Don't Let Firewood Bug You

[Paul J Hetzler]

All of us are using more heating fuel this season than in recent winters, and there is still plenty of cold weather to come. It’s bad enough that our wallets are thinner, but those who heat with wood have the additional burden of more time spent lugging in fuel. And to add insult to injury, uninvited guests occasionally show up with the wood.

Firewood, I’ve been told, comes from “trees” which seem to be covered in “bark,” under which insects can hide. As the wood we bring inside warms up, it feels to the critters under the bark like winter is over, and they gleefully sally forth. Inevitably, insects and homeowners are both disappointed.

The good news is that we can take steps to discourage critters from making our woodpile their condo in the first place. Also, most of these guests are merely a nuisance and just want out. The bad news is, well, they’re a nuisance. The two types of firewood crawlies are shelter seekers and wood borers. Both kinds of stowaways usually head for a window where you can let them out or...something.

Shelter-seekers just need a place to crash for the winter, and would be as happy in a brush pile as in a firewood pile. These include woolly caterpillars, ladybird beetles, and non-insects like spiders and centipedes.

Wood borers range in size from less than one-eighth up to three inches long, but most are on the small side. The largest are roundheaded borers, of which the white-spotted (pine) sawyer and its similar, sinister cousin the Asian longhorn beetle are examples. Flatheaded borers include the native bronze birch borer as well as the invasive emerald ash borer. Ambrosia and elm bark beetles are small and they stay just under the bark and don’t enter the wood.

With some notable exceptions, wood borers seek dying or just-killed trees, sometimes arriving to lay eggs hours after a tree is felled. Obviously they don’t need social media to keep up on news. They do need moisture, though, which plays into control options.

If conditions are right, it’s possible for certain insects to cause trouble. Tiny powderpost beetles (one-sixteenth of an inch) can infest bare hardwood in high-moisture environments. In the heating season your living space is too dry for them to survive. But firewood stored in a basement could be an issue if the joists are unpainted hardwood, which is sometimes the case in very old homes. Carpenter ants also need moisture to set up housekeeping. Unlike termites, they can’t eat wood, and can only make nests where moisture has initiated decay.

No matter what kind of wiggly passenger you see on firewood, never treat it with insecticide. Burning insecticide-treated wood poses a real health risk to those in the home.

The key to critter prevention is this: If they’re not in your firewood, they’re not getting inside. And they only like firewood if it’s damp. Seasoned wood that has been stored off the ground and out of the weather is unlikely to harbor insects. Keeping firewood out of garages and basements is highly recommended—ideally it should be stored away from the house in a non-attached structure on pallets or a dry floor. If possible, cut and split it at least12 months in advance of burning, and stack it such that it has good air circulation. Cover woodpiles on the top only, as side coverings will hamper drying.

Dry wood will also save you money. Compared to their dry weights, premium hardwoods like maple, hickory and oak may run between 60 and 70% water when first cut. In this state they may only give you 18-20 mBTUs per full cord compared to their potential 25-28 mBTU/cord when dry. You’ll have to chuck a lot more logs in the stove, and you’ll have to clean the chimney more often. And anyone living downwind from you will eat a lot more smoke. The bottom line is that when fuel wood is not dry, it can mean that up to a quarter of its heat value gets used to simply boil water out of firewood.

To burn even marginally clean, firewood has to be below 25% moisture. Wood is hygroscopic, that is it absorbs moisture from the air depending on the humidity. However, even in extreme humid conditions, wood that is covered for six months or more will not usually be over 20% moisture. If you’re curious about the actual moisture content of your firewood, hand-held moisture meters are available in the $25 to $50 price range at many hardware stores. It’s a small investment for peace of mind.

If you cut your own wood, timing can help minimize bugs, too. Trees cut between late autumn and early spring, especially if the wood is split right away, are less likely to garner wood-boring insect eggs. However, even if insects get a start, they’ll perish when the wood is fully dry.

For more information contact your local Extension office. They'll help you work the bugs out of heating with wood.