Invasive insects hitch a ride to Minnesota, Wisconsin on holiday decorations

An infestation of elongate hemlock scales
An infestation of elongate hemlock scales seen on hemlock needles.
Courtesy of Eric R. Day, Virginia Polytechnic Institute and State University via WikiCommons

Agriculture officials in Minnesota and Wisconsin are working to stop the spread of an invasive evergreen tree-eating insect, after it was discovered in holiday decorations sold at big box stores in both states.

The critter is called elongate hemlock scale, or EHS. Inspectors in Wisconsin first found the insect on wreaths and evergreen boughs, and in arrangements of boughs in hanging baskets, pots and other items, sold at Menards, Home Depot and other large retailers across the state. Inspectors did not find the bug on Christmas trees.

After consulting with Wisconsin officials, Minnesota inspectors on Thursday also discovered the insect on unsold decorations at Menards and Home Depot outlets in the Twin Cities area.

The infested evergreen materials were not grown in Minnesota or Wisconsin; all came from suppliers in North Carolina.

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But Wisconsin and Minnesota officials are concerned that if people compost evergreen decorations after the holidays, set them out for brush collection, or dump them in the woods, the bug could escape and infest neighborhoods, forests and Christmas tree farms.

"It's fine to keep your decorations up for the holiday season, but when it's time to dispose of them, don't put them on the compost pile or set the greens out for brush collection. Burn them if you can. If you can't do that, bag them and send them to the landfill," said Brian Kuhn, director of the Plant Industry Bureau in the Wisconsin Department of Agriculture, Trade and Consumer Protection.

Minnesota officials recommend taking similar precautions.

EHS feed on the undersides of needles of more than 40 species of conifer trees. Hemlocks, spruce and firs are among the most susceptible.

The bug doesn't kill outright, but it saps nutrients and weakens trees, making them vulnerable to other pests and diseases.

EHS was introduced into the U.S. from Asia and has now spread to 16 states, mostly along the East Coast, although it has made it as far as Ohio, Michigan and even Nevada.

"EHS has survived in the northeastern U.S., so winter weather will not kill it," said Kuhn. "As a result, if you compost this material, the insects may well attack conifers in your yard or neighborhood, and spread from there."

In the past five years, Wisconsin inspectors have found isolated cases of EHS at Christmas tree lots and other outlets selling cut trees and wreaths.

"This year, because we found it at so many different outlets, we really were very concerned that it could get out into the environment and become established here if we didn't move quickly," said Donna Gilson, a spokesperson for the Wisconsin Department of Agriculture, Trade and Consumer Protection.

In Minnesota, state agriculture department officials are still looking to see where the infested materials may have been sold beyond the Twin Cities area. "We suspect that there may have been other locations through these chains that these greenery have been distributed," said Minnesota Department of Agriculture spokesperson Allen Sommerfeld.

EHS can reproduce twice a year, and each generation feeds during multiple life stages unlike many other insect pests. The insects wrap themselves in a hard, waxy coating, which creates a distinctive "scale" visible on tree needles.

Those complexities make the insect difficult and expensive to treat with pesticides.

"When you bring it to a new habitat, it doesn't have predators," said Gilson. "It's just the Wild West for any invasive insect that way."