ITHACA, NY—Toss the name “hemlock woolly aphid” into a conversation and it’s likely to come up with a response like, “What did you just say?
The hemlock woolly aphid (HWA) is an invasive insect that, when able to swell in numbers, can kill trees in evergreen hemlock forests in the eastern United States. Without natural predators, the only thing that controls their populations are deep winter frosts. Mild winters mean big gains for their populations and the loss of more hemlocks, which are commonly found in the gorges.
“Basically the only control on their population growth is called weather right now,” said Mark Whitmore, director of the New York State Hemlock Initiative.
The warming effects of climate change are predicted to only increase the northern range of the HWA. 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 down a PLP infestation, but only in its early stages. of infestation.
The HWA reproduces asexually, increasing its population twice a year in spring and fall. A single hemlock woolly aphid lays 50 to 100 eggs each time it reproduces, so population growth can be exponential.
“If you come to an area and it’s been infested for years and it’s several hundred acres, there’s not much you can do,” Whitmore said. “But when you get into an early stage of infestation, you can get in there and maybe knock the population down and prevent it from growing locally.”
But to do that, you need to know where the woolly aphid is spreading – so that’s where citizen science comes in.
The New York State Hemlock Initiative is launching a “Hemlock Woolly Adelgid Winter Mapping Challenge” with iMapInvasives, an online database tool used to (you guessed it) map the spread of invasive species.
The mapping challenge will run from February 12 to March 12.
Caroline Marschner of the New York State Hemlock Initiative said, “This is a very good year for HWA research.”
The pests are sedentary, attach themselves to hemlock branches and suck their sap near the base of hemlock needles. In southern hemlock forests, PLP infestations kill their host trees in four to ten years. In New York’s harsher winters, it can take anywhere from six to twenty years. The hemlock woolly aphid is virtually invisible to the naked eye, only being identifiable by the waxy wool they form around them to survive the winter cold.
Marschner said recent winters have been mild enough for the HWA to increase its populations significantly. “And last winter was so mild that we had the lowest death rate we’ve ever seen,” Marschner said.
While this is bad news for hemlocks, the silver lining is that HWA will be clumped together in woolly clumps on tree branches, making them easier to spot.
The Finger Lakes Partnership for Regional Invasive Species Management (PRISM) manages volunteers for the mapping effort. The New York State Hemlock Initiative also offers volunteer opportunities.
The Cascading Effects of Hemlock Forest Decline
In the forests of the eastern United States, hemlocks are what is called a foundation species. They determine how an ecosystem works in monetary terms.
“They (the hemlocks) are super abundant, and that’s what makes the ecosystem the ecosystem it is,” Marschner said.
“Basically, they provide a base that many species need to survive in the environment,” Whitmore said.
Since hemlocks retain their needles year-round, their heavy canopies regulate temperature, slow spring snowmelt, and keep stream waters cool.
“Native brook trout and Atlantic salmon need cooler waters to reproduce. And hemlocks are a big part of that,” Whitmore said.
A growing body of scientific evidence shows that these humble conifers are important for regulating water levels in the watersheds they cultivate.
Marschner said, “There’s mounting evidence that watersheds that have lost their hemlocks are experiencing more flooding and just more fluctuation in their water levels.”
The reason for this, Marschner explained, is that hemlocks are “most active” in the spring and fall, when there’s an overabundance of water from rain and snowmelt.
Additionally, hemlocks grow well along steep slopes and along stream banks, retaining soil and slowing erosion.
“Hemlocks are just an irreplaceable part of eastern forests,” Whitmore said.
Hemlocks are shade-tolerant and when they die, Marschner said, “they’re usually replaced by material that doesn’t provide the same ecosystem services.”
Biological controls have been explored extensively at the NYS Hemlock Initiative. Biological control is the use of other organisms to suppress pest populations. So far, the NYS Hemlock Initiative has had success with the first establishments of the laricobius beetle, which feeds on developing and adult PLPs, but only its overwintering eggs. The NYS Hemlock Initiative began releasing these beetles in 2008.
Silver flies have been released as a means of biological control against the spring wave of HWA since 2015. The fly larvae feed on HWA eggs.
While these efforts show promise, it is unclear how long it will take for their populations to become established in New York State.
“There are short-term solutions and long-term solutions to an invasive pest. A short-term solution is to figure out where it is and treat it in places where you have significant hemlock trees that you want to save,” Marschner said. “And the long-term solution is biocontrol. But you will lose your hemlock if you sit around not doing the short term fix and waiting for the long term fix.