By David Shaw, Forest Health Specialist, OSU Forestry and Natural Resources Extension
Douglas-fir killed by drought
The summer of 2015 is shaping up as a big year for drought and drought related forest health issues throughout Oregon, but especially in the Willamette Valley, SW Oregon, and in Eastern Oregon.
In late summer, it can be very difficult to discern whether insects, disease, or drought and heat are causing tree dieback and deaths, but we are becoming pretty confident that drought and heat together are influencing much of what we see. In this report I outline and describe some of the more common problems we are seeing with conifers and hardwoods as of early September.
By David Shaw, OSU Extension Forest Health Specialist
Top left: western oak looper caterpillar; Top right: oak leaf eaten by looper; Bottom left: affected tree appears dead; Bottom right: caterpillars aggregating on a fence post. Photos: Dave Shaw
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. Continue reading →
