Facing a banner year for gray squirrels across the U.S., researchers are pulling out all the stops in their struggle to trim the tree dwellers' numbers—even contraception.

Due to a warm winter and a resulting boom in nuts, the small mammals' population is spiking, especially on the East Coast, in the Northeast, and in the Midwest. But despite their cute exterior, the animals can wreak havoc on their surroundings, devouring farmers' crops, chewing into building wires, and stripping bark—damaging, if not killing, trees outright.

To halt the growth, birth control is the best option. But no squirrel is willing to take a daily pill, and no IUD is small enough. So how do you put a squirrel on birth control?

Thanks to decades of contraceptive research for species such as white-tailed deer, squirrel managers have two options. They can use vaccines to stop the rodents from making sex hormones. Or they can lower the squirrels' cholesterol, the molecule from which sex hormones are made.

The major perk of vaccination is that a single injection does the job and lasts for years.

"But you have to spend time catching the squirrels, and it's hard on them to be handled," said Christi Yoder, a former researcher at the National Wildlife Research Center in Fort Collins, Colorado. Plus, at $50-plus per squirrel—including labor and dose—vaccination is expensive.

Sunflower Seeds With a Kick in the Nuts

That leaves option two: tempting squirrels with contraception-laced goodies. That's exactly the route researchers at South Carolina's Clemson University are taking with a cholesterol-lowering drug called DiazaCon.

Before testing the medication on campus squirrels, the team spent a year "capturing squirrels, drawing blood, [and] running bloodwork to see when hormone levels were at their peak," said Greg Yarrow, chair of the university's Division of Natural Resources.

The work isn't purely scientific: Yarrow's campus is in the throes of a nearly decade-long struggle with squirrels, and has already lost more than a hundred trees and over a million dollars in tree planting, care, and removal costs to the chew-happy rodents.

This year, the scientists began offering DiazaCon-coated black sunflower seeds—a squirrel favorite—at 16 campus feeders accessible only to gray squirrels. The drug's been tested in lab squirrels, but this is the first time it's been used in wild squirrels.

While the seeds aren't exactly the same as untreated seeds—they're pink because they contain dye and probably taste just a bit sweeter—the squirrels "don't seem to mind," said project leader Kristina Dunn, a graduate student of Yarrow's.

"I've seen them sitting there just eating, eating, eating" the treated seeds, Dunn said.

Squirrels in the Pink

The team will continue the treatment through another year, gathering reams of behavioral and biological information as they go. At the end, they'll assess how well the treatment worked, and whether any side effects turned up in squirrels or their predators. (Also see "New Sex Hormone Found—May Lead to Male Birth Control?")

Most of the work will be tedious data collection—but the researchers did build themselves one shortcut.

Mixed in with the DiazaCon coating is a nontoxic dye that, once inside the body, stains the squirrels' bellies pink—making it easy to identify treated squirrels at a glance.