Green Alder Sawfly

Green alder sawfly larva; photo from pest alert by Jim Kruse, 2010Green alder sawfly larva -- photo by Ken Zogas, USDA Forest Service.


Green alder sawfly, Monsoma pulveratum, was first detected in the contiguous United States by Andrei Karankou, who found adults on understory shrubs beneath red alders at a park in Vancouver, WA, in early April 2010. View photos taken by A. Karankou. During 2010, sawflies were found in British Columbia, Washington, and Oregon.

Recent Updates

News from Kathy Sheehan (US Forest Service, May 31, 2011):  Cooperators from 7 agencies have set out passive sticky traps this spring to catch adult green alder sawflies in Washington and Oregon.  As of mid-May, green alder sawflies had been detected in 6 WA counties (Stevens, Mason, Grays Harbor, Wahkiakum, Cowlitz, and Clark Counties) and 2 OR Counties (Multnomah and Clackamas Counties).  The cooperating agencies are: WA State Dept. of Agriculture, WA Dept. of Natural Resources, WA State University Extension, USDA Forest Service, OR Dept. of Agriculture, OR Dept. of Forestry, and USDA Animal Plant Health Inspection Service.

News from Eric LaGasa (WA State Dept. Agriculture; June 28, 2010): Green alder sawfly was recently identified in trap samples from northeastern WA (Stevens County) submitted by Mike Johnson, WDNR survey collaborator. This find represents the first detection of green alder sawfly in eastern Washington.

News from Lee Humble (Natural Resources Canada; May 26 and 28, 2010): Green alder sawfly adults have been found at several locations in the Fraser Valley and on Vancouver Island in British Columbia. Suspected GAS larvae were also found at most sites.

News from Eric LaGasa (WA State Dept. Agriculture; May 26, 2010): This sawfly has now been collected in 9 WA counties and 1 OR county. Examination of insect collections made by students at Western Washington University in Whatcom County revealed that this insect has been present since 1995. Eric also contributed the map showing where green alder sawfly has been found in WA and OR.

News from Jim Kruse (USFS Alaska; May 3, 2010): "It appears that Monsoma larvae are very capable of utilizing wood materials to an extent not previously imagined. They have been found overwintering in the most rotten stump to various branches down to less than 1 inch in diameter, dead on the ground, standing dead, and even standing live trees." Jump to additional photos.

Background Information

Green alder sawfly adult; photo by Andrei Karankou, 2010Green alder sawfly adult -- photo by Andrei Karankou

Green alder sawfly was first found in North America in eastern Canada in the mid-1990s, and subsequently collected in Alaska in 2004. This sawfly is native to Europe, North Africa, and the Near East, and generally feeds on alder (Alnus spp.) leaves as caterpillars (larvae). Adults emerge in early spring and lay eggs on the expanding alder leaves. Those eggs generally hatch within about two weeks. The bright green larvae feed on leaves during spring and early summer, then generally drop to the ground and pupate beneath the soil surface. In Europe, these sawflies may burrow into rotten wood to pupate, and this behavior has recently been seen in Alaska. View additional information from WSU Extension.

In Alaska, defoliation by this species was first detected in 2007, primarily on thinleaf alder (Alnus tenuifolia). In some riparian areas in Alaska, feeding on alder leaves by this sawfly and two other species (woolly alder sawfly and striped alder sawfly) plus stem cankers may lead reduced nitrogen fixation by alders or even alder mortality. A 2-page pest alert summarizes green alder sawfly identification, biology, and potential impacts; this pest alert was written by James J. Kruse, Ken Zogas, John Hard, and Nicholas Lisuzzo of the USDA Forest Service, Alaska Region, Forest Health Protection.

What's Next?

Not much is known about whether this sawfly will thrive on red alder or other alders found in the Pacific Northwest. Several other insects commonly feed on red alder leaves, so these insects might compete with each other for host foliage, and the additional feeding by this sawfly may have a minor net effect on the trees. On the other hand, 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 introduced sawfly.

Given the widespread distribution of confirmed and probable green alder sawflies detected in Washington and Oregon in 2010, eradication is not a feasible option. For 2011, we are focusing on delimiting the extent of this sawfly's distribution, and encouraging research on impacts and management options. The adult flight period (March through June, depending on latitude and altitude) is currently being monitored using passive sticky traps. The bright green larvae are distinctive, so we encourage you to check defoliation on alders, and report any likely green alder sawflies found in Washington or Oregon to Kathy Sheehan (, 503-808-2674).

Thank You!

Andrei Karankou, who reported the first green alder sawflies found in the contiguous United States

Eric LaGasa (WS State Dept. of Agr.), who responded to Mr. Karankou's detections, then found additional sawflies

David R. Smith (USDA Emeritus Research Scientist), renowned sawfly taxonomist who quickly identified the sawfly specimens

Cities of Portland and Vancouver, whose Parks Departments rapidly gave permission to set out traps in their city parks

Jim Kruse  and colleagues (USFS R10 FHP), who wrote a very informative - and timely! - pest alert for green alder sawfly

Lee Humble (Nat. Res. Canada), for contributing information about green alder sawfly occurrence in British Columbia.


Additional Photos of Green Alder Sawfly -- taken by Ken Zogas (USDA Forest Service)

photo of an alder stem with much boring by green alder sawfly Alder with boring by green alder sawfly
photo of holes chewed into a live birch tree by green alder sawfly

Entrance holes on live birch

close-up photo of a green alder sawfly larva, showing strong chewing mouthparts Larva -- note strong chewing mouthparts
photo of green alder sawfly prepupae burrowed into an alder stem Pre-pupae on alder
photo of pupal niches create by green alder sawflies on alder Pupal niches on alder
Photo of pupal niches created by green alder sawfly on alder Pupal niches on alder