Cold snap fatal to spruce beetles will 'knock back' infestation, say experts
'When it dips to –40, that's pretty lethal,' says provincial entomologist Jeanne Robert
Bark beetle experts say a recent cold snap has likely killed some spruce beetle infestations in northern B.C. — especially if the frigid temperatures hit fast, before the bugs could ramp up their protective "antifreeze."
That could make a dent in the worst outbreak of the spruce beetle this province has seen.
Provincial entomologist Jeanne Robert said temperatures must fall below –40 C for more than two days — as it did in parts of the province this January — to kill the hardy bark beetles.
"It won't cut the outbreak off at the knees. But I think this should knock them back," said Robert, a regional forest entomologist for the Omineca and Northeast regions of B.C.'s Ministry of Forests.
The wood-boring spruce beetles are only the length of a rice grain. But en masse they are powerful, infesting spruce from Yukon to Utah.
How many die will depend on how much snow was present and how sudden the change in temperature was, said Allan Carroll, the director of the University of British Columbia's forest sciences program.
Well-adapted spruce beetle larvae create a sort of antifreeze to survive cold, but Carroll said sudden, severe temperature dips can cause mass die-offs.
95% dead will end outbreak
Provincial surveys are underway now to gauge the recent cold snap's kill rate under tree bark. The beetles must be warmed up for several days to see if they reanimate, said Carroll.
"It's hard to tell a live one from a dead one. You need to warm them up to see if they come back to life and if they don't then they are most likely dead," he said.
Experts say about 95 per cent of the overwintering beetle larvae must die off to stop an outbreak.
"We won't really know until we survey in the summer how much [the cold] knocked them back," said Robert.
Environment Canada confirmed that Arctic air moved into Northern B.C. between Jan 5 and 19. In some spots it was close to –50 C.
"It was surprising that it wasn't record breaking," said Environment Canada meteorologist Matt MacDonald.
For at least two days this month — Jan. 14 and 15 — temperatures remained below –40 C in the northeastern part of the province most infested with spruce beetles.
"Good to see a potential benefit from that cold snap," said MacDonald.
Spruce vs. Mountain Pine beetles
Spruce beetles are nowhere near the threat of the infamous mountain pine beetle that hit peak infestation — the largest seen in the world — about 16 years ago, according to Robert.
Pine beetles killed off more than nine million hectares of forests by the peak of the infestation in 2004. In comparison, Robert said, the spruce beetle's unprecedented outbreak since 2014 has hit about 500,000 hectares.
But the spruce beetle is tougher to kill, according to experts.
Robert said it's easier for mountain pine beetles to hit the 95 per cent death rate for larvae because they remain under the bark over winter.
But spruce beetles are trickier because some of the bugs move down to the base of the tree or to the "duff" material, the decaying forest floor, where they remain better protected, often under snow, and able to survive colder snaps.
Robert said one more cool summer and another cold winter would be enough to kill off the infestation for good, even though beetles can adjust their life cycle to survive.
"But –40 is a lethal temperature. Yeah, when it gets to –40 I cross my fingers and my toes that that will knock the population back," said Robert, who doesn't often wish bugs death.
"But there is a lot of spruce beetles," she said.