It’s not easy earning the distinction of Worst Rat City in the World, but if there’s any city that can do it, New York can. And indeed, with rat population estimates ranging from 8 million to 32 million, the city may have more rat denizens than human citizens.

With the energetic ability to pump out 12 wee rats per litter seven times a year, each she-rat is capable of giving birth to around 84 rat pups a year … which starts to explain the numbers, and the presence of so many rodents infamously scurrying along the tracks of the city’s subway system.

The Metropolitan Transportation Authority (MTA) has long tried to fight the rodent warriors, to little avail. City strategies in the past have included the use of World War I-era poison gas and hunting forays led by gun-toting citizens. And then of course there’s always poison, but the rats always seem to win.

As it turns out, poisoning rats is not in itself a very tidy way of controlling them. In fact, according to rat scholar (yes, that’s a job) David Davis, when rats are killed off in numbers too large, the pregnancy rates of the surviving rats double and the survivors rapidly gain weight; the rats that survive become stronger.

With all of that in mind, the MTA has a new plan. Business Week reports that the agency will be teaming up with Arizona-based SenesTech in an effort to sterilize mama rats. The company invented ContraPest, a product that upon oral consumption accelerates egg loss and causes infertility within days.

Although city subway rats’ fondness for discarded pizza crusts, potato chip detritus and bagel edges is legendary, ContraPest is made up of mostly salt, sugar, fat, an herb, and an industrial chemical called 4-vinylcyclohexene diepoxide (VCD).

“In New York, rats have such a buffet available to them,” says SenesTech co-founder Loretta Mayer, “but they don’t necessarily get a lot of liquid, which is why we’ll be offering them … a semi-solid covered in kind of a cheese wax and also liquid from a bottle feeder.”

The hope is to decrease the number of rats by up to 75 percent, but not more than that. Concerns that the VCD may pose a risk to humans have little merit, Mayer says, adding that ContraPest does not affect people, nor should it cause any danger.

And ContraPest also doesn’t appear to cause rat hot flashes or mood swings, says Mayer. “As one person said to me, ‘Wow, don’t you worry? I mean, a whole bunch of large menopausal rats — aren’t they angry?’ Well, we’ve not seen that behavior change.”

Related stories on MNN: