BIG
SKY – In July of 2018, Scott Orazem, a member of Big Sky’s Porcupine Park HOA,
noticed a funny thing: many of the fir trees around the development had taken
on a rusted hue, their green tarnished by a perimeter of dead red needles as
the youngest needles on the trees, which should have been the healthiest, most
photosynthetic portions of each branch, had been munched by some evasive
culprit.
“Nobody
wants their trees to look like that,” Orazem said.
Forest
managers call these trees “singed” as if just their outer needles were breathed
on by fire; it’s the telltale sign of the native spruce budworm, which feeds on
the buds and new growth of Douglas and Subalpine firs.
Spruce
budworm moths lay eggs in the treetops in the fall, where the eggs overwinter until
larvae, which look like caterpillars, become active in the spring. Though they
begin feeding in late May and early June, the caterpillars don’t cause
appreciable damage to trees until around the beginning of July when they are at
their biggest—from a half inch to an inch long—just before they go into cocoons.
This year, 11 Big Sky HOAs, including Porcupine Park, paid to have their forests sprayed by helicopter to kill the insects and save their trees, but the spraying was less effective than past years due to the cool and moist summer weather.
A branch that shows a season’s work of the spruce budworm, which feeds on the new growth of firs, but won’t eat needles that budded in previous years. PHOTOS BY BAY STEPHENS
“It’s been a very, very odd year for the
spruce budworm,” said Chuck Gesme, a forester for Northwest Management Inc., a
private consulting firm that specializes in forest management and orchestrated
the spraying.
According to Gesme, the cooler summer led
to more variability in the stages of larval development within the spruce
budworm population. So, while most of the insects were at their largest,
chomping on this year’s new growth, he estimated a fifth of the population were
still the size of a grain of rice, nibbling on the insides of buds, and another
fifth had ceased eating as they moved toward pupating.
This poses a problem for spraying because
the organic biological insecticide uses to spray the trees, called BT, is only
effective for a very specific window when the caterpillars are big and eating
on the exterior of new growth. As an insecticide that can cover entire swaths
of forest without harming any insects other than the budworm, it’s a nifty substance.
“It’s a bacterium that is only harmful to
those caterpillars,” Gesme said. “I could spray it on an apple and immediately
eat the apple and I would be fine.”
BT lands on the limbs of trees and is
ingested by the spruce budworm larva, which causes the caterpillars die and
fall off the trees. But timing is critical, according to Gesme, because BT is
only active for 3-5 days before sunlight breaks it down.
After monitoring Big Sky’s forests twice
a week to ensure optimal timing for BT, Gesme gave the go-ahead in the second
week of July and a helicopter sprayed more than 16,077 acres of land in 11
different Big Sky developments with which Northwest had been working.
Despite hitting the population at the
point where the most budworms would be affected by BT—at the peak of the population’
bell curve—Gesme thinks they had about a 60 percent mortality rate for the
bugs, instead of the target range of 75-80 percent.
While checking the effectiveness of the
treatment on July 20, Gesme was astounded to find a larva and a cocoon on the
same tree weeks after any larva would normally be left.
“It was just ridiculous,” he said. “And
that’s just a measure of the strange weather we’ve had.”
Spraying isn’t the only way to mitigate
for spruce budworm, and property owners seeking to protect their trees can do
other mitigation work in tandem with spraying.
According to Nancy Sturdevant, forest
health specialist with the Missoula field office of the U.S. Forest Service,
property owners should thin their forests, removing trees that have been
significantly defoliated by spruce budworm and plan ahead to plant different
species of young trees.
“I understand people want to hold onto
their large, old trees … [but] trying to keep trees that are significantly
impacted by budworm or have very small crowns, that is not sustainable without
a lot of input,” Sturdevant said.
She also mentioned that homeowners could
site-spray carbaryl, a pesticide that is toxic to all insects, on treasured fir
trees, especially if they are not already too defoliated.
Gesme seconded that managing one’s forest
is important to managing spruce budworm, especially thinning, which creates
less competition between trees, and therefore less stress, and impedes larvae from
parachuting on silk threads carried by the wind from one munched tree to the
next food source.
“It’s kind of funny and I try to bring it
up, but you can actually vastly improve the health of your forest by killing [a
good percentage] of your trees,” Gesme said.
Years of repeated defoliation by the
budworm can leave trees haggard, stressed for nutrients and prone to other
insects, like the Douglas-fir bark beetle, which will finish the trees off.
Although the spruce budworm and bark
beetle may be pests to property owners who adore their trees, Sturdevant
stressed the insects’ roles in forest regeneration, especially in the largely
fir forests of Gallatin County.
“The spruce budworm is a native insect
that’s coevolved with these forests for forever,” she said. “So is the
Douglas-fir bark beetle. And the Douglas-fir bark beetle is called ‘the
recycler of mature forests.’ That’s its job: When we have too many mature trees
in an area, they hone in on that and kill those trees and reset succession.”
“When you see those two working in
tandem, that’s when you’ll see these large stand replacement events of a
sweeping hillside of all dead timber,” Gesme said. “There are places in Big Sky
that I see that happen currently,” such as in Jack Creek and on the slopes of
Fan Mountain, he added.
Once a forest gets to that point, it’s
time to start over, Gesme said, a process historically carried out by wildfire.