What are these Worms in my Firewood?
Have you seen these little grubs in your firewood??
If you’ve spent any time in the forests of the western prairies, you may know them by their adult form - the sawyer beetle. The males are characterized by their antennae which are typically twice the length of their body. Adults feed on coniferous foliage and bark, with a liking for pine, firs and spruce.
The larvae pictured here are round headed borers and are most likely the white spotted sawyer, or Monochamus scutellatus. These insects bore into the wood of the white spruce (amongst other conifer species), creating oval shaped tunnels into the sapwood and eventually in to the heartwood of the tree.
The larvae have a two year cycle, hatching from eggs left in slits chewed into the bark by the adult female mid-summer. The larvae then begin feeding in the cambium area of the tree for several weeks before excavating a hole and boring down into the sapwood. They will then spend their winter safe and protected within the sapwood of the tree.
Feeding resumes the following spring, with the larvae penetrating deep into the heartwood of the tree before eventually working their way back towards the surface. Pupation then occurs after yet another winter spent safe inside the tree, after which the adults emerge from circular shaped exit holes.
The larvae pictured here were found within a stressed and dying white spruce removed within Redwood Meadows, after splitting the rounds for firewood. These will be in their second year of growth, and would’ve spent the summer boring through the wood before overwintering and pupating early next spring.
Borers like the white spotted sawyer tend to seek out dead or decaying trees to lay their eggs, and the larvae can cause extensive damage to dead or dying conifers. While the creation of galleries and tunnels within the tree can certainly compromise the overall integrity, they’re typically a sign that the tree was already in a state of decline.
Where they can become particularly troublesome is in highly distressed trees, particularly those that have experienced defoliating insects and drought conditions. It is possible for these borers to kill a living tree under stress, conditions currently being felt by our white spruce through the bud worm infestation and several years of generalized drought conditions.
Trees with larvae within can be spotted by looking for the small holes in the bark where the adult females had laid their eggs. Active larvae will typically expel frass from these holes. Frass is essentially the excrement of the larvae, and looks like fine powdery sawdust. You can spot frass in a few of the galleries within the pictures. Frass and small wood chips will typically accumulate at the base of a standing tree in severe infestations.
Written by Sean Sterna - The Rocky Mountain Arborist & Used with Permission
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