Guam had a population of three million invasive brown snakes; then US dropped 2,000 mice stuffed with paracetamol in a $8M operation to curb the population
Wars are no longer just between humans, they are also against invasive predators that completely destroy ecosystems. In order to tackle one such enemy, a tropical island once waged an audacious chemical war to save its forests.Shortly after World War II and before 1952, brown tree snakes reached Guam from the Solomon Islands, hidden in ship cargo. The brown tree snake is an arboreal species native to Australia, Papua New Guinea, and several Pacific islands. The snake preys on birds, lizards, bats, and small mammals.A US Geological Survey synopsis of the invasion described what followed with admirable bluntness. Prey was abundant, predators were absent, and the snakes multiplied to numbers never seen before. The fallout included thousands of power outages, the loss of pets and domestic birds, and considerable distress for residents who kept finding snakes inside their homes. US scientists put the snake population at three million, with some areas containing 13,000 snakes per sq mile. The Guam Power Authority spends up to $4m annually in repair costs because they sneak into power stations and cause outages.A 2007 review published in Pacific Science found the snake eliminated 13 of Guam’s 22 native breeding bird species, including 10 of its 12 forest birds, and contributed to the loss of several native bats and lizards. The two forest species that hung on were left severely reduced.Places far from Guam have reason to worry too. The island is a large port and home to several military bases, which gives the snakes plenty of chances to hitch a ride elsewhere. A US Department of the Interior release cites estimates that an established population in Hawaii alone could cost between US$593 million and US$2.14 billion a year.
Dead mice to kill snakes
In 2010, dead mice packed with drugs were airdropped into Guam’s dense jungle canopy in an effort to kill the snakes. In the U.S. government-funded project, tablets of concentrated acetaminophen, the active ingredient in Tylenol, are placed in dead thumb-sized mice, which are then used as bait for brown tree snakes.In humans, acetaminophen helps soothe aches, pains, and fevers. But when ingested by brown tree snakes, the drug disrupts the oxygen-carrying ability of the snakes’ haemoglobin blood proteins.“They go into a coma, and then death,” said Peter Savarie, a researcher with the U.S. Department of Agriculture (USDA) Wildlife Services, which has been developing the technique since 1995 through grants from the U.S. Departments of Defence and Interior.Only about 80 milligrams of acetaminophen—equal to a child’s dose of Tylenol—are needed to kill an adult brown tree snake. Once ingested via a dead mouse, it typically takes about 60 hours for the drug to kill a snake. “There are very few snakes that will consume something that they haven’t killed themselves,” added Dan Vice, assistant state director of USDA Wildlife Services in Hawaii, Guam, and the Pacific Islands.But brown tree snakes will scavenge as well as hunt, he said, and that’s the “chink in the brown tree snake’s armour.”
The operation
Before the laced mice were airdropped, they were attached to “flotation devices” that each consist of two pieces of cardboard joined by a 4-foot-long (1.2-meter-long) paper streamer.The flotation device was designed to get the bait stuck in upper tree branches, where the brown tree snakes reside, instead of falling to the jungle floor, where the drug-laden mice might inadvertently get eaten by nontarget species, such as monitor lizards. There are few other species on Guam that could be tempted by the mouse bait, USDA’s Savarie said, because the brown tree snakes have eaten most of them.On September 1 USDA researchers performed a small-scale airdrop of about 200 baited mice onto 20 acres (8 hectares) of jungle around the U.S. Naval Base in Guam. USDA personnel flying low over Guam’s jungles in helicopters dropped the baited devices one at a time, to ensure even coverage.The drop was only the second in the project’s history, and was done to help refine the technique before a larger field test is conducted in late 2010 or early 2011. A small subset of mice in the latest drop was equipped with radio transmitters, which the team will use to determine the baits’ efficiency.“If we go out tomorrow and the radio signal from the bait has moved, it’s very likely that [it was eaten by] a snake,” Savarie said.In the end, targeted drops caused an impressive 80% to 90% drop in snake density in specific test zones within a few weeks
