Ask The Entomologist

Pine Bark Beetles and Fire Risk

The dense smoke currently hanging over our part of the country has had me thinking about beetles. Why beetles, you say? Pine bark beetles and relatives (genus Dendroctonus) are notorious for killing large stands of coniferous trees (Fir, Larch, Pine, etc.) – even their scientific name translates to “tree killer”.

As time goes by and environmental stressors increase, we’re seeing more and more tree die-offs related to these bark beetles. Death by bark beetle can triple the rate at which trees go up in flames. It’s not fair to put all the blame on the beetles, though – tree death is ultimately caused by infection from the fungi that the beetles spread. The one bright side of how destructive these beetles and fungi are is that they have been well studied. As a result, there are many ways of controlling their infestations.

Douglas-Fir Beetles I found under bark while splitting some Douglas Fir wood.
Note the “galleries” (tunnels) they’ve made – many bark beetles can be identified by the shape of their galleries.

The most effective way we can control pine bark beetles on our own is by good management of our forest resources. Keep tree stands resistant to pine bark beetles by thinning trees to a healthy density – beetles are most likely to become a problem in over-crowded woods. Beetle damage will become evident by “boring dust” – spots of reddish sawdusty powder along the trunk, and “pitch tubes” – globs of resin the tree has pushed out, trying to stop a beetle’s tunneling (red pitch tubes likely indicate infestation, while yellow ones often indicate successful defense).

Remove stressed, fire-damaged, and wind-thrown trees. Promptly clean up fallen timber. Damaged and downed trees are excellent places for pine bark beetles to reproduce, and should be removed before new beetles emerge! Where one beetle feeds and reproduces, there will soon be more – pine bark beetles attract each other to damaged trees by releasing aggregation pheromones.

A close-up of three Douglas-Fir Beetles – they have since been executed for their crimes.

My favorite method of control involves removing the pine bark beetles’ symbiotic fungi. Without these fungi, most of the beetles can’t reproduce well. Because it’s not practical to directly remove fungi from beetles in the wild, scientists have raised pine bark beetles in captivity. These captive-raised beetles can be inoculated with special aggressive fungi which can outcompete and kill the fungi that wild pine bark beetles carry. The thought is that captive beetles could be released and will interact with wild beetles, spreading their different fungi like a disease. The new fungus will prevent beetle reproduction and thus stop small infestations from becoming large and economically devastating ones.

Unfortunately, beetle-delivered fungal control treatments aren’t ready for public use yet – ask your Forest Service folks what beetle controls they are researching and testing! If you’re particularly concerned about pine bark beetle infestation, they may be able to use chemical measures like anti-aggregation MCH pheromone to lessen the risk.

Have you been seeing signs of pine bark beetle damage near your home?