Growing Concern in Willamette Valley about Spread of Western Oak Looper
Other tree species, including Douglas-fir and Oregon ash, have also been affected, primarily in areas where they are heavily intermixed with the more preferred oaks.
(SALEM, Ore. ) — Various State of Oregon agencies have been receiving an increasing number of calls in recent weeks concerning what is in some cases a second year of defoliation of Oregon white oaks in areas of the mid-Willamette Valley.
Other tree species, including Douglas-fir and Oregon ash, have also been affected, primarily in areas where they are heavily intermixed with the more preferred oaks.
The defoliation is due to the Western oak looper (Lambdina fiscellaria somniaria), a native moth that has periodic outbreaks in the Willamette Valley.
Affected trees are conspicuous in late summer by the scorched appearance of the leaves, and on closer inspection, by the presence of large numbers of small caterpillars. Defoliation often tends to be localized and can be highly variable; where high populations occur, trees may appear almost completely defoliated.
Previous outbreaks in the Willamette Valley have been recorded.
"In 2013 and 2014, the most heavily affected areas were generally within the extent of previously reported outbreaks, with the majority occurring west of the I-5 corridor in Yamhill, Polk, and Benton Counties along with other, smaller affected areas in Marion and Linn Counties," says Oregon Department of Forestry Entomologist Rob Flowers.
Due to the high tolerance of oaks to defoliation by this insect and the short-duration of most outbreaks, non-chemical management is recommended. Infestations of caterpillars on small, higher-value ornamentals can be manually removed by high-pressure sprays. Keeping trees healthy and vigorous is a good way to help trees survive defoliation outbreaks when they occur.
While there are some insecticides registered for control of the Western oak looper in Oregon, biological products such as Bacillus thuringiensis (Bt) offer the best option as they have less impact on natural enemies and other non-targets.
Pesticide applications are recommended in early summer as the caterpillars are emerging and beginning to feed. Treatments against older larvae in late summer and fall are generally ineffective as many have already completed feeding and moved from the foliage to pupate.
Douglas-fir or other conifers that are intermixed with oaks and become heavily defoliated are more at risk of dieback, top-kill, or even mortality. However, it is very difficult to predict which trees will survive and which ones will not, so the general recommendation for landowners wishing to maintain a conifer component in affected oak stands is not to harvest affected trees until it's clear they will not recover, indicated by no flush of new foliage in the spring.
Any conifers that are killed need not be removed if they pose no danger to life or property and can be retained for wildlife habitat.
Oak looper populations expected to decline in 2015:
In Oregon, outbreak levels and severe defoliation typically last only 2-3 years. During the first year of increasing populations, defoliation often goes unnoticed, but moths may be observed. During the second and third years of the outbreak, populations typically expand and the defoliation is most severe.
In the latter year(s) of the outbreak, some expansion and continued defoliation may occur, but damage tends to decline rapidly from the combined effects of naturally-occurring diseases, predators, and parasites.
Therefore, populations are expected to decline in 2015 and subsequent years as the outbreak returns to endemic levels.
For more information:
If you have questions or concerns about Oregon white oak in your area, please contact Rob Flowers at RFlowers@odf.state.or.us
Source: Oregon Department of Forestry
