Emerald ash borer able to withstand cold up to –40 C, new research indicates

Scientists in Sault Ste Marie, Ont., have made a discovery about the emerald ash borer that could help assess future risk to the region's — and Canada's — threatened ash trees.

Amanda Roe, a scientist at the Great Lakes Forestry Centre, studied how the ash borer was able to survive extreme cold temperatures and adapt its physiology to severe Canadian winters. 

The study, titled "Plasticity," was published in Current Research in Insect Science. Co-authors include Meghan Gray and Chris MacQuarrie from Great Lakes Forestry Centre, and Meghan Duell and Brent Sinclair from Western University in London.

Roe said the researchers isolated beetle larvae to study how they reacted under extreme cold temperatures similar to those in Canada, specifically, icy Winnipeg temperatures.

"These larvae actually can survive really cold temperatures, and they do that by keeping their blood from freezing," Roe said.

"As long as their blood stays liquid, they will survive, and they do that by using antifreeze … they use glycerol."

The larvae Roe tested survived through –40 C, with one beetle surviving into –50 C, before freezing and dying.

"They're significantly more cold tolerant than anything we'd seen before," she said.

The amazing part, Roe said, was the beetles harvested from Winnipeg appear to be adapting to their location, becoming more acclimatized to deep dips in the temperature. 

"These bugs from Winnipeg, if they're given a southern Ontario winter, they behave like other bugs from southern Ontario, so they don't get that cold," Roe said.

"But those Winnipeg bugs, if they experience a Winnipeg-like winter, they are the ones that are able to get those really low survival temperatures." 

Roe said that means the emerald ash borer can adapt its physiology to the conditions the bug is experiencing.

"They're responding in a flexible way," she said. "It's priming them to be able to survive those more extreme temperatures."

And that's not good news for Canada's ash tree population.

Ash borers a risk to 'urban canopy'

According to an Ontario government website, ash borer infestations now exist across much of southern Ontario. The species has also been found on Manitoulin Island, and a separate infestation exists in the Sault Ste. Marie and east of the city around St. Joseph's Island.

Emerald ash borer feeds on all ash species in Ontario, including green, red, white, black and blue ash, as well as some non-native species such as European black ash. The beetles leave S-shaped tunnels under the bark of the tree, while the leaves of an infested tree will turn yellow before dying.

But despite their presence in Ontario, Roe said the real concern is the spread of the species to the west.

"Places like Winnipeg or further west, like Saskatoon or Edmonton and Calgary, have a significant proportion of ash trees in their urban canopy," she said.

"In some of the places, at least 50 per cent or more of their trees are ash trees, and if these beetles get into those urban forests, those cities are going to lose a significant number of their trees.

"That's a real concern for anyone who wants to try to maintain that urban canopy." 

Roe said the next step will be for researchers to develop an approach that recognizes the beetle's unique ability.

"One of the things that we want to understand is what are the cues that are telling them to prepare for these really cold winter temperatures," Roe said.

"There's surprisingly little known about how this bug responds to winter. So this is really an indication that there is a lot more work to do."