BOSTON – They burned it, mowed it, sprayed it and flooded it.

But nothing killed the pretty purple loosestrife weed, which has become a regional plague, until officials at the Parker River National Wildlife Refuge set a European beetle loose on it.

Deborah Melvin, a wildlife biologist at the Newburyport refuge, said the once ubiquitous loosestrife was all but gone five years after the beetles were introduced.

“It works great,” she said. “We know it takes time.”

State scientists are hoping the beetles, called Galerucella, do the same at a dozen different sites across Massachusetts where the beetle has been released on the invasive weed as part of a pilot project.

The beetle’s progress at the 12 sites will be examined in a state report, scheduled for release this fall.

In Maine, where purple loosestrife also flourishes, beetles have been used experimentally in some areas to defoliate the plant.

The purple loosestrife, native to Europe and Asia, was introduced to North America in the late 1800s in ship ballast, imported wool and by people who used it for its supposed healing properties, according to biologists.

The weed has no regional enemies, and rapidly takes hold of wetland areas, choking out indigenous plants and affecting plant, animal and fish life up the food chain.

An area overrun by the weed is a striking swath of purple. But apart from its beauty and the testimony of beekeepers – who like the honey produced by bees that pollinate the plant – it has no documented benefit, said Tim Smith, a wetlands scientist at the Massachusetts Office of Coastal Zone Management.

The weed-eating beetle, native to the same regions as the loosestrife, has been tested extensively at Cornell University to determine whether introducing another foreign species would do more harm than good.

Smith said years of research showed the beetles ate other plants only as an incidental snack and didn’t reproduce on other plants. The beetle is also hardier than other insects and has proved it can survive a New England winter.

The beetles have been introduced to various regions around the country, including the 1996 release on Plum Island as part of cooperative effort between Cornell and the U.S. Fish & Wildlife service.

A minimum of two or three years, with 5,000 to 15,000 beetles released annually, are needed for the beetle to establish a sustainable population and make a dent in the loosestrife population, Smith said.

The state pilot program began in 2000 at Turner’s Pond in Walpole, where researchers now report an 80 percent drop in the loosestrife. Tens of thousands of beetles have also been let loose in Acton, Ashland, Beverly, Boston, Hanover, Lexington, Lincoln, Longmeadow, Monterey, North Reading and Woburn.

The beetle feeds on the loosestrife’s leaves, putting severe stress on the plant until it can’t grow or flower.

Smith said scientists don’t want to eradicate the loosestrife because the beetles would also die without their food source and the weed would later re-establish itself. The long-term goal, he said, is “a small amount of loosestrife consumed by a small amount of beetles.”

Plenty of unknowns about the long-term effects of the beetles remain, which is why the state has moved ahead on a limited basis, Smith said.

But early progress, as well as reports from other states, indicate the beetle may be one of the most effective biological solutions ever in the fight against an invasive species. Smith said he envisioned a day when the beetles were controlling the loosestrife as a natural part of the state ecology.

“From what we know, there’s no downside,” he said.

On the Net: Cornell University’s Ecology and Management of Invasive Plants Program: www.invasiveplants.net