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Vermont Officials Tracking Outbreak Of Caterpillar That Feeds On Sugar Maple

During aerial surveys of the state’s forests this summer, the Vermont Department of Forests, Parks and Recreation found evidence of the forest tent caterpillar in trees in northeastern Vermont.  Officials are concerned because the bug feeds on the sugar maple, a key agricultural industry of the state.
The forest tent caterpillar has been seen in Orange and Windsor counties for the first time in 10 years. It primarily feeds on the leaves of sugar maple and white ash trees. Vermont Department of Forests, Parks and Recreation officials say the caterpillar lays its eggs in masses of 150 to 200 in tree branches each July. But because the eggs are laid so high above ground, it's unknown how widespread the outbreak might be.

This infestation was discovered during aerial surveys the state conducts under a national program from the U.S. Forest Service.  Vermont Department of Forest Parks and Recreation Protection Forester Dan Dillner says summer surveys found evidence that the native insect pest has emerged in northeastern areas of the state.  “It’s scattered somewhat throughout the northeast and in central, sort of eastern area.  But some of the, without looking at the full list in front of me towns would that stick to mind,  a lot in Orleans County. So Irasburg, Lowell, Albany, Craftsbury, Eden, Belvedere, Waterville . There’s a bunch of other towns. Quite a bunch down in Orange County as well.”

Full results of the aerial surveys are being compiled for the state to review the extent of the outbreak.  Dillner notes that forest tent caterpillar outbreaks are cyclical.  “The last big outbreak was around 2004 – 2005 and a little into 2006. So it’s common.  And it will cycle again. It doesn’t usually kill the trees one hundred percent. Usually they’ll be defoliated and most of the time the trees will refoliate somewhat.  Throughout the summer this year because of the drought a lot of the trees haven’t refoliated all the way. But most of the trees are recovered. The trees that are already weakened they may die, but primarily they’ll recover.”

While the trees don’t necessarily die, Vermont sugarmakers keep close track of the  pest’s emergence.  UVM Extension Maple Specialist Mark Isselhardt has been in the field checking the trees and photographing the defoliation caused by the caterpillar.  “We started getting reports about browning or defoliation in June.  And when you go out to these woods in some areas the defoliation can be close to one hundred percent.  They favor sugar maple, white ash and a few other species. Although when those are not very abundant or they've already consumed those leaves, they’ll eat just about anything. The important thing to remember is that these populations are here year after year. But only on a cycle of maybe ten to twenty years do you see a large outbreak and a widespread defoliation in a given area.”   

Isselhardt says the science is limited as to the impact on maple sugar production.   “The big impact on sugar maple, or really any tree that’s been defoliated, will be a pretty dramatic decrease in the radial growth. There will be a slight increase in mortality. There are some indications that sugar sweetness will be lower in the coming year, the coming season. It's really hard to say for sure. Some people have seen no difference in the sap sweetness.  There just hasn't been a lot of really controlled studies looking at that. And part of it has to do with the cyclical nature of this insect.”

The last major outbreak in Vermont resulted in about 10 percent defoliation of the state’s forests.