The silk moth
& the fly
| (Elizabeth Pols illustration) |
The convoluted saga of the gypsy moth took yet another turn this winter, when Conservation Biology published a study by UMass entomologists George Boettner and Joseph Elkinton and Cynthia Boettner of the Fish and Wildlife Service.
The study is evidence of the unforeseen foul-ups that can result from our efforts to control nature by natural means – for example, by releasing non-native predators.
The forest-ravaging gypsy moth is itself not native to America. Its ancestors escaped around 1869 from an amateur entomologist in Boston, who’d imported them from Europe as part of a quest to breed a better silkworm.
By 1906, in any case, the gypsy moth was running rampant, and to combat it scientists turned to another European insect, Compsilura concinnata, a fly with a ferocious attraction to moth larvae.
The fly has now joined the moth as a naturalized American. Compsilura releases took place in 30 states over a period of 80 years, and the fly undoubtedly destroyed many of its intended targets by depositing its larvae in the emergent caterpillars.
Unfortunately, it’s now the prime suspect in the decline of moths no one intended to target.
The native wild silk moths of the Northeast were once so abundant “that people could gather them by the basketful,” according to a report on the UMass study in the London Times. These native moths are now threatened to an extent exceeding what scientists can attribute to usual factors like habitat loss and pesticide use.
Especially sad is the plight of Hyalophora cecropia, the giant silk moth. With a wingspan of up to six inches, it is the country’s largest moth and among its most beautiful.
To confirm the suspicion that Compsilura is the culprit, the UMass team baited trees in the Cadwell Memorial Forest in Pelham with silk moth caterpillars. When the caterpillars were retrieved, as many as 80 percent were found to have been destroyed by Compsilura maggots.
“When you see that kind of mortality, it’s a wakeup call” for the exercise of caution with biological controls, said Boettner.
“The advantage to biocontrol is that when you do it right, it’s a permanent fix. The problem is when you make a mistake, that’s permanent too.”
