Double trouble

A deadly moth and a fly create havoc

Published: Tuesday 15 May 2001

-- around 1869, an amateur scientist named tienne Lopold Trouvelot staked his claim to entomological infamy when he released imported gypsy moths from his Massachusetts home in the us . The price paid for this little mistake has been more than a century of outbreaks of the forest- ravaging pests. Research shows that the gypsy moth may be responsible for yet more unsuspected damage: the decline, and in some areas the disappearance, of species of giant silk moths, a spectacular group of insects native to the us. The problem, says a study published in Conservation Biology , is not the gypsy moth itself, but a parasitic fly brought from Europe and released into the wild to get rid of it. The fly, it turns out, is not only killing gypsy moths but also huge numbers of wild giant silk moths.

"High mortality rates are being recorded," says George H Boettner, entomologist at the University of Massachusetts, usa, and the lead author of the research paper. It remains unclear whether the fly has caused harm to any of the 200 or so other species of insects, including swallowtail and tortoiseshell butterflies, which it is known to attack.

Scientists say the study provides some of the first compelling evidence of the damage that can be done when foreign species are released into the wild to get rid of pests.

Scientists say these findings will add to the controversy over the practice, which involves the release of hundreds of foreign species throughout the country.

The disappearance of wild silk moths was first noticed in the 1950s. Insects such as Hyalophora cecropia , a Godzilla among moths, more than 15 centimetres across, became rare to find. Today several of the moths are on the endangered species lists and the imperial and regal moths have nearly disappeared. Scientists considered habitat loss, pesticide use and city lighting, factors that disrupt mating among moths, as possible causes, but none explained the timing and extent of the declines adequately.

To test whether the fly, known as Compsilura concinnata , might be at fault, entomologists placed cecropia and promethea moth caterpillars in the field. They found that the fly fatally attacked most of them. And, the researchers found a wild population of a state-listed silk moth known as the buck moth and discovered that the fly also killed more than a third of its caterpillars studied. The fact that Compsilura concinnata attacks so many species besides gypsy moths makes it the perfect example of the kind of species that should never be used for biological control. "The advantage of biocontrol measures is that when you do it right, it's a permanent fix," says Boettner, adding that biological control is sometimes not only the best, but the only solution. "The problem is that when you make a mistake, it's permanent too," he adds.

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