Rats

These four commensal species of rat together destroy about one-fifth of the world's food harvest each year. In the United States alone, the Norway and black rat damage or destroy a billion dollars worth of property each year, not counting the accidental fires that start when they chew through electrical insulation.

These commensal rats succeed because they are generalists and opportunists. The Norway rat, for instance, adapted its natural ground-dwelling habit to take advantage of many environments: cellars, sewers, even among the bushes in front of nicely landscaped homes and apartment buildings. In some buildings, the basement is home to Norway rats while black rats inhabit the upper stories.

Rats are present in almost every major city in the world. A study of Baltimore during World War II (done in reaction to fear that the Axis would attempt rat-borne germ warfare) discovered that many blocks in "good residential areas" harbored 300 or more rats. In poor, rundown neighborhoods, the number was doubtless much higher. Some cities in North America have an estimated population of two rats for every person.

Sanitation is the major contributing factor to the number of rats that will be found in a city, but new construction in an urban area will also force rats into areas where they have not traditionally been found, as digging unearths their traditional burrows.

Most rat control efforts involve poison bait. The most common type is an anticoagulant, usually rotenone, which causes fatal internal bleeding after the rat eats it.

However, there are formidable obstacles to effective rat control. First is the rats' innate fear of anything new. Even if something as innocuous as a brick is placed near a rat colony, they will go out of their way to avoid it. So merely placing the poison does not guarantee results. In 1960, rats that were apparently unaffected by anticoagulant poisons were found on a farm in Scotland. They had evolved a genetically based resistance to the anticoagulants. These so-called super rats are now found in several places in Great Britain.

Rat-control experts in New York City's Central Park noticed something curious about the rats they had been poisoning: the rats abandoned their normal shy, nocturnal habits and began appearing in the park in broad daylight. Rather than killing the rats, the poisons apparently acted like a stimulant to them.

Poisons obviously have their limits. The most effective method of rat control has proved to be a general clean-up to reduce the habitat quality for the pest rodents. Members of the Inspectional Services department must supplement their poisoning efforts with the education of local residents, telling people how to store their trash in rat-proof containers and how to rat-proof buildings by plugging all entry holes with steel wool.