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. The company says that if the rats continuously eat the bait, they stop having pups, a finding confirmed in a laboratory setting.
But that’s a big if. A prior attempt to use ContraPest in D.C. was inconclusive, according to Andre Pitman, the program manager for the city’s Rodent and Vector Control Division. In rat hot spots such as Adams Morgan, where evening revelers have been known to throw greasy pizza plates on the ground, rats simply had too much other food around for the birth-control liquid to hold much appeal. “We couldn’t get the rats to drink it,” Pitman told me.
So this time, Pitman said, D.C. is making what, to his knowledge, is the most aggressive push against rats in a very long time. First, the city is going to try to kill rats by heaving poison directly down their burrows, which will then be filled in. City workers will return multiple times, looking for burrows that the rats have reopened, adding more poison, then filling them again. The next week, the same procedure will be followed with a different poison, which clings to rat fur and is ingested as they attempt to lick themselves clean. Finally, starting in week three, the city will put out ContraPest for 15 days. All the while, city workers will clean alleys and encourage residents to remove ground cover, containerize trash, and generally boost hygiene levels so that rats are more likely to take the bait. Post-bar snackers are requested to keep things tidy too.
It is understandable that the nation’s capital, facing a high number of resident complaints, has chosen to throw everything it’s got at the rats. But the downside to the guns-blazing approach, from a scientific standpoint, is that if it succeeds, no one will have any idea why. If burrow numbers are down by the new year, no one can say that birth control works—only that the combination of two poisons, ContraPest, hygiene, and other measures works. And Pitman acknowledges as much. “It is not a fertility pilot,” he said. “It is a rodent blitz.”
Fertility control of wildlife, which is used in elephants, wild horses, kangaroos, and pigeons, among other species, does work sometimes, and advocates for this method worry that premature deployment of oral contraceptives for rodents risks making the entire field look bad. Ultimately, no real independent studies show that SenesTech’s products are safe or effective, Lawrence Green, the interim managing director for the Botstiber Institute for Wildlife Fertility Control, told me. (SenesTech CEO Michael Edell responded, “We have been doing this a long time and have case studies and data to back up our claims.”) Assuming ContraPest can work in cities, rats would likely have to take it for longer than 15 days to see lasting results. As people who take birth control generally know, if you miss a dose, it’s less effective, and you have to keep taking it for as long as you want to not have babies. ContraPest is really meant to be used indefinitely: SenesTech even offers a “Subscribe & save” option.
And that familiar e-commerce option points to a larger truth that everyone I spoke with agreed on: There is no end to this war. The quest to control rats is not about winning. It is about nobly devoting oneself to an unwinnable fight. Because rats spread disease, we must struggle against them, clear-eyed in the knowledge that, unless we are dealing with a small island, eradication is not possible.
The reason that rats will never be conquered, at least not in our lifetime, either with poison or with birth control, is easily explained by two facts of rat biology: They have many babies, and they love to explore. You can nuke rats with poison or slowly usher them into a
population crash with contraceptives, but all you will have done is create unoccupied territories for young, exploratory rats to come occupy. D.C.’s current blitz is focused on Adams Morgan, Chinatown, and Eastern Market, but rats live in other places in Washington. They will be happy to move in once those neighborhoods have space.
Niamh Quinn, a human-wildlife-interactions adviser at the University of California Division of Agriculture and Natural Resources, puts it this way: “We live in an infinite sea of rats.” Just beyond the boundaries of any rat-control project lie more rats, waiting to move in. Pitman, a longtime rat expert since before he joined the city, knows this too. “Rodents were here long before humans,” he said. All we can hope is to control them.
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