Infestation of looper moths stresses North Shore trees for third year in a row

An infestation of hungry moths has returned to Metro Vancouver for the third straight year, threatening to defoliate coniferous trees already stressed from drought conditions.

The Western hemlock looper moth is native to B.C. and sees outbreaks every 11 to 15 years on the South Coast due to environmental conditions, with outbreaks lasting one to four years. 

However, after last year's "significant" outbreak, the return of the moths and their worm-like larvae could have a severe impact on conifers like hemlock, Douglas fir and red cedar.

A rash of red and brown trees, caused by the larvae feeding on needles and causing defoliation, have been observed in North Vancouver, Stanley Park, and on the Sunshine Coast.

Babita Bains, a provincial forest entomologist, says the effects of the infestation are occurring much earlier than usual.

"A lot of that has to do with the high population [of moth larvae], slightly higher than last year, but also it's associated with the droughts — the cumulative years of drought stress that these trees have endured," she said. "We're seeing greater impacts than we have in the past couple of years."

Bains stresses that insect infestation is normal and healthy for the ecosystem, and the insects are not harmful to humans.

The tree mortality caused by the moth infestation allows younger trees to grow, according to Bains, and allows the ecosystem to recycle nutrients.

"These disturbance events are normal. Unfortunately, with climate change these disturbance events might become more severe and probably are becoming more frequent," she said.

"I guess we don't really want to panic, but it's a good reminder that climate change is real and it is going to impact other processes." 

Robyn Worcester, a natural resource management specialist with Metro Vancouver Regional Parks, says the recent heat wave could have stressed native trees more than usual.

"I certainly have noticed this year [there are] more trees under stress. So even trees that normally would be fine are looking a little brown," she said.

Worcester also says that while it might be "shocking" to see trees go brown due to the moths, there will be long-term biodiversity improvements that emerge as a result.

"There are no plans to spray pesticide — it's a natural occurrence and [moths are] part of the ecology of our forests," she said. "We don't want to be spraying that in our watersheds, anyway, or our parks. So there are no plans for that at this point."

Worcester says authorities will let nature take its course and look at the repercussions of the moth infestation on the watershed and forests afterwards.

The larvae feasting on the conifers now will finish metamorphosis and emerge as moths in September, according to Worcester.

"It sounded like rain in the forest today, like you could hear the needles dropping," she said. "It was quite amazing."