SALEM — Washington state is monitoring the wild pig populations in Oregon, where the Department of Fish and Wildlife has ordered farmers to determine the size of the destructive pig populations on their land and get rid of the invaders.

The fish and wildlife department knows feral pigs are a problem. As an invasive species, they threaten crops and cause headaches for farmers.

The state doesn’t yet know how big of a problem. That’s because most feral pigs in the state live on low-precipitation private land. But they’re inching closer to high-value cropland, and neighboring states in the Pacific Northwest are worried.

The introduction of feral pigs in Oregon is largely blamed on California, where they are game mammals, meaning hunters have to get tags to shoot them. In the meantime, they root up cropland, destroy hillsides and generally wreak havoc on the environment.

A bill passed last year in Oregon requires landowners to trap or shoot any feral swine known to roam their land, or at a minimum allow someone else to shoot or trap it.

“Whether or not we’ll be successful really depends on private landowners,” said Keith Kohl of the Oregon Department of Fish and Wildlife. “They are on private land and we can’t do it without the landowners’ cooperation.”

The hairy pigs are native to Europe but have spread to every continent except Antarctica, usually introduced by humans.

The pigs prefer to forage in areas around rivers or streams and can hinder timber growth, tear up irrigated fields, damage white oak stands and erode stream banks. Though mostly vegetarian, they will eat about anything, even small mammals, such as fawns or ewes.

Kohl said feral pigs make it to Oregon in three ways: migration, escapes from exotic-animal ranches and hunters, who set them loose to shoot for sport.