Unwelcome Immigrants: Can the U.S. Thwart Asian Moths?

The Asian cousins of North America's tree-munching gypsy moths are crossing the Pacific on cargo ships and could establish a beachhead in the U.S.

Photo courtesy USDA

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In a major step toward controlling the spread of tree-destroying gypsy moths, China has agreed to allow scientists to inspect forests near shipping ports to gauge the risk of the pests there hitching rides on ships to the U.S.

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In exchange, the U.S. agreed to share its expertise on exterminating other invasive bugs, such as fire ants. Officials at the U.S. Department of Agriculture (USDA) say the move could help prevent the international transport of a faster-spreading and particularly destructive Asian gypsy moth variety.

The deal with China is the fourth the U.S. has cut in an attempt to get a handle on the problem; it has more extensive agreements with Russia, Japan, and South Korea. Mike Simon, an offshore pest mitigation expert with the USDA's Animal and Plant Health Inspection Service (APHIS), says scientists in China are set to begin trapping and studying gypsy moths this month.

The North American gypsy moth is blamed for defoliating trees in national forests and residential areas in at least 20 states as well as across southeastern Canada. U.S. Forest Service officials say that aerial pesticide spraying of hundreds of thousands of acres each summer has slowed but not stopped the moths. They add that they are trying to keep a lid on the native population as well as prevent the more virulent Asian variety from reaching U.S. soil.

The Asian moth made two minor incursions into U.S. ports during the 1990s, but both times were quickly eradicated. Over the past year, inspectors have again found Asian egg masses on foreign ships arriving in U.S. ports. U.S. Customs and Border Patrol agents are authorized by law to bar entry to foreign vessels carrying invasive bugs or their eggs and to order them offshore to be scrubbed of the pests before unloading cargo. But Simon says it would be more effective to verify that no eggs, caterpillars or adult moths are aboard ships before they set sail from overseas ports.

The main concern: that the virulent strain of Asian gypsy moth, if introduced to the U.S., could exacerbate damage to North American forests.

"The west coast states are basically terrified; it would be devastating to their timber resources, both for industry and recreation," Simon says. That's because, other than a few isolated and quickly squashed outbreaks, the U.S. west coast is generally free of gypsy moths. Without competition from local populations, any Asians that arrived there could thrive.

At least 10,000 cargo containers and 100 ships arrive daily in the Port of Los Angeles, most of them from Asia, which poses a real risk, he adds.

The North American strain, Lymantria dispar, now stretches from Canada into North Carolina, Minnesota and Wisconsin, with pockets of infestation beyond its contiguous range. It is currently expanding its  reach by about five miles (eight kilometers) a year, according to ecologist Patrick Tobin, head of the technical advisory committee* for the U.S. Forest Service's Slow the Spread, a program to manage the gypsy moth.

As its name suggests, the program, which was first funded by Congress in 2000, is trying to decelerate the expansion of the gypsy moth's range. It monitors 10 states along the region's leading edge for outbreaks and decides which ones to combat with insecticides. It also educates truckers, plant nurseries, the timber industry, motorists and recreational forest users about ways to avoid transporting the pest. Human movement helps the gypsy moth move faster than it does on its own.

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.

The current agreement calls for scientists to set similar traps in forests around Chinese ports. The traps could confirm large populations, providing the evidence U.S. officials need to back up arguments for cargo inspections.

Past studies have shown that gypsy moths are attracted to light; they swarm from infested forests to the bright lights of nearby port cities and docked ships. Once an infested ship docks in the U.S., its infested containers can also disperse quickly.

This summer, the U.S. is seeing massive gypsy moth outbreaks in Pennsylvania and northern New Jersey. But nowhere is the gypsy moth's attraction to light more apparent than in far eastern Russia, which is in the second year of its worst outbreak since the early 1990s, says Steven Munson, the U.S. Forest Service’s team leader for the overseas monitoring program. He notes that larval caterpillars have been chewing through the vast forests around two major shipping ports—Vostochnyy and Nakhodka—near the Chinese and North Korean borders. U.S. scientists headed there last week to help check traps and search for egg masses, he says.

A naturally occurring virus helps structure gypsy moth infestations into cycles: An outbreak may last three years before the virus sends a population into dormancy for more than 10 years, Munson says. By mid-August, when the female Russian moths typically emerge from pupae and start laying eggs, researchers should know whether to step up ship inspections there or whether the virus is causing a new die-off.

*Note (7/14/08): Patrick Tobin's title has been modified since the original posting.

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