They float down from the sky on tiny cardboard parachutes over Andersen Air Force Base in the U.S. territory of Guam. The battle for Guam has begin! But the rodent commandos didn't know they were on a mission: to help eliminate the brown tree snake, an invasive species that has caused millions of dollars in wildlife and commercial losses since it arrived a few decades ago. That's because they were dead. And pumped full of painkillers.This invasion is the fourth and biggest rodent air assault so far, part of an $8 million U.S. program approved in February to eradicate the snakes and save the exotic native birds that are their snack food. The snakes cause big problems when they wriggle their way into electric substations — an average of 80 a year, costing as much as $4 million in annual repair costs and lost productivity. The U.S. has tried lots of ways to eliminate the snakes, which it says likely arrived in an inadequately inspected cargo shipment sometime in the 1950s. Snake traps, snake-sniffing dogs and snake-hunting inspectors have all helped control the population, but the snakes have proved especially hardy and now infest the entire island. Guam is home to an estimated 2 million of the reptiles, which in some areas reach a density of 13,000 per square mile — more concentrated than even in the Amazonian rainforests, the government says. But brown tree snakes have an Achilles' heel: Tylenol. For some reason, the snakes are almost uniquely sensitive to acetaminophen, the active ingredient in the ubiquitous over-the-counter painkiller. If you can get a tree snake to eat just 80 milligrams, you can kill it. Brown tree snakes also love mice. It's easy to bait mice with acetaminophen. Drop them from helicopters in tiny cardboard parachutes and let the battle begin!