At the peak of its migration, in the 1980s the gypsy moth was spreading southward and westward across the U.S. by about 13 miles (20 kilometers) per year. But dead trees aren't the sole concern: Gypsy moths' hair and scales contain histamines, which can cause allergic reactions in humans sensitive to them.
As it turns out, L. dispar females cannot fly (because they lack the muscle strength and wing length of their Asian cousins); they advance when larvae suspended on strands of silk are carried by the wind—or even farther and faster by cars, trains and trucks. The adult males, meantime, are long-distance flyers that establish new colonies by sniffing out the pheromones of adult females that grew from transported larvae.
The adult female's inability to fly has helped limit the spread of gypsy moths across North America. An L. dispar egg also requires a long winter chill and the hatched caterpillars generally eat the foliage of oak, poplar, and birch trees. Their Asian cousins can hatch more quickly and have a more eclectic diet that also includes Douglas fir, red maple, pine and cottonwood, says entomologist Melody Keena, a Forest Service supervisory researcher. The introduction of the Asian variety, should it mate with the local population and pass along those traits, could accelerate the spread and endanger additional tree species, she adds.
In experiments that began more than a decade ago (after minor incursions of flying strains were quickly eradicated in the southern and western U.S.), Keena and her group demonstrated for the first time that, when Asian and North American gypsy moths mate, the offspring female develop the ability to fly. That could help them advance into forests that are currently free of infestation.
Gypsy moths are believed to have been introduced into North America in 1869 after an amateur entomologist brought larvae back with him from Europe, most likely from France. He accidentally allowed some larvae, presumed to have been L. dispar, to escape from his backyard outside Boston, where he was keeping and studying his new colony. From there, it began inching across the continent.
Now, it appears Asian strains are gradually moving across Europe and interbreeding with L. dispar there. Resulting hybrid females in Croatia, Portugal, France and Greece still cannot fly, but major populations in Lithuania, Poland and Germany can, according to Keena.
Although Keena and other scientists say that suggests a scenario in which L. dispar interbreeding with Asians might produce flying offspring that could accelerate the spread, Tobin notes that their dispersal could be limited by still larger populations of native L. dispar.
If there were to be an influx of Asians, the areas most affected would likely be those where the L. dispar population is scarce—west and south of the Virginia–North Carolina border, eastern Wisconsin and northern Minnesota—and especially in the west where it's nonexistent, Tobin says. In those places, where the Asian traits are less likely to be diluted by mixing with L. dispar, the Asians could dominate.
Vic Mastro, director of the APHIS research lab in Otis, Mass., says agriculture officials have set around 350,000 traps in the western part of the country. But there's a hitch: Baited with pheromone, they only attract males and leave scientists guessing whether any flying females are in the area. So the traps alone are not enough, and it's more crucial to keep the Asian variety out altogether by inspecting ships in ports before they enter the U.S., Mastro says.
"Right now, if we find egg mass on a ship, we assume the worst," he says.