A limited ban introduced three years ago on a veterinary drug used for treating farm animals has given wildlife a welcome boost.

The drug, Ivermectin, is used to control worms and other internal infestations in cattle, and is also effective against some external parasites, such as lice. The problem is that it can remain effective even after it has passed through the animal, so the resulting cowpats are toxic to the insects which would normally feed on them.

This means the nutrients in the dung are recycled more slowly. It also means that species which feed on the dung-eating beetles and flies have less food available.

Some scientists say the drug can be administered in ways which leave the cowpats nutritious.

In 1996, the National Trust banned the use of Ivermectin on some of the farms it owns in the west of England. The ban was introduced to try to encourage the return of the chough, a bird that had disappeared from Cornwall.

But the trust has found that other birds and bats are increasing because there are more insects available for them.

Boost for bats

The greater horseshoe bat, one of Britain's rarest mammals, is doing especially well.

It has increased at two sites in Cornwall - one, a church in Boscastle, is now the bats' second biggest roost in Britain. The trust says the ban was not the only reason for the bats' success, but was "a significant factor".

The greater horseshoe bat (Photo Phil Richardson/Bat Conservation Trust)

The floor of a barn on the Devon/Cornwall border, which has been specially modified as a bat roost, is now regularly strewn with moth wings and the outer skins of insects commonly found living on cowpats.

About 100 of the trust's tenant farmers across the country have agreed not to use Ivermectin, and have turned to alternative wormers.

The trust believes the ban explains why the rare hornet robber fly, an insect which resembles the hornet and feeds on fresh cow dung, has been found at two new places on its land in Cornwall.

English Nature, the government's wildlife adviser, recommends not using the class of drugs to which Ivermectin belongs near a bat roost. But the Ministry of Agriculture allows their use, even in areas where it is paying farmers to protect the bats.

The National Office of Animal Health, which represents the UK animal medicines industry, said it was unhappy with the prohibition.

"We do not like this sort of ban on a product which has been through a rigorous evaluation process, including environmental safety."

Ivermectin's maker, Merial, said the National Trust's comments "lack any scientific basis".