By Linda Williams, forest health specialist, Woodruff. Linda.Williams@wisconsin.gov; 920-360-0665

The tops of aspen leaves will appear off-colored when aspen blotch miner caterpillars feed within. Photo: Linda Williams

If you’ve noticed aspen trees seem a little pale lately, you are probably seeing damage from aspen blotch miner caterpillars. Typically, these leaf-mining insects only affect young aspen trees, but this year I’ve found uniform damage on bigger trees with large crowns. Symptoms include thinned crowns, off-color leaves with blisters on their undersides, and, later in the summer, curling and browning leaves. Tiny caterpillars spend their entire lives feeding within the leaf; they then pupate into the tunneled-out areas. Moths emerge in August and spend the winter in protected places.

I have reported this insect each year since 2012 in northeastern Wisconsin. This year, there were aspen leaf blotch miners in Marinette, Florence, Forest, Oneida, and Vilas counties, which is similar to where they were found last year. Although defoliation can be severe, aspen trees usually tolerate the situation well. Many affected aspen trees will send out new leaves after feeding by caterpillars ends. Although damage may appear severe, the effects on the trees’ overall health seems negligible.

By Linda Williams, forest health specialist, Woodruff, Linda.Williams@wisconsin.gov, 920-360-0665 and Paul Cigan, forest health specialist, Hayward, Paul.Cigan@wisconsin.gov, 715-416-4920.

Severely defoliated balsam fir in Vilas County.

In many areas this summer, damage from spruce budworm (Choristoneura spp.), a native insect, is less noticeable than in past years. Although heavy defoliation is evident north of St. Germain in Vilas County and at a site in Shawano County, only light to moderate defoliation has been seen in other areas. Light defoliation was observed in Bayfield, Florence, Forest, Marinette, Oneida, Shawano, and Vilas counties. Defoliation was less predominant last year as well, probably because of unusually heavy rainfall in spring 2017 which led to an increase in tree growth. This year’s spring was also unusually wet, resulting in increased tree growth.

Spruce budworm outbreaks typically last about 10 years; the current outbreak began in 2012. The last two years of exceptionally robust tree growth may help some of the damaged trees to at least partially recover. Since 2012, some areas of northeastern Wisconsin experienced three or four years of heavy defoliation; affected trees are either dead or declining despite good growing conditions.