Flatheaded fir borer life cycle

Asked March 20, 2019, 12:27 AM EDT

This time of year, we've been seeing quite a number of adult flathead borers, which strikes me as a bit unusual. My understanding is that the adults come out in the summer, not the winter. Down here in Eugene, we've had quite a Doug-fir dieoff, mostly due to drought. We've done some logging to keep only the healthy trees, but chipped the infested logs, rather than burn them. Was this a mistake? Have we done something that will serve to propagate the borers?

Lane County Oregon forestry

2 Responses

Flatheaded fir borers are right on time. The adults emerge in the spring March-April. So you might just be noticing them more because of the Douglas-fir die off and all the stressed trees we have as a result of the drought. Chipping the infested logs isn't a mistake this dries them out fairly quickly and shouldn't attract beetles to the area especially if you aren't chipping during the time when adults are flying. The only thing that may have slightly increased the borers introduction is if there was a lot of damage to the residual trees when your thinning was done. If there was a lot of damage this increase the tree's stress and that can attract the borers. Otherwise you are doing the right thing. Douglas-fir is only a moderately drought tolerant tree, so this prolonged exposure to drought in soils with heavy clay is particularly tough on the species. Depending on your objectives for your property, if you are having major areas of die off you may consider favoring more drought tolerant species such as Willamette Valley Ponderosa Pine, Incense-cedar, and Oregon White oak.

If you'd like to read up on the borer a bit more. Here is a great resource from our Oregon Department of Forestry - Forest Health Team - https://www.oregon.gov/ODF/Documents/ForestBenefits/FFB_2016.pdf

We planted a few acres of ponderosa pine about 25 years ago ("Fox Hollow" strain) and they've done better than expected, although the ice storm of a couple of years ago did some real damage. Same for the white oaks that we had--the heavy snow and ice did us no favors.

The drought, in addition to stressing the Doug-firs, manifested itself in the utter lack of "volunteer" seedlings--we can remember when they sprouted like weeds.

Thanks for your response--the chipping was done during the summer last year (water truck in attendance). We were pretty unforgiving when assessing tree health, so even the live trees that were showing stress effects were taken.