Winnipeg's –36 C forecast could be bad news for emerald ash borers

Extreme cold temperatures in the forecast could be good news for Winnipeg's ash trees and very bad news for emerald ash borers.

The city started battling the invasive species even before its presence was confirmed last year; the beetle could cost the city millions of dollars and devastate the tree canopy, which includes more than 350,000 green ashes.

But now researchers are probing whether temperatures below –30 C could kill or slow the development of the emerald ash borer. Chris MacQuarrie, a research scientist with Natural Resources Canada, is trying to determine what it might take to stop the borer as it mounts an invasion of Western Canada.

"The cold question is one that's really interesting especially for, as it moves — we've sort of expected it's going to keep moving — and so as it gets into Western Canada, that question of what does the cold do, and what does the climate do, is really interesting," said MacQuarrie, who's working with a postdoctoral student to get to the bottom of that question. 

A map that marks a random sample of 2,000 ash trees with green dots shows they're planted across the city, with a higher concentration in newer areas:

This week, beetles from Winnipeg will be shipped to Western University, where the theory that they're susceptible to freezing will be put to the test.

There's evidence from Minnesota that extreme cold temperatures — like the lows of –35 C, –36 C and –34 C forecast for Monday, Tuesday and Wednesday nights in Winnipeg — could freeze the borer out. 

A 2014 study for the U.S. Forestry Service found that when the mercury dips to about –34 C, up to 90% of the beetles die.

'Knocks the population back'

MacQuarrie notes that even if 90 per cent of the beetles are wiped out in an extreme cold winter, "90 per cent of a lot still leaves a lot of insects left. Even that 10 per cent is enough to keep that population going and to keep killing trees, they're still there," he said.

"It might suggest why a population might go slower. If it gets cold and it knocks the population back every winter, but they're still there, it might make the population grow slower."

MacQuarrie says that's some of the information their studies hope to uncover, including finding how much the Winnipeg-born bugs can handle. He notes there are emerald ash borers in Moscow, which hit −42 C in 2007, and Thunder Bay, which hit –40 C in 2014.

"Before they found emerald ash borer in Winnipeg, if you'd asked me that question [if cold would kill emerald ash borers] I probably would have said yeah … that would have killed it," said MacQuarrie. "But ... it can probably handle a Winnipeg winter, an average Winnipeg winter. When you get these extreme cold temperatures, it might kill some of them — but it's not cold enough for long enough to kill all of them."

Wind chill doesn't count, either.

And like any hardy transplant to Winnipeg, they may be adapting. 

"One of the things we're studying is whether the emerald ash borer has evolved at all in the time that has been in North America to deal with cold temperatures, because certainly its had a chance to experience it as its moved across the country."

Shorter summers, more time to fight back?

Winnipeg's shorter summers could have an effect too. Borers need a certain number of summer days to mature and MacQuarrie said it appears that Canadian beetles are more likely to need two years to become adults. 

"If a bug takes one year to go through its life cycle, its population can grow really fast; if it takes two years to go through its life cycle, it'll grow slower — which means it takes longer to kill trees, which means you maybe have more time to do something about it," he said.

That could have implications for how the city fights the beetle, but first, they have to do the studies. 

A year from now, MacQuarrie says there may be an answer about how cold kills or slows the bugs.

"It's a fun insect to work on," MacQuarrie said.