"[Passengers] get a little bit more uptight when I start to ask them for their declarations and maybe their passports," said Matthes.

Comet is one of more than 50 dogs in airports around the U.S. trained to sniff out plants and animals from foreign countries. Every day Comet and inspection officers at Dulles confiscate 100 to 200 pounds of prohibited products.

"The only time [Comet] gets a treat is when he finds the right thing," explained Matthes. "That means he's got to find the fresh fruits, vegetables, meats, plants, seeds, snails, things like that…and not the things that are allowed in like chocolate, cheeses, breads, gummi bears, German beer, Swiss cheese, and sourdough bread."

This is the front line against invasive species.

"A lot of times they understand," said Matthes. "A lot of times [passengers] get mad. There's not much I can do about that, I try to make them as happy as possible."

Keeping invasive species at bay is a serious and expensive business.

American farmers spread a billion pounds of herbicides and pesticides each year to keep down invasive pests. Utility companies spend about $U.S. 30 million annually to control zebra mussels in the Great Lakes. Southern states are introducing insects like the Brazilian phorid fly to stem the invasion of fire ants. And in cities like New York, officials have been spraying mosquitoes to prevent outbreaks of the deadly West Nile Virus.

On Maryland's Chesapeake Bay, wildlife biologists lead by Mark Sherfy of the U.S. Fish and Wildlife Service are trying not just to control, but to eliminate, the nutria—rat-like creatures brought here from their native Argentina for their fur.

It is a daunting task.

"One of the big questions is how intensively do we need to trap to remove these animals," said Sherfy. "Given that this whole marsh is full of nutria, do we really need to trap throughout the entire marsh? Or can we trap in a smaller subset of the marsh and pull animals in from more remote areas?"

Right now, the biologists are attaching tracking devices to 90 of the animals and following their movements, trying to figure out the best way to capture, control, or kill off the entire population.

"I'd love to see this state be devoid of nutria," said Sherfy, "[but] can it be done? I don't know. That's a big question."

It's a big question that isn't likely to go away any time soon. The more people travel and trade in the growing world economy, the more invasive species are given free rides all around the globe.

And there's only so much one little dog can do.