December 17, 2017
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Low trap counts for spruce budworm doesn’t mean outbreak isn’t looming

By John Holyoke, BDN Staff

OLD TOWN, Maine — As scientists continued to monitor the state’s forests in advance of another battle with the voracious spruce budworm, data gathered this summer show that there were far fewer of the insects on the landscape this summer than a year before.

That’s the good news.

The bad news: That doesn’t mean another outbreak that could defoliate spruce and fir trees isn’t looming.

“I don’t think it changes our predictions,” Dave Struble, the state entomologist for the Maine Forest Service, said. “It may set [an outbreak] back a year.”

Major spruce budworm outbreaks are cyclical and typically attack a forest every 30 to 40 years. The arrival of the insects in Quebec and New Brunswick, Canada, and the defoliation of millions of acres there have been seen as a sign that spruce budworm may show up in large numbers over the next several years.

As reported in a previous Bangor Daily News story, the Maine Forest Service estimated that the last major outbreak of spruce budworm in the 1970s and 1980s killed between 20 million and 25 million cords of fir and spruce worth hundreds of millions of dollars.

Since then, the Maine Forest Service has been closely monitoring conditions in the woods in hopes of providing landowners as much advance warning as possible before a future outbreak. So far, that hasn’t happened.

In February, Allison Kanoti, a forest entomologist for the Maine Forest Service, took over administration duties for part of that research — a pheromone trap survey.

Those traps, which are deployed at more than 400 sites around the northern and eastern regions of the state, use a scent-based lure that draws male budworm moths. At the end of the season, in August, the moths are sent to Kanoti and counted.

This year’s totals were eye-opening. “Last year we had a number of sites where we were up around 100 moths per trap, and that’s [a level] where you can expect to see some defoliation,” Kanoti said. “Our maximum [last year] was around 320 moths per trap.”

But when Kanoti examined this year’s samples, she learned that far fewer moths were actually captured in the Maine woods in 2016.

The average per trap in 2015 was about 25 moths per trap. This year, that average dropped to about 7 moths per trap.

“And about 10 percent of the traps had zero [moths] this year, which was up quite a bit since last year,” Kanoti said. “Last year, I think about 2 percent of the traps had [no moths].”

As she began looking at the data, Kanoti initially was a bit alarmed.

“I would say I was a little bit taken aback by it, because this was my first year of running the surveying,” she said. “My first thought was, ‘What did I do wrong?’”

After consulting with scientists in New Brunswick and looking at results of light traps — those that capture male and females that flock to the light — she became more comfortable with the sudden decrease in spruce budworm moths.

Kanoti said survey data have shown that there’s not necessarily an increase from year to year, and changes can be expected.

“That fluctuation can be unnerving, but I don’t think you can say they’re unexpected,” Kanoti said, explaining that historical records show similar fluctuations from year to year, rather than a constant building of budworm populations year over year until a full-fledged outbreak that begins to affect stands of trees.

Struble said this year’s surveying leads him to believe that Maine landowners and timber managers are still a few years away from seeing the budworm take a toll.

“It doesn’t appear that populations are to a defoliation-causing stage. That’s the easy way to say it. We’re still looking at the buildup.” Struble said. “We’re still leading up to seeing defoliation. I’m thinking two to five years out.”

Struble has been studying spruce budworm since the last outbreak and bases his estimate on that work, along with data shared by Canadian researchers based on the outbreaks there.

Kanoti preferred not to predict when defoliation might start happening but said there might be one scenario in which Maine could dodge a large-scale event.

“The outbreak [of spruce budworm] is inevitable unless we get real warm falls. That could potentially lead to us maybe getting a pass,” Kanoti said.

Kanoti explained that warm weather in the autumn months can disrupt spruce budworm populations during a phase in which the insects are trying to prepare for winter.

“They are entering a period of dispause, or rest,” Kanoti said. “They go into the fall with a certain amount of energy reserves. Insects are cold-blooded, so their metabolism is affected by that outside air temperature. So what [researchers] have seen is that when you do have warmer falls, you don’t have as good of a winter survival of the larvae. They seem to use up their reserves before winter is over.”

 


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