Sawfly Moves in; Sitka Alders at Risk

Sentinel Staff Writer
    A new invasive species has begun hitting Sitka’s alders, but federal biologists say they don’t expect it to cause the same harm in Southeast as it has in Southcentral Alaska.
    The green alder sawfly is native to Europe, North Africa and East Asia, and was first positively identified in Alaska in 2009. By the time it was identified, it was found to be actively defoliating thin-leaf alder along riverbeds in Anchorage, Kenai, Seward, and in the Matanuska-Susitna River Valley, says the local Cooperative Extension Service.
    “The sawfly has done a lot of damage to alders in the Anchorage area,” said Bob Gorman, the CES extension agent in Sitka. “They’ve been just about wiping out the alder in the streambeds which is important for shading salmon habitat in streams.”
    Ship Creek, the main stream that runs through downtown Anchorage, has been particularly hard hit, he said.
    “It’s pretty well cleared out of alder,” Gorman said.
    The presence of the green alder sawfly in Sitka was brought to the attention of the Cooperative Extension Service here by Michelle Putz, a writer and editor for the U.S. Forest Service and formerly a field biologist. Putz said she was working in her yard a couple of weeks ago when she noticed one of her red alder trees didn’t look healthy.
    “I was thinking, ‘why does my tree look so bad?’” On closer inspection she saw the tree was covered with hundreds of pale green caterpillar-like insects.
    She collected a sample and called Gorman, who went to her Shelikof Street house to see for himself.
    “It was, like, whoa,” said Gorman, who was familiar with the insect and the damage caused in Anchorage. A Forest Service entomologist from Juneau was curious enough to fly in soon after to take samples. In the meantime, Putz destroyed the tree to prevent the bugs from spreading.
    Gorman said he doesn’t see the same threat to the Sitka alder – and Sitka stream beds – as the thin-leaf alder in Anchorage and farther north, where there are no other species to take the place of the thin-leaf alder. “Here there is no shortage of trees,” he said. “If one species died out another would be moving in.”
    But he said it’s something to keep an eye on.
    “It’s just going to be another creature on the landscape,” Gorman said. (He noted that there is also a native species of sawfly in Sitka that is distinguishable from the green alder sawfly.)
    James Kruse, a Forest Service entomologist in Fairbanks, said it was just a matter of time before the insect showed up around the state. He said the earliest records show its arrival in Palmer in 2004 “but it may have been sooner than that,” he said.
    “Once we raised the flag in 2009, we found it pretty much everywhere,” he said, including Fairbanks, Oregon, Washington and British Columbia. “It’s not super-surprising (to find it in Sitka), because it’s been in Juneau, and as early as 1994 in the Pacific Northwest.”
    The Cooperative Extension Service issued a handout to raise awareness here, stating that the species has been “recently identified” on red alder trees in Sitka along Halibut Point Road, Sawmill Creek Road and Jarvis Street.
    The larval stage of the green alder sawfly resembles a caterpillar, said the Cooperative Extension Service handout. The larvae are  very pale green and approximately 5/8-inch long, and 1/8-inch thick with a darker green lateral stripe.
    “It is a voracious feeder, which defoliates leaves, leaving only the leaf veins and stem,” CES said. The sawfly is in the insect order Hymenoptera, which includes bees, wasps and ants.
    At the end of the insects’ growing season, the larvae fall and burrow several inches into the ground or wood to overwinter. In early spring, the larvae pupate and change life form into a winged adult about 3/8-inch long with a black head and black and white – or yellow striped – abdomen, resembling some wasps. The larvae feed on alder leaves within two weeks of emerging, and mature in six weeks.
    The Cooperative Extension Services encourage those finding the green alder sawfly to destroy them “using any reasonable means like hosing off infested tree leaves, picking off leaves, crushing or drowning larvae. Birds readily feed on the larvae. The biological control B.T. is not effective on sawflies. Chemical control options will be provided as information becomes available.”
    Those with further questions may call the CES at 747-9440.

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