By David Shaw, OSU Extension Forest Health Specialist
Insects of Oregon white oak are causing some damage this summer, and you may be seeing trees that look completely brown or have scattered dead branches (distinct brown foliage clumps all through the crown). There are two different issues that have emerged around the Valley this summer: whole tree defoliation/leaf eating by the western oak looper (Lamdina fiscellaria somniaria); and scattered branch death caused by the combination of a twig gall wasp (Bassettia ligni) and the western grey squirrel.
The western oak looper (a type of inchworm) is a flashy defoliator that is a native in the Willamette Valley. The caterpillars are messy feeders, hang by silken threads, and leave browned mostly consumed leaves all over the tree, giving the tree the appearance of being dead. But the oak trees usually come back the next year. Historically, the outbreaks have been of short duration in any single area (typically one or two years) and the oak trees rarely suffer long term damage even though the defoliation may be spectacular. However, when conifers such as Douglas-fir are intermingled with affected oaks, they can also be defoliated in an outbreak and impacts to the Douglas-fir may be more severe.
The photos above show the western oak looper and were taken near Sheridan this week (click photos to enlarge).
There are registered pesticides for control of western oak looper with a biological control using a bacteria (Bt) being a most common approach. However, this year, it is likely too late to apply any pesticides. The caterpillars are present from June to September, but most pupate in late August. The moths will fly in late September, and can be very abundant in areas where the outbreaks have occurred.
The oak twig gall wasp lays its eggs under the bark surface and the grubs develop and pupate under the bark, causing small galls, but possibly many of them. The galls may cause girdling of the branchlets. However, the abundant damage we observe in oak is from the western grey squirrel that removes the bark from the entire twig area to access and eat the grubs and pupae. The debarking by squirrels causes the branches to die toward the tip and this is the common symptom we see. There is no known control for this combination of gall wasp and squirrel.
Here are some photos of damage from the twig gall wasp and squirrel, from north of Corvallis (click photos to enlarge).