The Mystery of Grubs in Your Firewood, Solved

Have you spotted little white grubs in your firewood lately? If you live in Calgary, Redwood Meadows, or Bragg Creek, you’re not alone. These “worms” are actually round-headed borer larvae, most likely from the white spotted sawyer beetle (Monochamus scutellatus).

They’re common in coniferous firewood — especially white spruce — and while they might seem gross, they’re also an important indicator of tree health and even larger issues in our local forests.

Meet the White Spotted Sawyer Beetle

You’ve probably seen the adult version before, especially in summer. These longhorn beetles:

• Prefer pine, fir, and spruce
  
  
• Have long antennae (males’ are twice their body length)
  
  
• Feed on bark and foliage
  
  
• Lay eggs in dead, dying, or heavily stressed trees
  
  

It’s what happens after the eggs hatch that really impacts your firewood.

The Larvae: Round-Headed Borers and Their Life Cycle

The white spotted sawyer beetle has a two-year life cycle, most of which is spent as a larva inside the tree.

Year 1:

• Eggs are laid mid-summer in bark slits
  
  
• Larvae feed in the cambium (the tree’s vital growth layer)
  
  
• By fall, they bore into the sapwood to overwinter
  
  

Year 2:

• Larvae resume feeding, going deeper into the heartwood
  
  
• By summer’s end, they begin working back toward the surface
  
  
• A second overwintering occurs, followed by pupation and emergence the next spring
  
  

Telltale Signs:

• Oval galleries inside the wood
  
  
• Circular exit holes from adults
  
  
• Frass (fine sawdust-like excrement) near holes or firewood pile
  
  

Are These Firewood Worms Dangerous to Trees?

Yes — and no. Here’s the breakdown:

Typically Not a Problem If…

• The tree was already dead or severely stressed
  
  
• The firewood is stored properly and dry
  
  

They Become a Concern When…

• The tree was still barely alive, but under extreme stress (drought, defoliation, etc.)
  
  
• Borers infest living trees, which can happen if populations grow or other pests (like spruce budworms) are weakening trees in your yard
  
  

In these cases, larvae can speed up a tree’s decline or even kill a living one — especially in combination with other stressors.

How to Identify Borer Activity in Live Trees

Red Mountain Rigging regularly inspects trees for early signs of borer infestation. Here’s what to look for:

• Small, round holes in bark (adult exit holes)
  
  
• Frass and wood chips at the base of a tree
  
  
• Galleries under the bark if removed
  
  
• Dieback or browning foliage, especially from the top down
  
  
• Visible stress after drought or defoliation events
  
  

Spotting this early can mean the difference between tree removal and tree recovery.

Real Case: Borer Larvae in Redwood Meadows Firewood

We recently removed a stressed white spruce in Redwood Meadows and found numerous larvae while splitting the rounds. These were clearly in their second year, boring deep into the heartwood and prepping to pupate by spring.

This tree had:

• Bark inclusions trapping moisture
  
  
• Structural damage and internal decay
  
  
• Clear signs of secondary pest invasion
  
  

It’s a perfect example of how seemingly minor insect damage can be part of a bigger tree health issue.

What Should You Do If You Find Worms in Your Firewood?

If the firewood is dry and stored properly:
You’re safe. The larvae won’t infest other trees or cause structural damage indoors. Just burn the wood.

If you notice active holes or frass on standing trees in your yard:
That’s when to call Red Mountain Rigging for a free tree assessment. We’ll check for pests, decay, and structural risks — and recommend safe, economical solutions.

FAQs About Worms in Firewood and Tree Borers

What are the white worms in my firewood?
They’re most likely the larvae of the white spotted sawyer beetle, a type of longhorn beetle common in conifers.

Can these worms infest my home?
No. They can’t survive long indoors or infest finished wood structures — just burn them safely.

Can sawyer beetles kill a living tree?
While they prefer dead or dying trees, they can harm stressed trees during drought or insect infestations.

How do I know if my spruce tree is infested?
Look for round holes in bark, frass at the base, or call us for a free inspection.

Should I treat my firewood for insects?
No need to treat. Just keep it dry, off the ground, and rotate it so older wood is burned first.

Final Thoughts: Firewood Grubs Are Just the Tip of the Iceberg

Finding “worms” in your firewood might feel a little creepy — but it’s also a valuable warning sign about the health of our urban forests.

If you’re worried about pests in your trees, or simply want to learn more about what’s living inside your firewood, we’re here to help.

Contact Red Mountain Rigging today for a free tree risk assessment or expert advice on Calgary firewood pests and tree health.