City Rats Are Going on Birth Control

A city can have one of two levels of rat.

There are cities where rats are glimpsed at night, furtively scurrying into a dark corner or among the train tracks.

Then there are cities where the rats are so numerous and so brazen that they work out private disputes by brawling midday on a train platform, or they lounge on your front porch, eating fallen fruit.

When rats reach that second level, city officials need some answer, any answer, to their rat problem.

Washington, D.C., has reached that level, and Mayor Muriel Bowser’s answer involves rat birth control.

Contraception for rodents has become more popular over the past few years because the most commonly used rodenticides kill painfully , as the poisoned rodent internally bleeds to death over a period of days.

Not a lot of people love rats or mice, but people don’t love causing that much suffering, either.

Many are fond, too, of cats, owls, and hawks, which can be poisoned when they eat poisoned rats.

New York City’s Flaco, the beloved Eurasian eagle-owl sprung from a cage in the Central Park Zoo, was found to have high levels of this kind of poison in his system when he died.

And to top it off, some rodents are evolving resistance to poisons.

D.C. hopes that rats will lap up a liquid contraceptive called ContraPest from plastic bait stations.

It is not the only city to try the approach: Chicago is having luck with rat birth control in Wicker Park; Boston had mixed results in Jamaica Plain.

Most of the available research on ContraPest has been done by the staff of the company that makes it, SenesTech , sometimes in collaboration with external researchers.

One study at a poultry farm found that a year of continuous baiting with ContraPest led to a 94 percent reduction in rat activity as measured by camera traps.