What’s behind those patches on the mountain?
I am bad at keeping in touch with people, which partly explains why I only talk to Clive Jones every seven years.
Jones is an ecologist who works at the Cary Institute in Millbrook and a specialist in the gypsy moth. In 2016, when patches of brown appeared on the north side of Mount Beacon, I called Jones to find out why. A year after we published a story, the trees had recovered, as Jones suggested they would.
When brown patches reappeared on Mount Beacon a few weeks ago, I dismissed it as the return of the gypsy moth. But local Facebook groups filled with panicked queries from people who wanted to know why there were suddenly swarms of these strange brown moths flinging themselves against porch lights. Where did they come from? Were they dangerous?
Female spongy moths laying eggs on Mount Beacon (Photos by B. Cronin)
I called Jones. Some significant things have changed since the last time we spoke. For starters, they are now called spongy moths, at least among scientists. In 2021 the Entomological Society of America stopped using the name because “gypsy” is a slur against Romani people.
(The Entomological Society of America is also considering a wider renaming of all invasive, destructive insect pests named after races or nationalities, e.g., the Japanese beetle.)
The gypsy name is thought to have been attached because the caterpillars that become moths are migrants. Every May they travel great distances by inching their way up trees, hanging from the high branches by a silk thread and being gently blown by the wind to locations farther away, with their furry bristles helping to slow their descent. (This is known as “ballooning.”) Once they settle, they chomp every leaf they can find, defoliating vast swaths of forest.
Spongy moth was chosen because of the spongy, tan sacs that hold the moth’s eggs for 10 months. The French Canadian name for the moth is spongieuse.
Why are they back? In fact, they are always here, but for a notable outbreak to occur — for the caterpillars to have a good year — its two main predators need to have a bad year.
The first of its enemies is a fungus that, like the spongy moth, is native to Asia. The absence of this fungus in North America is one of the reasons the spongy moth population exploded so quickly after a scientist imported them to Massachusetts in the late 19th century. In the 1980s, the fungus mysteriously appeared in the Northeast — clearly an act of human intervention, although no one has claimed responsibility.
The mission was a success. The fungus spread, infecting the caterpillars with spores. When the caterpillars die and decompose, the spores spread from their mummified corpses to infect new caterpillars. Science! Sometimes it’s gross!
The spores can rest in the soil for quite some time but need a warm, wet spring to get going. This year, we had a cooler than usual May, as well as an unusually dry one. It’s hard to remember after a month of getting hammered by historic storms, but by the end of May there were worries that we were heading into a drought. Usually, the Highlands get 3.5 to 4.5 inches of rain in May; this year we had ½ inch.
The second predator that keeps the moth in check is the white-footed mouse, which feasts on the caterpillars in their pupae form. When I asked Jones where the mice were this year, he discussed acorns.
In 2021, the Highlands went through a heavy “mast” year, when the trees drop an unusually high number of acorns following a few seasons of low activity. One of the side effects of a heavy mast is that the population of animals that eat acorns, including white-footed mice and squirrels, explode, only to crash in the years following when acorns are harder to find. With so few mice around this year, there wasn’t enough to make a dent in the pupae population.
The next time you notice unusually abundant acorns, mark your calendar to check the weather in two years. If it’s cool and dry, get ready for a lot of spongy moths.
A tree on Mount Beacon, defoliated by spongy moth caterpillars
Once you notice them, it’s too late to do anything about it that year. The damage is done, because the caterpillars are responsible for the defoliation. (The moths are harmless.) Those swarms around your porch light are packs of needy males, waiting for the flightless females to hatch.
You can see the fruits of this romance now on the lower slopes of Mount Beacon, where last week I spotted several females laying spongy sacs for next year’s brood.
Each year, the state Department of Environmental Conservation flies over forests to search for defoliation. Along with Mount Beacon, northeastern Dutchess County, the lower slopes on the Shawanagunks and Bear Mountain have all suffered this year from spongy moth outbreaks.
The 2023 outbreak actually began in the Finger Lakes three years ago and has been spreading every year, taking root in whatever microclimates prove amenable due to low mouse and fungal activity, says Jessica Cancelliere of the DEC. “It’s like playing whack-a-mole,” she said. “These outbreaks keep popping up in different areas.”
If you notice egg sacs on your lawn, the DEC has a wealth of information about what to do at dec.ny.gov/animals/83118.html. Wrapping tree trunks with burlap sacks in late winter will stop the caterpillars from reaching the leaves, Cancelliere said. Another option is to encircle the trunk with sticky tape, but Cancelliere cautioned against that approach because the tape will ensnare pollinators, as well.
The DEC policy is typically to let nature take its course. In some areas, they’ll spray vulnerable stands but otherwise trees usually recover. Some of the defoliated trees on Mount Beacon are already sprouting new leaves, and as long as we don’t have a cool, dry spring and a low mouse population in 2024, most of the egg sacs on the mountain won’t survive.
“The moth populations crash eventually,” said Cancelliere. “There are pockets of trees that die here and there, but most of them can withstand even a couple of years of defoliation and they recover. That’s happening right now.”
There’s still much to be learned about spongy moths, but there’s less research being done. Even when there’s an outbreak, it usually doesn’t last long. Scientists tend to focus on species that are in danger of being wiped out or the species that are doing the wiping out.
“We stopped long-term monitoring of the moths in 2010, partly because at that point we understood what was going on,” Jones said. “But also, out of sight, out of mind. Most people think it’s not a problem.”
With climate change, that may change. What if cool, dry springs become the norm in the Highlands? What if masting years get more erratic? “The combination of more droughts due to climate change and continuing defoliation from spongy moths could result in something that we would not like,” said Jones.
