Heads Up to Invasive Pests: Poison Mice Falling From the Sky

Earlier this week, scientists led by the U.S. Department of Agriculture dropped 2,000 mice—dead and stuffed with Tylenol—over the island of Guam.

The hail of mice is part of a long-term plan to rid the island of as many as two million brown tree snakes, an invader that came to Guam as early as World War II in ships from East Asia and within a generation wiped out 75 percent of the island's native birds and are now targeting geckos and lizards.

Brown tree snakes are fatally allergic to acetaminophen, the active chemical in Tylenol. To deliver the bait, the researchers dropped the rodents from a helicopter.

The USDA and the Department of Defense have spent more than three years developing the dead mouse strategy, using Guam as a testing center for combating invasive species. Invaders like the tree snake, which reproduce quickly and have no natural predator on the island, are of particular interest.

The image of dead mice falling with parachutes—although they're not actually parachutes but long pieces of paper that get caught in trees—has earned international attention. "It's a strategy that's very easy to understand and with a chemical [acetaminophen] that has already been widely tested by the FDA," Craig Clark, a USDA biologist on Guam, told me when I visited the island last month. (Related: "Guam: Can Invasive Species Be Stopped?")

But this isn't the first time poison bait has been dropped to eliminate invasive species. The tactic has been tried in other areas, many of them islands, with varying success.

Target: Palmyra's Black Rats

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Black rats that had come from Asia were killing geckos and bristle-thighed curlew birds in the Palmyra atoll, a small group of uninhabited islands south of Hawaii.

Biologists dropped pellets of rat poison, formally known as rodenticide, from helicopters and shot it into palm trees using slingshots.

The two-year project was completed in August 2011. This year the project was declared to have been largely successfully, greatly reducing the population of rats, which at its peak was believed to have hit 30,000. Birds and geckos that had been taken into captivity for protection were returned to the wild.

Target: Galápagos Rats

Charles Darwin called the Galápagos one of the most biodiverse places on Earth. But many of the species on the islands are at risk of extinction because of invasive rats. The rats arrived aboard merchant ships beginning in the 17th century. On Pinzon Island, more than 180 million rats, roughly one every square foot, have eaten away at the native flora and populations of tortoises, lizards, and birds.

In what was called the biggest rodenticide in the history of South America, conservationists dropped 22 tons of potent poison pellets over several of the islands in 2012. Critics of the plan feared that the bait would wreak havoc on an already fragile ecosystem if ingested by other species such as hawks and iguanas. But scientists with the Galápagos Conservancy said the rats posed more harm than the toxicity of the bait did.

The vast size of the project makes it difficult to assess the impact. And it was unlikely that all 180 million rats would be eliminated at once. But the Ecuadoran government and some conservation groups believe the strategy significantly reduced the rat populations on several islands—although other drops will likely be needed because the rodents reproduce every three to four months.

Target: Social Wasps of New Zealand

No one knows exactly how they arrived in New Zealand, but four species of social wasps, which live together in colonies with one queen, have invaded New Zealand since at least the 1990s, attracted by mild winters and plentiful insects.

In some areas, especially around farms with abundant fruit for the wasps to eat, biologists have counted as many as 60 nests per hectare (a hectare is about the size of two football fields). Over time, the voracious wasps have been responsible for a moderate decline in the local populations of caterpillars, ants, and bees every year. As a result, honey production in New Zealand has fallen by more than 30 percent each season over the past 20 years.

At least one research firm has experimented with aerial baits made primarily of dried sardines (a popular food among social wasps) mixed with a toxin. The abundance of other invertebrates that could feed on the poisoned bait poses challenges to the strategy, compounded by the fact that wasps have many other options—from fruit to insects—for food other than the sardine bait.

After the poison was dropped, mainly in the north end of New Zealand, researchers saw some success in reducing the wasps' impact. They can't be eliminated completely, the country's government has concluded, but can be controlled in certain areas from year to year with continued drops.