Brown birch trees in both Southcentral and Interior Alaska this summer aren’t signaling an early fall. Rather, they are subject to an infestation of small, leaf-munching larvae.
Alaska is in the midst of a birch leafminer outbreak. The caterpillar-like insect has munched its way through birch leaves near both Anchorage and Fairbanks this summer, said Sydney Brannoch, an entomologist with the Forest Service based in Fairbanks.
Alaska has seen several outbreaks over the past decades. Last year’s aerial surveys showed nearly 48,000 acres of damage in Southcentral and Interior Alaska. Scientists are still working to process this year’s data, but say the outbreak around Fairbanks has grown more and more obvious.
Brannoch said that this summer’s hot and dry weather in the Interior may have been optimal “conditions for these insects to really go wild.”
The state’s first major outbreak was in Anchorage in 1996, which lasted through 2006. Current outbreaks have been occurring for several years in and around Anchorage, Wasilla and the Interior. Particularly heavily-hit areas this summer include Fairbanks, Fox, North Pole and Eielson Air Force Base.
An infested tree is left with a brown crown, Brannoch said, appearing as they would right before dropping their leaves in fall. But the trees won’t look yellow and vibrant with fall colors. That’s because the leafminers, true to their name, mine out all of the internal tissue of the leaf, causing it to shrivel up, die and fall off. An infested tree can lose 90% of its leaves.
The invasive insects have been in Alaska since the ‘90s, likely introduced through the transport of live plants. They aren’t native to the state, and have flourished since they don’t have any natural checks and balances, Brannoch said.
Insect outbreaks can be driven in part by climate change, Brannoch said, as certain conditions, like a warming climate, can correspond with ideal insect population growth and can lead to bigger outbreaks that happen more often.
The birch leafminers look like small caterpillars, are a half-inch in length and cream-colored with a slight green tint given their leaf-only diet. The larvae feed on the insides of the leaves before eventually emerging the following spring as adult sawflies. They lay their eggs into birch leaves.
But before that, some of the larvae drop into soil to pupate in the winter, a process that is supposed to begin in the next few weeks.
“People are going to start to see larvae in the beds of their pickup trucks or on the ground underneath birch trees,” Brannoch said.
It’s possible to see birch leafminer larvae or exoskeletons and even poop when holding an infested leaf with a dead splotch up to the light.
But even amid the ongoing outbreak, birch lovers need not despair. Unlike the spruce beetle, another destructive Alaska insect which has been responsible for killing spruce trees all around Southcentral Alaska, the leafminer damage is aesthetic.
“It just doesn’t look as nice,” Brannoch said. “But so far there has not been a documented case of tree mortality or tree deaths associated with these.”
But repeated years of heavy damage has shown some tree growth decline, Brannoch said. Also, heavily-hit trees could be more susceptible to other diseases and insects.
To help the Forest Service track the outbreak, people can upload photos, videos or sightings to iNaturalist.com, where they will be part of the Alaska Forest Health Observation Project.