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The hitchhiker’s guide to the galaxy: invasive insect edition

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A beagle finds food that may bring pests into the U.S. (Photo courtesy of CBP)

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Destructive insects and plants might come to the U.S. via passengers' luggage. (Photo courtesy of CBP)

If you were a destructive foreign plant looking to move to the U.S. or an American insect seeking to relocate overseas, June 2010 would be the ideal time to make the journey. That’s when at least two key factors will align to create a hospitable travel environment: An increased number of flights will connect distant airports, and similar weather conditions will exist in many global destinations.

More air traffic routes will link more similar climatic regions in June 2010 than in any other period since 2007, according to a study recently published in the journal Ecography.

“Now that the world is so connected by transport methods,” says the study’s author, Andrew Tatem, an associate professor at the department of geography and the Emerging Pathogens Institute at the University of Florida, “there aren’t really any restrictions on actual invasive species moving” and “finding a suitable climate at their new destination.”

These invasive species are outside organisms—typically insects, plants and pathogens—that can cause serious damage to people and the environment. They can trigger epidemics, decrease crop values, kill forests and lead to the extinction of animals. These uninvited visitors are estimated to cause at least $120 billion in damages a year, according to a 2004 Cornell University study.

The destructive Mediterranean fruit fly has been hitchhiking to the U.S. since 1910, usually traveling in infested fruit. The emerald ash borer, an exotic beetle native to Asia, was identified in 2002 in Michigan and has killed several million trees in nine states and Canada. The light brown apple moth, originally from Australia, can damage a large number of crops and plants, including redwood trees. It was first seen in California in 2007.

Airline passengers and their luggage make good pest couriers—often unwittingly. On a typical day last year, according to a U.S. Customs and Border Protection report, customs agents seized an average of 435 unwanted pests among the meats, plants and animal products confiscated at the nation’s airports, seaports and border crossings.

A study published in the journal Biological Invasions in 2006 found that of the pests seized between 1984 and 2000 in the U.S., 73 percent were intercepted at airports, 13 percent at U.S.-Mexico border stations and fewer than 10 percent at maritime ports.

Damaging organisms can cross the border in many ways, even on tiny bits of soil stuck to a passenger’s shoes. And flying insects have been known to survive long-distance flights. But most pests seized at airports come in via fruit brought by passengers traveling from developing countries, says Andrew Liebhold, a research entomologist with the U.S. Forest Service and author of the Biological Invasions article.

“In many cases, insects will move in alive inside the fruit, or they’ll have eggs on the outside of a fruit,” Liebhold says.

Travelers often don’t realize the damage some pests can cause, says Nolan Lemon, a public affairs representative with the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service. When agents at Miami International Airport stopped a woman carrying a sugarcane stalk from Barbados, Lemon remembers, the traveler explained that she intended to plant it, not eat it. Planting it could have caused severe problems.

For the Ecography study, Tatem compared the climate change model developed by Britain’s Hadley Centre for Climate Prediction and Research with passenger and flight data from OAG Worldwide and the International Air Transportation Association. These models included more than 35 million scheduled flights, 3,570 airports and 44,000 different routes.

Uninvited pests that land in a familiar climate have a greater chance of survival, but it’s usually difficult to track how an organism entered a country. “There have been only a few cases where it has actually been proven where invasive species came from and the route they’ve taken,” Tatem says.

The gypsy moth is one of those few cases. Étienne Léopold Trouvelot, an amateur scientist living in Medford, Mass., brought the species back from a trip to France in 1869, hoping to develop a stronger type of silk caterpillar. Some of his moths escaped and began to colonize their new land, but it took years before anyone realized they were causing trouble. And they are still doing so. Sine 1984, the gypsy moth has defoliated more than 90 million acres of forest land, according to a Forest Service database.

While insect or plant pests that cross borders in great numbers have a better chance of propagating in the new territory, it’s possible for even a lone organism to spread. Laurence Packer, a biology professor at York University in Canada, says that a species of bee found in eastern North America most likely descended from a single female bee that came to the U.S. hundreds of years ago and had probably been fertilized abroad. In the case of asexual plants, “all you need is just a single individual,” Packer says.

Several methods are currently used to predict the propagation of invasive species, and Tatem’s study could help government scientists craft new tools to prevent pest entry.

“It’s kind of a different type of analysis,” says Daniel Borchert, an entomologist for the Animal and Plant Health Inspection Service, of Tatem’s study.

Tatem’s study, done while he was at Oxford University, only projected to 2010. But it allows for more similar forecasts and could help border inspectors pinpoint risky times of year when they should be increasingly vigilant. “In this country, we should be using that type of information to make more targeted inspections,” says Liebhold, the entomologist.

Raising public awareness is also crucial. “As traffic levels keep increasing, things are only likely to get worse in terms of risks,” says Tatem. “The more traffic there is, the more chances that these invasive species will be transported in large enough numbers to survive at a new location.”

E-mail: aea2125@columbia.edu