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Hitchhiking at Christmas

This post is part of the Science Tuesday feature series on the USDA blog. Check back each week as we showcase stories and news from USDA’s rich science and research portfolio.

Even in a tropical paradise like Hawaii, Christmas just isn’t Christmas to some folks without an evergreen tree decked out with twinkling lights and sparkling ornaments. But some USDA scientists worry about that Yule tree being decked out with something else: invasive western yellowjackets.

These black-and-yellow-striped predators normally build their nests in the ground. But for close to a century, they’ve also been successful at sneaking into Hawaii aboard holiday trees harvested in the Pacific Northwest.

First spotted in Hawaii in 1919, the western yellowjacket (Vespula pensylvanica) remains a target for researchers and regulators who have observed the stinging invader competing with Hawaii’s birds for native insect prey.

Applications of insecticide before the trees are harvested have proven 100 percent effective in killing the yellowjackets, and residues of insecticides applied 6 weeks earlier were still killing the pests at harvest.

A process of literally shaking these trees, however, offers a pesticide-free alternative to ridding them of yellowjackets.  In the past, Hawaii’s agricultural regulators have required that 10 percent of Christmas trees bound for the islands be shaken manually to detect hitchhiking pests if an inspector is present at harvest, or 100 percent of the trees be shaken mechanically if no inspector is present.  But there’s no precise rule about how long the tree should be shaken, and the duration could make a big difference in the number of pests dislodged.

Scientists with USDA’s Agricultural Research Service teamed with colleagues from Washington State University, the University of Hawaii and the Hawaii Department of Agriculture to evaluate the efficacy of shaking the trees.  Although their research demonstrated that current shaking techniques won’t dislodge all the yellowjackets, Hawaiian authorities have since stepped up protection efforts by requiring that all incoming Yule trees undergo shaking.  These scientists are working to make pesticide-free shaking treatments more effective to help ensure Hawaii’s holiday celebrants don’t find more than they wanted under their Christmas trees.

Washington State University plant pathologist Gary Chastagner uses a mechanical tree shaker at a commercial shipping yard in western Oregon.Washington State University plant pathologist Gary Chastagner uses a mechanical tree shaker at a commercial shipping yard in western Oregon.

Washington State University plant pathologist Gary Chastagner uses a mechanical tree shaker at a commercial shipping yard in western Oregon.

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