Larch needleworm outbreak discolouring Banff’s picturesque larch trees

Larch season is underway in Banff National Park, but an infestation of native moths is changing what some popular hiking areas will look like this fall.

Larch needleworm, scientifically named Zeiraphera improbana, is a species of moth native to North America. The moths’ caterpillars feed on larch needles, causing affected trees to appear reddish-brown, scorched or sparse instead of their normal golden-yellow colour.

Larches are one of the few coniferous trees in the Rockies that lose their needles each year, with their needles turning yellow in the fall before dropping, creating the golden colour that draws visitors to areas such as Larch Valley and Healy Pass.

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The current outbreak is affecting approximately 4,400 hectares, with some patches also found in Yoho and Kootenay national parks, according to Parks Canada.

The majority of the defoliation is in Banff, between the Sundance and Bow mountain ranges, roughly from Healy Pass to Lake Louise.

“There have been some previously reported outbreaks in adjacent areas in B.C. around Invermere and Kimberley, but we have not seen defoliation on such scale in the parks yet, so this is kind of the first one on a big scale,” Parks Canada forest health specialist Jakub Olesinski said.

Despite the extent of the defoliation, Parks Canada has not observed larch trees dying as a result of the infestation.

Larches produce a new crop of needles every year, meaning the trees can withstand the loss of their needles, but severe defoliation can weaken trees and reduce their growth and development.

Parks Canada expects the affected larches to survive and return to their normal appearance in future years, as these outbreaks are typically short-lived, but if the infestation continues, there could be potential impacts like branch mortality.

“If the defoliation occurs again over the next few years, that’s when we start seeing some effects on trees and if this actually repeats over several years, then we would be looking closely at tree mortality,” Olesinski said.

For visitors, the change could be difficult to miss during a season when people travel to the Rockies, specifically to see the larches turn gold.

Rania Nazzal, a Calgary resident who regularly visits the mountains, said the fall colour has become something she looks forward to each year.