This plant invaded Michigan with ‘chemical warfare.' Can beetles defeat it?

Native to Europe and Asia, invasive knapweed has been choking out Michigan’s native ecosystems for more than a century
State officials are considering whether to let land managers import beetles to kill it off
That solution may not be popular with beekeepers, who prize the plant for its late-season flowers

Despised for killing off native plants but beloved for its nectar-rich flowers, invasive spotted knapweed has been spreading across Michigan for more than a century.

Now, state officials want your opinion about whether landowners should be allowed to release imported weevils to devour the plant.

The Michigan Department of Agriculture & Rural Development is collecting public feedback until March 6 about whether to permit the release of seedhead weevils (Larinus minutus) and root boring weevils (Cyphocleonus achates), two Eurasian beetle species that evolved alongside spotted knapweed and feed exclusively on the plant.

Officials expect support for the concept among landowners who’ve been struggling to keep the plant from destroying native ecosystems and pushback from beekeepers whose hives produce prized honey from its pinkish-purple flowers.

For all the damage spotted knapweed does, “it is an excellent nectar plant,” said Mike Philip, director of the agriculture department’s Bureau of Environment and Sustainability. “It blooms at a time in the summer when little else is blooming, and so the bees really like to go to it.”

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Recognizable by its slender pale green leaves and thistle-like late summer blooms, spotted knapweed was first documented in Michigan in 1911, having arrived in the Pacific Northwest in the late 1800s on shipments of alfalfa and clover from Europe or Asia.

Each plant produces thousands of seeds that can remain viable for up to eight years. Knapweed also releases toxins that suppress the growth of other plants — a survival trait Phillip describes as “chemical warfare.”

Add it all up, and it’s no wonder knapweed has spread into every Michigan county, taking over roadsides, farm fields and sandy-soiled forests, prairies and dunes.

The beetles attack the weed in tandem, with one slowing its spread by feeding on the seed head, while the other attacks the roots to weaken the plant.

If combatting one introduced species with another sounds like a recipe for disaster, Phillips said to rest easy.

“These insects will not, cannot feed on anything other than knapweed,” he said.

Extensive studies have shown that the bugs die off when there is none of the plant left to eat. What’s more, the seedhead weevil is already widespread in Michigan, having been brought in for past scientific studies.

That’s little reassurance for Gale Moore, a Shiawassee County beekeeper who called spotted knapweed — also known as star thistle — a godsend for all manner of pollinators.

“It isn’t just honey bees,” he said. “So when you’re talking about one species versus something else, you’re opening up the wrong can of worms.”

This wouldn’t be the first time Michigan has killed off invasive plants with imported bugs. One example is purple loosestrife, a plant that was rapidly colonizing Michigan wetlands until Galerucella beetles from Europe helped knock it back.

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This plant invaded Michigan with ‘chemical warfare.’ Can beetles defeat it?

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