ITHACA, N.Y.—Drop the name “hemlock woolly adelgid” in a conversation and it’s likely to yield a response similar to this: “What the heck did you just say?”
The hemlock woolly adelgid (HWA) is an invasive insect that when able to swell in numbers, can kill the trees of the eastern U.S.’s evergreen hemlock forests. Without any natural predators, the only thing that controls their populations are the deep freezes of winter. Mild winters mean big gains for their populations, and the loss of more hemlocks, which can be commonly found in the gorges.
“Basically, the only control over their population growth is called weather right now,” said Mark Whitmore, the Director of the New York State Hemlock Initiative.
The warming effects of climate change are projected to only increase the HWA’s northern range. While long term solutions, like biocontrols are being worked on by the NYS Hemlock Initiative and others, there are chemical treatments that can be effective in slowing an HWA infestation, but only in its early stages of infestation.
HWA reproduces asexually, adding to its population twice a year in the spring and fall. A single hemlock woolly adelgid lays 50 to 100 eggs each time it reproduces, so the population growth can be exponential.
“If you get to an area and it’s been infested for years and it’s spread over many hundreds of acres, there’s not a lot you can do,” said Whitmore. “But when you get into an early infestation stage, you can get in there and maybe knock down the population and keep it from growing locally.”
But doing that means knowing where the woolly adelgid is spreading—so here’s where citizen science steps in.
The New York State Hemlock Initiative is launching a “Hemlock Woolly Adelgid Winter Mapping Challenge” with iMapInvasives, an online database tool used for (you guessed it) mapping the spread of invasive species.
The mapping challenge will run from Feb. 12 to Mar. 12.
Caroline Marschner of the New York State Hemlock initiative said that, “This is a really great year for looking for HWA.”
The pests are sedentary, fixing themselves to hemlock branches and sucking out their sap from near the base of the hemlock needles. In southern hemlock forests, HWA infestations kill their host trees in four to 10 years. In New York’s harsher winters, it can take six to twenty years. The hemlock woolly adelgid is practically invisible to the naked-eye, only being identifiable by the waxy wool they form around themselves to survive the cold of winter.
Marschner said that the last few winters have been mild enough for the HWA to build up it’s populations substantially. “And last winter was so mild, we had the lowest mortality we’d ever seen,” said Marschner.
While this is bad news for the hemlocks, the silver lining is that the HWA will be clustered into woolly clumps on tree branches, making them easier to spot.
The Finger Lakes Partnership for Regional Invasive Species Management (PRISM) is managing volunteers for the mapping effort. The New York State Hemlock Initiative also has volunteer opportunities.
The Cascading Effects of Declining Hemlock Forests
In eastern U.S. forests, hemlock trees are what’s known as a foundational species. They determine the way an ecosystem functions on money levels.
“They (hemlocks) are super abundant, and they’re the thing that makes the ecosystem the ecosystem that it is,” said Marschner.
“Basically they provide a foundation upon which many species need to survive in the environment,” said Whitmore
Since hemlocks keep their needles year round, their heavy canopies regulate temperature, slowing snow melt in the spring time, and keeping creek waters cool.
“The native brook trout, and the Atlantic salmon require cooler waters to reproduce. And hemlocks are an important part of that,” said Whitmore.
There’s also a growing body of science showing that these humble conifers are important for regulating the water levels within the watersheds that they grow.
Marschner said, “There’s growing evidence that watersheds that have lost their hemlocks are seeing more flooding, and just more fluctuations in their water levels.”
The reason being, Marschner explained, is that hemlocks are “most active” during the spring and fall, when there’s an overabundance of water from rain and snowmelt.
In addition, hemlocks grow well along steep slopes and along stream banks, holding the soil and slowing erosion.
“Hemlocks are just an irreplaceable part of the Eastern forests,” said Whitmore.
Hemlocks are shade tolerant, and when they die back, Marschner said, “they’re usually getting replaced by hardwoods that don’t provide the same ecosystem services.”
Biological controls have been explored extensively at the NYS Hemlock Initiative. Biocontrol is the use of other organisms to suppress the populations of pests. So far, the NYS Hemlock initiative has found success with early establishments of the laricobius beetle, which feeds on developing and adult HWA, but only it’s winter spawns. The NYS Hemlock Initiative started releasing these beetles in 2008.
Silver flies have been released as a biocontrol for the spring wave of HWA since 2015. The fly larvae feed on HWA’s eggs.
While these efforts are showing promise, it’s unclear how long it may take for their populations to establish in New York State.
“There are short term solutions, and long term solutions to an invasive pest. A short term solution is figuring out where it is and treating it in places where you have important hemlocks you want to save,” said Marschner. “And the long term solution is biocontrol. But you’ll lose your hemlocks if you sit around not doing the short term solution and waiting for the long term solution.”