February Was Cold, But Was It Enough to Cause Bark Beetle Die-off?

Just how cold does it have to get to kill those pests?

Many of us like to look for silver linings, and the shining hope of a frigid winter is that overwintering insects suffered more than we did. If there are fewer harmful creepy-crawlers this summer, then maybe that chilly February was worth it.

I can’t speak to how the yellow jackets fared (hopefully poorly), but I do know that our problematic bark beetles—specifically the fir engraver and Douglas fir engraver—took it all in stride. February was hardly a blip on the radar for their sleeping larval selves. It was not cold enough to cause beetle die-off. Thus, expect more browning firs in the months and years to come.

When it comes to winter, beetle larvae are way tougher than we are. They use snow around the base of a tree and the tree’s bark as insulation. The larvae excrete all their stomach contents so the liquid won’t freeze up. Then they fold over on themselves and generate a kind of internal antifreeze. Finally, they wait, able to withstand temperatures of up to -40 degrees.

February’s chill didn’t even come close to that.

This is the characteristic “mining” pattern the Douglas-fir engraver beetle larvae makes when eating the tree’s inner bark. Each beetle has a pattern all its own. Image courtesy of Wayne Brewer, Auburn University, Bugwood.org.

The best hope for temperature-related beetle die-off is to have a hard freeze—below zero—very early or very late in the season. Like in October or now, in March or April. If the freeze comes early, the beetles won’t have yet generated enough antifreeze in their systems to survive; and if the freeze comes late, the larvae will have already woken up with the warmer weather and shed their defenses. However, even in these scenarios, beetles can always fly in from areas that didn’t get as cold.

Thus, it seems, the beetles—and their detrimental effects—are with us for at least another year.

So, in the absence of such beetle die-off, what is a landowner to do? First, keep an eye on the health of your trees. Beetles are opportunists. They’ll go for stressed trees whose defenses are down, and then they’ll spread from there. Cull the diseased trees from your forest to limit infestation opportunities. And while you’re at it, just thin your forest in general. Trees living in competition with one another for light and nutrients don’t have the necessary resources to fend off beetle attacks. Give your trees the room they need to thrive. Finally, if you do have beetle-kill on your property, remove it.

To learn more about beetles, check out these Forest Service documents on the fir engraver and the Douglas-fir engraver beetles. Also, our last post was about bark beetles in North Idaho. Finally, don’t hesitate to give us a call if you have any questions about the health of your trees.