SCIENTISTS descended on a Lancaster primary school to set up a test bed in a bid to fight back against an uninvited guest.

A water weed has settled in the pond at Quernmore Primary School and starting to kill plants and wildlife.

The fairy fern, Azolla, is an attractive aquatic plant, which has made the pond its home but scientists at CABI Bioscience hope to solve the problem by putting a number of North American weevil into the water to fight back.

The scheme is a pilot and if their initiative works it will be rolled out to all problem ponds in the UK.

"Azolla has no native natural control agents in the UK and if left untreated can rapidly choke watercourses," says Dr Rob Reeder, a senior diagnostician at CABI Bioscience who has been researching the problem.

"Our work at the school involves boosting the numbers of a North American weevil, just 2mm long, which eats the Azolla and destroys it."

The weevil has proved a success in South Africa where Azolla is also a problem.

He says there is no danger of the weevils themselves becoming an invasive species - once the Azolla fairy fern has disappeared the weevils die out with no other known host in the UK.

"It is a very neat, environmentally-friendly solution," says Dr Reeder. "Although mechanical control of Azolla has been used in the past, it is not cost-effective. And using herbicides is not desirable from an environmental standpoint.

"Without control the Azolla can shade out other important indigenous plants and reduce the oxygen levels in the water to harm fish and aquatic life. The mat of weed can also mislead children into thinking it is firm ground rather than water and therefore poses a real danger."

Martin Wain, one of a group of parents involved with managing the wildlife area at Quernmore School, is delighted this novel approach is being tested.

"The children and I are looking forward to getting our wildlife-friendly pond back to health and by using this method there is no danger of harming anything other than the Azolla. It is a useful lesson to all of us on the dangers of introduced species escaping in to the wild."