Spruce Bark Beetle

Welcome back to Ask a Naturalist, your own personal Google for information on all things natural on PEI! Today’s topic comes from my own woodlot. Like many landowners, I’ve noticed that White Spruce on parts of my property are dying, so I took a hike to find out why. The answer was easy to find: Spruce Bark Beetle (Dendroctonus rufipennis).

Photo 1: Bark Beetles have affected this White Spruce on PEI.

Spruce Bark Beetles are native insects present throughout North America wherever Spruce trees are found. They’re common here on the Island, but you may not notice them until they’ve been in a tree for more than a year and the tree’s needles start turning yellow. Eventually, the Spruce will start losing bark (Photo 1). On this particular tree, notice the odd, flat look to the remaining bark; some insect-eater – likely a Woodpecker or Nuthatch – has flaked off the outer bark scales in order to get at the tasty beetles and larvae underneath. 

Photo 2: Initial signs of Bark Beetles are holes in the bark where the females entered, along with piles of resin (called pitch tubes) the tree produces to try and seal the holes.

Spruce Bark Beetles have a two-year life cycle. In late spring, adult females fly to new trees and begin by boring holes through the bark. Early signs of beetle presence are these holes, as well the piles of resin (called pitch tubes) that the tree produces in an effort to seal them up (Photo 2). Male Bark Beetles follow the females under the bark where they mate, and the female lays eggs. 

Photo 3: Spruce Bark Beetle larvae on PEI.

Within a few weeks, the eggs hatch into white-bodied, tan-headed larvae that grow to about a quarter-inch (6-7 mm) in length and create shallow grooves under the bark as they feed (Photo 3). Spruce Bark Beetles spend their first winter in this larval form, pupating the following spring to emerge as adults. Those adults spend their second winter either under the bark or in groups at the base of the tree where they hatched, insulated and protected by snow cover until flying off to new trees the following spring.

Many people have noticed new patches of dead and dying Spruce across the Island. I can’t confirm Spruce Bark Beetle as the culprit in every case, but I expect they account for most. The insects are known to increase in number following landscape-scale stresses such as drought or extreme wind. Given their two-year life cycle, it makes sense that there are more of them around now, following 2022’s Hurricane Fiona.

Fiona’s winds not only toppled many trees but also damaged and stressed some of those still standing. Most plants, including trees, have tiny root hairs that greatly increase the surface area of their roots. Root hairs play critical roles in providing water and nutrients to the trees, as well as forming important relationships with certain types of fungi. It’s likely that the violent rocking back and forth during Fiona’s winds damaged root hairs of some standing trees, weakening them.

On my own land, Bark Beetle sign is much more prevalent where the Hurricane effects were greatest, including in the lowlands along my streams. Beetles are certainly making use of the fallen trees, but it’s likely that the taller Spruce in these softer riparian soils suffered root hair or other damage, making them more susceptible to Beetle attack. All my younger Spruce (shorter trees that would have been less affected by wind) as well as trees in areas less impacted by Fiona are healthy with no sign of Bark Beetles.

If you’re managing your woodlot for lumber or other wood products and have signs of increased Bark Beetle activity, you may want to harvest those areas of Spruce while they’re still sound. For the rest of us, there’s no real action we can take nor reason to be especially concerned.

Spruce Bark Beetles don’t affect other types of trees. PEI’s patchwork landscape and mixed forest mean we won’t see the huge outbreaks that occur in western Canada, where there are extensive stands of Spruce covering areas the size of PEI (and larger!). Increases in Bark Beetles typically only last a few years and will likely soon subside without major impact. There will be some dead Spruce trees, of course, but these provide food for other insects and decomposes (as well as animals that feed on them), habitat for cavity nesters, and – eventually – become recycled into forms that can be used by future generations of trees, ground plants, animals, and fungi.

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