By Sheilla Jones
The Manitoba Woodlot Issue 103
The polar vortex that brought record-breaking cold temperatures to much of North America this winter may have warmed hopes for a chill on the spread of forest pests. But experts say that the bitter cold may have had little impact on the emerald ash borer, mountain pine beetle and elm bark beetle.
“It takes substantially colder temperatures to cause substantial insect mortality in the middle of winter,” said Jordan Bannerman, an instructor in entomology at the University of Manitoba. “Cold weather in the fall or winter can have a larger impact. A warm spell in early spring followed by a cold snap may also cause increased mortality, since the insects begin reducing their cold tolerance.”
Manitoba, Saskatchewan and the Northwest Territories all recorded sustained severe cold in January, with sustained lows ranging from -37C to -28C.
But it takes more than cold temperatures to kill larvae overwintering under the bark of trees.
“The actual temperatures,” said Bannerman, “that these insects experience are higher than ambient temperatures which can be misleading. It takes sustained cold to drop the temperatures under the bark. Factors like the amount of snow in forests and tree density are also important in determining temperatures experienced by the larvae.”
Where sustained cold does impact insect mortality, noted Bannerman, it affects more than just damaging pests.
“Although in the case of the emerald ash borer the cold weather may have caused some mortality, the mortality to the natural enemies that are being used as an attempt to control their populations will also experience significant and potentially higher levels of mortality. This is likely relevant for both the mountain pine beetle and elm bark beetle as well.”
While extreme weather events, such as record-setting cold temperatures, may raise hopes for a natural constraint on insect populations, Bannerman said this winter has not likely had much impact beyond those experienced in an average winter.
“Really, the take-home message is that although these periods of cold weather likely killed some insects, these insects are cold-adapted. And it is highly unlikely that these weather events will have any influence on the movement of these insects and their population sizes.”
Mountain pine beetle moves into Boreal forest
There is no evidence of the mountain pine beetle spreading into Manitoba; however, it may be only a matter of time. The beetle was at one time confined to lodgepole pine forests in the BC mountain regions, but it has now developed a taste for jack pine, a staple of the northern boreal forest. Jack pine stands cover about two-thirds of Manitoba’s land mass.
Scientists conducting genomic research have confirmed that the mountain pine beetle (MPB) “just loves jack pine”.
“Scientists expect the beetle to continue expanding its geographic range, moving into the boreal forest and Canada’s northern and eastern pine forests,” according to Natural Resources Canada. “Today the MPB occurs well beyond its historic range, extending into northern British Columbia and eastward in the boreal forest of north-central Alberta. Not just limiting itself to lodgepole pine any longer, the beetle is also reproducing in jack pine, the dominant pine species of the boreal forest.”
The mountain pine beetle infestation has decimated 18-million hectares of forests in BC, and crippled independent lumber businesses across the country as provincial governments slash stumpage fees to get rid of the dead and diseased trees as fast as possible.
