(CBS) — Chicago has re-claimed a title it held in 2014: The “rattiest” city in America, so determined by Orkin pest control.

The No. 1 ranking is based on the number of rodent control treatments the company has performed — both residential and commercial.

Chicago’s Department of Streets and Sanitation wouldn’t respond directly to Orkin’s rat ranking. But spokeswoman Jennifer Martinez did say the city itself has gotten more calls so far this year than the previous time frame last year.

“We have received about 30 percent more calls for rodents, this year over last,” she says.

Martinez says the city believes a mild winter allowed a lot of rats to survive.

The city has been trying a new approach: dry ice.

“So the dry ice –you actually place it into a rodent’s burrow. And you close up the burrow,” Martinez says.

The dry ice becomes carbon dioxide gas, and it’s toxic to the rats. And they die.

Streets and Sanitation is trying the new dry ice approach in four parks and in planter boxes downtown.