A beetle colony is being used by the Environment Agency to tackle an invasive water fern in Lincolnshire.
The fast-growing azolla weed has covered a 2.5 mile (4km) stretch of the River Till near Saxilby.
It has formed a dense mat on the surface of the water, depriving other plants, fish and invertebrates of light and oxygen.
The azolla weevil, stenopelmus rufinasus, feeds off the plant without harming native species, experts said.
Nick Kite, from the Environment Agency, said: "We've put 8,000 weevils in at this particular site.
"In a couple of weeks they will go up to millions of weevils, more than enough to control the amount of weed that we've got here."
Agency officers said the weevils often die out naturally once they have eaten their way through the azolla, eradicating the weed without the need for dredging and chemicals.
The azolla was imported from North America in the 1840s as an ornamental pond plant.
It started to pose a problem on the River Till, a tributary of the River Witham, earlier this year.
Officers cleared the weed in June but the plant, which can double in size every four to five days, returned and spread downstream, invading the River Witham.
Experts said the warm, dry weather had helped the weed grow.
Invasive species cost the UK economy an estimated £1.7bn every year, the Environment Agency said.