HELENA, Mont. — Ranchers and government officials here are keeping watch on an enemy army gathering to the north, along the border with Canada. The invaders are big, testy, tenacious — and they’ll eat absolutely anything.

Feral pigs are widely considered to be the most destructive invasive species in the United States. They can do remarkable damage to the ecosystem, wrecking crops and hunting animals like birds and amphibians to near extinction.

They have wrecked military planes on runways. And although attacks on people are extremely rare, in November feral hogs killed a woman in Texas who was arriving for work in the early morning hours.

“Generally an invasive species is detrimental to one crop, or are introduced into waterways and hurt the fish,” said Dale Nolte, manager of the feral swine program at the Department of Agriculture. “But feral swine are destructive across the board and impact all sectors.”