Iowa has a number of feral swine - it's just kept fairly mum. I work with the USDA investigators who follow up on the calls and kills.
Not only do they do a number on crops, but they can also rut up sensitive areas, causing soil loss and displacing vegetation. But, the biggest problem here in the #1 pork producing state are the diseases they can carry which can spread to humans and captive swine. I know of a documented brucellosis case in SE Iowa where a neighbor caught brucellosis from his herd after a wild board got into the pens and bred the sows - giving them brucellosis. Read about the symptoms - you don't want it.
When feral swine are caught/killed, they are tested for brucellosis and pseudorabies. If positive, all (ALL!) swine in a 5 mile circle around that kill must be tested. If you're convicted of being the idiot that brought the feral swine in, you'll be the one paying the bills on all that testing.
In the past couple of years USDA has watched reported areas with night vision surveillance - and have confiscated Russian pigs from petting zoos, pens on farm, and a backyard in Des Moines.
A video from DNR (
IA DNR: Video) is pretty good except it's old and it is now illegal to bring feral swine to Iowa.