Hemlock looper moth outbreak

An outbreak of western hemlock looper moths is occurring throughout North Vancouver, which can result in damage to trees and forested areas.

The western hemlock looper is a native species part of the natural coastal forest ecosystem that feeds on trees, particularly in the Fromme and Lynn Valley area where the community borders the forest.

We've been receiving inquiries from the community about the outbreak as the moths and impact on trees become more visible, so we put together a summary of frequently asked questions. 

How long will the outbreak last?

Outbreaks can last three to four years and we are currently in year two.

While outbreaks are not uncommon and populations build every 11 – 15 years in our region, the moth populations could diminish next year or we may see another year of defoliation next summer. This depends on weather and other environmental factors.

Can we control the outbreak?

There are no practical measures we can take. It's best to let the outbreak naturally run its course, usually within three years. 

Which trees are being damaged?

The moths primarily feed on Western Hemlock trees, but also feed on other hosts including Douglas-fir, subalpine fir, amabilis fir, grand fir and spruce when populations are high.

Deciduous trees such as native maples and forest understory vegetation may also be hosts.

Will the trees die?

Some trees that are less tolerant of being defoliated, western hemlock in particular, may succumb to the damage and there is evidence already of mortality occurring in the hardest hit areas. 

Are the dead trees a hazard?

Not immediately, as generally the trees were healthy prior to being defoliated and dying. It will require years of decay before trees become structurally weaker.

Trees that have pre-existing major structural defects and decay, on case by case bases, may have to be removed sooner if there is a nearby target at risk. 

What is the impact to forest health?

The killing or removal of these susceptible trees is an important component of ecosystem dynamics and essential in recharging ecosystems by allowing younger trees to emerge, while supporting the recycling of nutrients. This is a natural and important process.

Will there be an increase in the wildfire risk?

This will depend on the severity of the outbreak and resulting damage. Once the outbreak runs its course, we will assess the forested areas along the wildland urban interface — where our community meets the forest — to determine the risk.

Will dead trees on a steep slope hazard area result in instability issues?

This will depend on many geotechnical factors and the severity of tree mortality. As the trees were generally heathy prior to dying, the roots systems and especially the main structural roots will take a long-time to decay, therefore providing stability for many years while new native vegetation and trees establish.

A geotechnical engineer will make recommendations to determine the risk associated with significant tree loss on slopes with known stability and erosion issues.

What can we do to help?

There is not a lot we can do to control the situation. We do ask that you have patience while the outbreak completes its natural cycle and see whether defoliated trees recover their health.

If it appears that trees on your property have died and become hazardous, please report the problem using our online tool.

Report trees issues on your property