DEC: Eastern larch beetle outbreak affecting healthy trees
By Megan Plete Postol
Recent reports of dead and dying Eastern larch (commonly known as tamarack) trees in the Adirondacks have been traced to Eastern larch beetle attacks, with a new twist. Researchers have found the beetles are attacking and destroying healthy trees, which is uncommon as the insects were previously believed to prey on trees that were already injured or compromised.
“The Eastern larch beetle has long been thought to only be a secondary pest, infesting weak and dying larch trees,” said NYS Department of Environmental Conservation Entomologist Liam Somers. “Historical outbreaks of small infestations throughout the Eastern larch’s native range have been linked to environmental stressors, leaving these trees vulnerable to Eastern larch beetle attacks.”
The Eastern larch beetle is a bark beetle native to North America that feeds on and colonizes in the phloem — tissue — of larch trees. Their main host preference is the native Eastern larch tree (tamarack) but they have also been found inside European, Japanese, Serbian, and Dahurian larch, all of which are non-native species.
Hardest hit areas
As of right now, almost all reports of symptomatic trees have been in DEC regions 5 (Eastern Adirondacks/Lake Champlain) and 6 (Western Adirondacks/Eastern Lake Ontario). This is likely due to the fact the Adirondack Park has the highest concentration of Eastern larch in the state, according to DEC.
“Small pockets of Eastern larch beetle are likely existing outside of these regions that have not been reported yet,” Somers said. “We are hoping the recent higher volume of Eastern larch beetle outreach will lead to a more representative infestation map.”
DEC’s Forest Health Diagnostic Lab has received a total of 30 symptomatic tree reports from regions 4 through 7, starting at the beginning of June with roughly one to three reports a week.
The U.S. Forest Service has historical records of a few scattered infested larch stands in the 1970s through 1980s. DEC started keeping track of reports this May when the first infested area was discovered.
At this point, the DEC is in monitoring mode, as there is little that can be done in the short term to thwart the beetle’s destructive path.
“Unfortunately, there is no control for Eastern larch beetle outside of cutting and burning or chipping infested trees to help contain the spread,” Somers said. “Minnesota has been battling Eastern larch beetle for over 20 years with little to no success, losing thousands of acres of larch. The Minnesota Department of Natural Resources Forest Health Unit is experimenting with silviculture forest management practices by thinning out dense larch stands, diversifying the forest tree species, and promoting the health of uninfested larch. These practices show promise but aren’t a quick solution to a fast tree-mortality-causing pest.”
Signs of Eastern larch beetle infestations include two millimeter bore holes in the trunk and branches of larch, defoliation before larch’s natural deciduous needle drop in the fall, sap runs not caused by physical damage, bark stripped from the tree by woodpeckers, and vertical boring galleries in the phloem with occasional branching, Somers said.
It is important to note that due to the wet summer of this past year, Mycosphaerella needlecast has also been found impacting the health of European larch, Somers added. This species of fungal needlecast causes larch needles to turn yellow/brown and drop prematurely. According to DEC, most trees can survive a year of this defoliation. Symptoms of needlecast can resemble those of an Eastern larch beetle infestation. However, with Eastern larch beetle, there will also be visible entrance or exit holes in the bark, and maybe some frass (sawdust like material) collected in branch and bark crevices.