Forest Tent Caterpillar Defoliation in the North Country
The forest tent caterpillar (Malacosoma disstria) (FTC) is a forest pest that is native to New Hampshire. It can be seen every year in the woods or on a tree in your backyard. It’s a distinct caterpillar with characteristic markings on its back often described as white keyholes or boot prints or, if you look closer, a penguin with its flippers sticking out (see photos 1 & 2). FTC is found throughout the United States and commonly feeds on the foliage of ash, birch, poplar, oak and sugar maple. In NH, sugar maple is the most common host tree during outbreaks of FTC. The FTC is often confused with the Eastern tent caterpillar. While both caterpillars produce “tents” to pupate in, the Eastern tent caterpillar produces a large tent with many caterpillars inside on tree branches that is easily seen from afar. The FTC makes small, single occupancy tents in bark crevices, rolled up leaves, or any place they deem stable (photo 3). Occasionally, as in 2005-06 and 2016-18, FTC populations can grow to outbreak levels and strip its host trees completely of their foliage in late spring/early summer.
Outbreaks can be dramatic and last 2 to 3 years. In forests dominated by sugar maple, you can see 75% to 90% of the forest canopy defoliated (photo 4) in June and July. Following defoliation, trees break the buds created for the next spring and refoliate in late July into August. This second round of leaves sent out by the tree can be smaller and malformed (photo 5). At this point, the trees are trying to continue their growing season to produce and store energy and set new buds for the following spring to replace the ones they used to refoliate. This process of refoliation is taxing on the trees.
Defoliation from FTC occurs during the high season of photosynthesis when trees are growing and replenishing the starch reserves (stored energy) they use to grow new leaves, wood, etc. When the leaves are eaten by FTC, the tree misses the opportunity to build up its starch reserves. On top of that, the trees need to spend even more energy to refoliate, again using stored starches in the tree. Most trees can handle this defoliation-refoliation for a year or two—they have enough starch reserves stored in the roots and stem. FTC defoliation is a natural event by a native insect that has evolved over time with the host trees.
After 2 to 3 years, the outbreak dies down through natural population decline, the predation of FTC pupae by the friendly fly (Sarcophaga aldrichi), or a disease from fungi or viruses (photo 6). The cycle of defoliation and refoliation and energy the tree uses to stay alive often leads to dieback in the canopies of trees from a single branch to whole tops of trees lost. Occasionally, an individual tree dies from the stress. Rarely are whole stands of sugar maples killed due to FTC alone.
Coös County Defoliation
The 2016-18 outbreak located in Coös County covered roughly 20,000 acres of defoliated forestland. Sugar maples observed on a property in Randolph, NH, during this outbreak seemed to be going about their normal response. In the spring of 2018 however, sugar maples in certain areas of the Randolph FTC outbreak were not leafing out during the spring months (photo 7). On the same property but on a different site, sugar maples with the same amount of defoliation looked as they should—leaves in the canopy with the occasional dieback from FTC defoliation stress (photo 8). The stark differences in tree response to this recent FTC outbreak led to a closer investigation of why trees on the same property would respond so differently.
Rarely does a FTC outbreak result in the mortality and dieback seen in photo 7. The stands of maples with the highest mortality and dieback showed a number of pre-existing conditions affecting the health of the trees. The sugar maples that died had been subjected to a long list of stresses that set them up to be killed by FTC defoliation.
This property was previously owned by an industrial paper company that harvested red spruce and balsam fir. This practice led to the release of long suppressed sugar maples that were already old and not vigorous. During the harvesting of spruce and fir, there was an excessive amount of residual logging damage that led to stem and root decay and dieback that can still be seen over 35 years later. This type of damage, especially in trees of less vigor that were suppressed, leads to poorer root function and introduction of fungi and insects. Evidence of decay and sugar maple borer damage is throughout the stand.
In 2016 all of NH experienced a drought of some sort. The North Country was no exception. Fortunately for this stand, the drought was much less severe than what was experienced in the Seacoast region. Nevertheless, sugar maple are not as drought tolerant as white pines or red oaks. Any deviation from a normal rain pattern for sugar maple can be a stress on the species.
The stand’s elevation ranges from 2,000 to 2,300 feet. Sugar maple is rarely found above 2,500 feet of elevation in northern New England. The elevation, and the climate these trees are subjected to is a constant stress.
Soils play an important role in tree health and a tree’s ability to tolerate stress. According to the U.S. Forest Service’s Agriculture Handbook No. 271, Silvics of Forest Trees of the United States, sugar maple “...thrives only on fertile moist and well-drained soils of all soil types. Yield and quality of sugar maple stands increase as soil fertility and moisture conditions improve. It does grow on poor, dry, shallow soils but is never thrifty there.”
As it turns out, the trees with the most severe dieback and mortality were found in a specific soil type—Tunbridge-Lyman-Marlow association, gently sloping, very stony. This soil isn't fertile and moist. It may have some fertility, but it is often well to excessively drained and the sugar maples may be stressed from droughty soil. Further down slope in more moist, fertile soils, similarly defoliated sugar maple have full canopies.
In the case of FTC defoliation in a forest already stressed by a number of factors, soils matter. It seems that FTC hit a stand that was already stressed and not on the best site for growing sugar maple (offsite). The sugar maples were there due to the harvesting history. Under natural development of the forest, the sugar maples may have been out-competed by other trees better suited to the site. Now, since those other species were previously harvested, the stand has little to no future with the sugar maple dying from the combinations of stresses put on them.
Species can grow on most sites and soils. Some are more “thrifty” on certain sites. If given the chance, even a sugar maple can grow on a poor site, but only for a while. It's only a matter of time before multiple stresses add up, causing the tree that's offsite to die. The more suitable a site is for the species, the longer it will last at that site and thrive.
Also, it was easy to rush to judgment that the FTC killed these trees. Yes, the trees died after the outbreak, but the defoliation was only one of many stresses these trees were subjected to over their lives. Trees subjected to the same stresses, but on better sites, were not killed and in fact look like they will bounce back from this outbreak just like sugar maple has done after other FTC outbreaks.
What’s going on in your woods? Contact your County Forester
The forests of New Hampshire have a long and storied past. For over 400 years we have been harvesting, regrowing, neglecting, and tinkering with our forests. Natural events such as forest tent caterpillar defoliation, ice storms, and hurricanes have also made their mark on the landscape. What is observed on one property can be a totally different story on the other side of the stone wall. If you are curious about your woodlot’s history and the story it tells, contact your local County Forester for a walk in the woods.