Dr. Adamec said he and other European researchers believe rescuing endangered species will increasingly depend on such transplants, “so that the case of Aldrovanda shall not be exceptional, but normal and ordinary in a near future.”

That can still feel like a gamble to wildlife officials in the Northeast, who are already scarred from battles with other invasive aquatic species, such as Eurasian watermilfoil and hydrilla, Mr. Doyle said. From their perspective, early prevention is cheaper than managing an explosion.

“Once it gets out of the barn, you might not be able to do much about it,” he said.

Once waterwheel is entrenched, management options are limited. While it can be hand-collected, just one or two individuals can seed an entire population. The alternative is an herbicide that kills every plant in the area.

With few practical options, Dr. Cross, Mr. Doyle and other experts have suggested the plant should remain monitored, but mostly left alone.

What should not happen again, according to Eric Lamont of the Long Island Botanical Society, are campaigns to introduce plants into environments where they did not evolve — however well intentioned.

“You don’t want to just willy-nilly introduce it to areas,” he said. “This is not the way to try to save a species.”