Hemlock sawfly in its larval form. (Photo courtesy of the U.S. Forest Service)
The hemlock sawfly is native to Southeast Alaska. But for the past few years, the tiny insect has been causing some big problems. Bug scientists think drought conditions played a major role in a recent outbreak.
And it’s alarmed some residents who’ve noticed more brown trees in their rainforest backyards.
Elizabeth Graham doesn’t get what you’d consider normal text messages from her friends.
She’s an entomologist with the U.S. Forest Service. So, rather than pictures of people’s cats or puppies, she might get a text with a question mark and a wolf spider. And sometimes she has to deliver bad news.
“I like it a lot better when someone sends me a picture and says, ‘Is this a bedbug?’ And I can tell them, ‘It’s not.’
Graham does this for the public, too: identifying weird insects or insect behavior. And in the summer of 2018, her office received something much more descriptive than a text message: a bag of frass. That’s hemlock sawfly poop. A sandy, green-looking concoction.
It was collected outside a cabin on Killisnoo Island in Southeast Alaska. The residents had lived in the cabin for 40 years, and they’d never seen anything like this.
They explained to Graham the insects were causing them — and the trees — major stress.
“It was so dry and there were so many sawflies in the canopy, that it was literally raining down,” Graham said. “And you could hear the frass hitting the devil’s club and skunk cabbage.”
Here’s how Mary McDowell — the resident who gathered the insect poop — describes it:
“If you took a walk in the woods, you could feel the frass on your scalp and in your hair,” McDowell said.
In their squirmy larval form, sawflies devour old hemlock and, occasionally, spruce needles.
Last year, the sawfly munched on about 40,000 acres of hemlock in the region. This year, that number ballooned to close to 400,000 acres of damage.
It got so bad McDowell stopped collecting rainwater from her roof to drink. Her gutters were often clogged with the sawfly poop.
She said this event was upsetting for a variety of reasons.
“One of my favorite little rituals is: First thing in the morning, I’d open the curtains in this lush green forest,” McDowell said. “One of my joys is to open that curtain and look out. And now, the two trees that are right in front of that window are totally brown.”
If the trees lose all their needles, they can die. McDowell is anxious about that happening to her beloved trees.
“It’s just heartbreaking to see the changes, and we’re so fearful,” McDowell said.
This is one of the worst hemlock sawfly outbreaks the Forest Service has documented in Alaska since the 1950s.
To understand why and how it could be remedied, you need to understand the lifespan of a sawfly.
Outside on a hiking trail, Graham plucked the insect off a hemlock branch. This sawfly was wrapped up in a cocoon.
“They’re just real small. I like to say Tic Tac size,” Graham said.
Like butterflies, sawflies start out looking like tiny caterpillars. They eat, cocoon and reemerge with wings. Their lifespan is short — only a few months — but it’s enough time to reproduce and ensure the cycle continues.
To make it through this is a gauntlet of horrors. It’s nature’s way of keeping the sawfly numbers in check.
Normally, with average rainfall, a type of fungus covers the trees and the sawflies eat the fungus. It can bloom inside them — killing some of the sawflies.
But with the recent drought in Southeast Alaska, Graham said that plan has gone a bit wonky.
“Because of that, those sawflies that should have been killed weren’t,” Graham said. “And we just ended up with a huge population.”
Fungus isn’t the sawfly’s only nemesis. In fact, Graham has some sinister hopes for this Tic Tac-sized sawfly in the cocoon.
Entomologists are rooting for a parasitic wasp to help knock down next year’s population.
“A lot of people compare them to aliens, because they bore inside and feed on them,” Graham said.
Still, what if the drought conditions are as terrible as they were for the past few years? The parasitic wasp alone probably won’t drastically reduce the sawfly’s numbers.
Graham said the sawfly’s voracious appetite could ultimately be its downfall. Remember: The insects only eat the old needles, and they munched on much of that this year. A lot of them have already starved.
Graham thinks the sawfly population has likely peaked in some places.
“It’s sort of like a blip,” Graham said. “You kind of think in the long run it looks dramatic. But hopefully it’s not going to have long lasting impacts.”
But there’s at least one more frightening scenario that could play out. An insect called the western blackheaded budworm eats the new needles on hemlock trees. It could present a double whammy after the sawfly has eaten so many of the old needles.
Scientists like Graham are monitoring carefully and hoping the trees in the rainforest have time to heal.
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