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New Pest Presents Potential Danger to Western U.S.
Government researchers find green alder sawfly in Western states, raise concerns about the potential impact.

By Chaille Brindley
Date Posted: 6/1/2010

           Government scientists recently found a new wood pest that could cause problems for forests in the Western United States. They discovered the green alder sawfly (Monsoma pulveratu) in Washington state this year. It has also been sighted, although not collected, near Tillamook, on the Oregon coast. The green alder sawfly prefers alder trees, which are key species on the West Coast. Nobody knows for sure what kind of impact this sawfly can have because a lot depends on how it adapts to conditions in the United States.

            “This pest is brand new to the United States…We don’t know if this is going to be big yet,” said, Kathy Sheehan, entomologist with the U.S. Forest Service.

            The green alder sawfly is native to Europe, North Africa, and the Near East, and generally feeds on alder leaves as caterpillars (larvae). It was first detected in North America in eastern Canada in the mid-1990s and subsequently collected in Alaska in 2004. The pest has been found in willow and birch species although alder seems to be its preferred target.

            Researchers don’t know if the pest will thrive on the red alder and other alder species found in the Pacific Northwest. It appeared to spread in the thinleaf alder stands in Alaska. In some areas in Alaska this sawfly combined with two other varieties of pest (woolly alder sawfly and stripped alder sawfly) and a stem canker problem to reduce nitrogen fixation by alders and even kill some trees. Researchers have had a difficult time assessing how much of the problems in Alaska were caused by the green alder sawfly compared to the other factors. Conditions, species, weather and other key factors tend to vary from region to region.

            Alder can be an important species for adding nitrogen to the soil, a vital ingredient to enhance plant growth and biodiversity. Alder is one of a limited number of tree species that can take nitrogen from the air and inject it into the soil through a tree’s roots. By eating the leaves, the green alder sawfly can reduce the ability for alder trees to fix nitrogen into the soil. According to the U.S. Forest Service, up to 70% of the available nitrogen in the most productive forests may have its origin in stands of alder and that the rate of nitrogen fixation can decline by as much as 73% following defoliation.

            The goods news is that in the long run the green alder sawfly may not be as bad as the emerald ash borer or the Asian longhorned beetle. These two pests, which have become the poster children for wood pest epidemics, are known to burrow into live trees and cause tree mortality. Based on current research, the green alder sawfly doesn’t generally burrow in live trees and is much less likely to cause tree mortality than some other invasive species.

            Researchers recently found in Alaska something that they have not seen before in the North America. The green alder sawfly pupated in wood that had been damaged instead of the soil, which is common for the pest. This could be a problem if the sawfly adapts to pupate in wood like it has done in some incidents in Europe. It could lead to people unknowingly spreading the pest by moving firewood outside of the infestation area. Human activity is known to have been a major cause in other pest outbreaks, such as the emerald ash borer.

            The life cycle of the green alder sawfly starts when adults emerge in early spring and lay eggs on the expanding alder leaves. Those eggs generally hatch within about two weeks. The distinctive, bright green larvae feed on leaves during spring and early summer, then generally drop to the ground and pupate beneath the soil surface.

            Looking at the potential danger in the Pacific Northwest, the situation could go either way. Several other insects commonly feed on red alder leaves, so these insects might compete with each other for host foliage. If this happened, the net effect of the new sawfly might be negligible. However, the green alder sawfly generally begins feeding earlier in the spring than the other species, so it might out-compete the other species if foliage becomes limited. Also, parasites are important natural enemies for many other sawfly species, and we do not know if any native parasites will switch over to this newly introduced sawfly.

            Sheehan commented that given the widespread distribution of confirmed detections, the green alder sawfly has likely been here for a while, which makes eradication highly improbable. Government entomologists are working to determine the actual scope of the problem and ways to mitigate the risks.

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