North America's largest, most spectacular moths are being decimated by a foreign fly introduced to control gypsy moths, according to a report in the December issue of Conservation Biology. The researchers urge extreme caution when introducing new species to control pests, as they can easily have undesired side effects.
With a wingspan of up to 15 centimeters, the cecropia moth (Hyalophora cecropia) is North America's largest moth. It dwells in the forests of central and eastern North America, along with other members of the silk moth family. At the turn of the century, people reportedly gathered cocoons of silk moths by the bushel, just to watch them hatch in the parlor. Now trained entomologists are hard-pressed to find silk moths in the Northeast, and at least four species of silk moths are listed as threatened by the state of Massachusetts.
George Boettner, a wildlife biologist at the University of Massachusetts in Amherst, and his colleagues suspected that the culprit might be a European fly (Compsilura concinnata), introduced as a biological control for gypsy moths--a species introduced from Europe whose caterpillars have long been wreaking havoc in U.S. forests. Up to 1986, the fly was released in 30 states with gypsy moth problems. But the fly doesn't just kill gypsy moths; it's a generalist that attacks at least 180 species of insects, Boettner says. As early as 1919, scientists noted that another silk moth, the promethea moth, seemed to become rare in the areas where the fly had been loosed.
To test their hypothesis, the team put out promethea moth caterpillars at densities varying from one to 100 per tree. Flies killed between 52% to 100% of the promethea caterpillars. In another series of experiments, the team reared cecropia moth caterpillars and placed 300 of them, five per tree, in several spots in the Cadwell Memorial Forest in Massachusetts. After a week, the caterpillars were recaptured and reared further in the lab. Eventually, 81% of them had alien flies bursting from their bodies. "When you see that kind of mortality, it's a wake-up call," Boettner says, adding that it was unwise to release a predator with such a broad host range in the first place.
"It's a wonderful study," says Francis Howarth, an entomologist at the Bernice P. Bishop Museum in Honolulu, Hawaii. "I think it's the first that uses experimental techniques to figure out, in hindsight," what unintended consequences can occur during a biological control campaign.