Dead mice don’t get headaches. So why are scientists on the island of Guam stuffing them with acetaminophen, the widely used painkiller? The hope is that the acetaminophen will give a major headache to brown tree snakes. More than a headache in fact. Should the snakes dine on the toxin-filled rodents, it will be their last meal. And how will the last supper be served up? It takes a military operation!

The idea is to air drop the acetaminophen-laden rodents into an area around Guam’s Andersen Air Force Base. Each mouse will sport a long streamer designed to intertwine with branches, delivering a delectable meal to the tree snake. The snakes, estimated to number two million, have become an enormous problem since they were inadvertently introduced to the island as ship stowaways during the Second World War.

The island’s forests are eerily quiet because the snakes have wiped out bird populations, including native species like the koko. The decimation of the bird population is only part of the problem. Spiders, free of natural predators, overrun the island. Electrical “brown outs” are frequent as reptiles, which grow up to three metres long, crawl along power lines and bring them down. Although only mildly venomous, the snakes, which crawl into homes and bedrooms, can cause serious problems in children who are bitten.

There is real concern that the problem may be exported if snakes take refuge in cargo that is being loaded aboard planes on the Air Force base. Should they make their way to Hawaii, the economic impact would be horrendous. Not only are attempts being made to poison the snakes with acetaminophen, which is a good choice because it is harmless to other wildlife, but dogs have been enlisted to sniff out snakes in every item of cargo leaving Guam. Whether the mouse drop will be effective isn’t clear yet. But there is hope. A test in which the dead mice were equipped with radio transmitters indicated that all the rodents had been eaten by snakes.

The brown snake problem is just one example of what can happen when novel species are either accidentally or purposely introduced into a new habitat. Possums in New Zealand are another example of unforeseen consequences. Originally imported by 19th century European settlers who hoped to establish a fur trade, possums tend to feed on the newest shoots, with the result that many plants and trees die. These predators also disturb nesting birds and eat their eggs and chicks. They are partly responsible for the decline of the beloved kiwi, a flight – less bird that comes out of hiding at night. Unfortunately, possums are also nocturnal creatures and a kiwi-possum encounter is not to the bird’s advantage.

The use of sodium fluoroacetate- laced bait has been shown to reduce possum numbers by disrupting the energy-producing mechanism in mammalian cells. It can be added to carrot or cereal bait dropped from helicopters, achieving a kill rate of 98 per cent. Fluoroacetate actually occurs in nature in a variety of plants that grow in high-fluoride soils such as black tea leaves from India or Sri Lanka. A cup of tea brewed from these leaves will result in a concentration of about five parts per billion of fluoroacetate, which is 1½ times the drinking water limit. In the environment, fluoroacetate is degraded by soil microbes and fungi into non-toxic substances. That doesn’t mean there are no possible problems. Dogs can be poisoned either by eating the bait or biting into the carcass of a possum or rabbit that has been dispensed with by fluoroacetate.

Such collateral damage, unfortunately, is often a risk in any chemical warfare battle.

Joe Schwarcz is the director of McGill University’s Office for Science and Society. 
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