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Environment | Flora and Fauna

Non-Native Bugs Could Be Answer To Controlling Scotch Broom



Mail Tribune

The recent discovery of a non-native insect in the Rogue Valley has native-plant protectionists buzzing, but not in the way they normally react to a new invader.

Two clusters of gall mites found this month near Eagle Point and in Ashland’s Siskiyou Mountain Park are being welcomed by plant devotees because these microscopic bugs exist solely to prey on Scotch broom, one of the valley’s most troublesome exotic plants.

Gall mites attack Scotch broom’s buds, reducing their ability to reproduce and even can kill large swaths of the bushy plants during especially large infestations — thus potentially reclaiming hillsides for native shrubs and grasses choked out by brooms. “It seems to be spreading itself — and rather quickly,” says Kristi Mergenthaler, program coordinator for the Siskiyou Chapter of the Native Plant Society.

“In the insect-control world, it’s a big excitement,” Mergenthaler says.

The Ashland mites are the most southerly known population since biological control entomologist Eric Coombs discovered them on Governor’s Island in the Columbia River in 2006, says Coombs, who works on noxious weed control for the state Department of Agriculture.

The most southerly population previously had been discovered in Sutherlin in 2010, Coombs says.

“Finding them way down in Ashland was really a surprise to me,” Coombs says.

While these mites likely rode the wind into Jackson County, more mites might get packaged and shipped here as early as next year, Coombs says.

A Washington State University researcher is studying whether gall mites harm any native plants such as lupine, Coombs says. Their hope is to persuade the federal Agriculture Department to permit the mites’ use as a biological-control agent.

Until then, those who wage local battles against noxious weeds will just sit back and watch these mighty mites in action.

“This is like a silver bullet, almost,” says Bob Budesa, a retired Bureau of Land Management weed-wrangler who is also a member of the Jacksonville Cooperative Weed Management Area. “If this thing gets (approved), it can help us in a big way.”

Scotch broom was imported and for years was sold as an ornamental bush and a soil-stabilizing plant. But it has turned into the most expensive weed to deal with in Oregon, Coombs says.

Its ability to produce nitrogen for itself in low-nitrogen soils gives it a competitive advantage over native plants, allowing it to take over huge areas. It produces large volumes of seeds that help it spread quickly, and the seeds can sprout into plants 80 years later.

It’s native to Western Europe, where the gall mites (Aceria genistae) are also native and keep broom in check.

The tiny mites, which look “like little white sausages with white legs,” attack Scotch broom’s bud tissues, causing the creation of little fluffy balls, called galls, over the bud in which the mites live and reproduce.

Their presence reduces Scotch broom’s ability to grow and produce seeds, Coombs says. In heavy infestations, it can kill branches or complete plants.

“We’re hoping this will have a big impact on broom,” Coombs says.

The veteran of many Scotch broom-pull parties, Mergenthaler acknowledges the irony of those like her who stick up for native plants now welcoming the discovery of a non-native insect.

“It is a strange path to be jumping up and down on,” Mergenthaler says. “But I’m happy to know that Scotch broom has at least one predator.”

Reach reporter Mark Freeman at 541-776-4470 or email at

This story originally appeared in Medford Mail Tribune.

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