Environment
8:49 am
Thu May 3, 2012

Invasive species success story: Purple Loosestrife

Purple Loosestrife is a widespread invasive plant. It’s taken over wetlands in every state in the US except Florida. But now, scientists consider Purple Loostrife an invasive species success story.

Purple Loosestrife are the tall bright purple flowering plants you see mixed in with cattails lining the edge of many lakes and wetlands.

A long road before success

It was first recorded in Michigan more than 160 years ago near Muskegon. “But like many invasive plants, once they get a foothold they become much more aggressive invaders,” Michigan State University Entomologist Doug Landis said. (See details of Landis' peer reviewed research on the plant here.)

Purple Loosestrife can grow up to ten feet tall. And with each plant producing 2.5 million seeds a year, it quickly crowded out other native plants. People began to notice in the 1950s that ducks, geese, and other waterfowl hate nesting in ponds overrun by loosestrife. And other native species have a hard time finding food.

“Basically every method to control Purple Loosestrife was tried. Ultimately they decided that all the conventionally means were failing and they really needed to look at biological control,” Landis said.

That means they had to find something to eat it.

Biological control

About 20 volunteers gather around mid-morning last Friday at Huff Park in Grand Rapids. It’s sunny, maybe 50 degrees; just warm enough for volunteers to begin spotting their targets moving around in the park’s wetlands.

“You have to kind of pick a spot, kind of watch it a little bit,” said Jacqueline Bilello, the stewardship coordinator at the Land Conservancy of West Michigan.

She crouches down in front of a newly budding Purple Loosestrife plant. She points out the tell-tale signs she’s looking for… little holes in the leaves.

“Got our first beetle!” Bilello said with a laugh. She carefully gathers the tiny black and red loosestrife beetle in a homemade bug trap. It’s passed around so the rest of the volunteers can see what they’re hunting for.

In a couple of hours the group has captured about 80 beetles; plenty to establish a new population at a nature preserve about ten miles away.

Beetle keeps the plant under control

Entomologist Doug Landis says this beetle loves eating Purple Loostrife. That’s all it eats in its native home in Europe. The beetles’ larvae makes the most damage; burrowing inside the plant to eat it.

The beetle was first introduced in Michigan in 1994.

“The beetles become very abundant. They knock down the population of loosestrife but in doing so they’ve kind of eaten themselves out of house and home,” Doug Landis said.

The beetles travel to find more Purple Loosestrife. Or, if they’re lucky some people, like this group in Grand Rapids, help them out. Landis says groups like this one regularly capture and release beetles to target specific areas.

So far, the beetle hasn’t adapted to eat any other plants. And Landis says it hasn’t caused any known secondary problems.

“15 years ago Purple Loosestrife was pretty much unchecked in southern Michigan. And now where I find Purple Loosestrife I almost always find the beetles,” Landis said.

We can never get rid of Purple Loosestrife, Landis said. But the beetles are keeping the plant under control most of the state.