.