Due to the past hot, dry summers, the area is experiencing an increase in pest activity such as bark and engraver beetles. Stressed trees are more likely to be targeted by insects. Below are tips from local entomologist, Erika Eidson with the Idaho Department of Lands, to keep your trees healthy to prevent tree loss and the spread of these harmful insects.
Keep trees healthy and vigorous. Apply supplemental water. Avoid soil compaction. Avoid damage to the trunk. Don’t over-fertilize, especially in dry years. Don’t use pesticides unless you need to so you don’t unnecessarily kill off important insect predators. Maintain appropriate tree spacing to reduce competition for water and sunlight.
Keep your site clean. Clean up winter storm damage. Remove slash material from the site or dry it out as fast as possible by lopping and scattering it. If you can’t do those things, wait to create slash piles or to cut green firewood until between August and Nov, so it dries out before the next season. Don’t treat firewood with pesticides, it’s against the label. Cutting dead trees is not a problem (since the insects have already emerged). Covering piles with plastic can help, but logs must be rotated to effectively heat all sides and kill the beetles. Beetles can chew through plastic, but the plastic will slow them down if you have no other options. Large downed Douglas-fir and larch should also be cleaned up.
Remove/destroy infested materials, as mentioned above. If you see a red tree, check the green trees around it for pitch tubes or boring dust. Often people chase the red & dead trees that the beetles have already left, and they miss the currently infested trees because it takes a while for the needles to fade.
Pesticides only work PREVENTATIVELY. Trees that have already been successfully attacked cannot be saved. Carbaryl or pyrethroid full coverage bark spray is usually the standard. It is expensive and short-term. It isn’t recommended unless there is a viable threat nearby. As far as pheromones, MCH works great for Douglas-fir against Douglas-fir beetle only. Verbenone works moderately well against mountain pine beetle only, and it may also have some effectiveness against red turpentine beetle, but that is usually not a tree killer. Pheromones are also short-term, and must be applied at the appropriate time of year, before the beetles fly. Homeowners should seek professional advice from an entomologist when using pheromones.
Do not keep infested firewood onsite. The beetles can come out of the infested firewood and attack nearby trees. Infested material needs to be taken to the dump, chipped, burned, or debarked. If it’s western pine beetle, the bark itself also needs to be destroyed, because they actually develop in the bark, not the phloem.
The large beetles often seen in firewood piles and on dead trees are wood borers, not bark beetles. They do not need to be the target of your efforts! They are generally harmless recyclers here in Idaho. People hear/see them and become alarmed.
Attached is a fact sheet with more information. Since each beetle has a different life cycle and treatment, it is important to know what you are dealing with if a pest has been confirmed. As mentioned, keeping trees healthy to begin with is the best option for pest prevention.