Storm damage may lead to beetle problems in ponderosa pine

By Brad Withrow-Robinson, OSU Forestry & Natural Resources Extension – Benton, Linn & Polk Counties

Not to be a fear-monger, but there is talk about last winter’s storm damage leading to some future beetle problems for ponderosa pine in the Valley.

Now, bark beetles are generally weak predators of trees.  Damage is often limited to marginal sites, with beetles usually attacking trees weakened by other stresses, such as drought or flooding.  Generally this does not pose a great  threat to the other, healthier trees in the area.

But I recently spoke to a couple landowners concerned about bark beetle attacks in their ponderosa pine.  Their thinking goes as follows: storm events that cause blowdown can create lots of weak or dying trees- prime beetle rearing habitat!-which sometimes allow beetle populations to grow to a point that they are numerous enough to attack adjacent healthy trees.  This is particularly true of the  California fivespined Ips (Ips paraconfusus) in ponderosa pine.  The tiny Ips beetle needs weak or dying trees to rear brood and is very fond of fresh storm damage just 3 inches in diameter and up.  And because Ips have two generations a year in the Willamette Valley, they can have explosive population increases when conditions are right.

Their logic is good, and the landowners I was talking to were worried that conditions this year are lining up for an Ips beetle increase that might harm their young ponderosa pine plantings.  I have not seen much storm damage directly to the ponderosa pine plantings, but I have seen some of scots pine plantings which sustained significant damage.  Much of it is above 3 inches, freshly down and so potentially ideal breeding material.

Why “potentially?”  Well, it is certainly the right size wood to cause problems, and Ips have been collected from many non-native ornamental pine species (including scots pine) here in the Valley.  But Rob Flowers, ODF State Entomologist tells me it is not clear how well scots or other exotic pine species support Ips brood production.  Nonetheless, he expects we may see an up-tick in Ips damage here and there in the Valley where there was storm damage.

Here is why.  The first IPs flight of the year is probably about to start as the overwintering generation of Ips emerges to look for breeding sites.  Their flight generally peaks around early-May here in the Valley.   If there is lots of down breeding material laying around for too long, we might see a large emergence of the first summer generation in early-mid July, and the second flight period of the year. It is this generation that could pose a significant threat to even healthy ponderosa pine stands in the neighborhood.  The July beetles cannot find any fresh slash, so they are more likely to attack standing green trees, just when trees are typically starting to be under some drought stress. This flight will in turn lead to the second generation of the summer, emerging around mid-October to be the overwintering generation.

Generally, the key to preventing large brood build ups  of the California fivespined Ips is to clean up damaged stands early.  Trunks should be cut from the roots, made into firewood, and stacked to encourage rapid drying.  Larger slash needs to be chipped, burned or spread out to dry, rather than left in slow-drying piles. Timeliness is important to prevent larvae from completing their development in June.  Rob figures one could theoretically stretch the clean-up period until then, but cautions that every year is different, and our understanding and prediction of development rates and emergence dates is not an exact science.

I’ll close by referring you to two resources for more information about the bug and slash treatment recommendations, ODF Forest Health Note on the California Fivespined Ips  and  WSU Pest Watch publication .