They say no worms are native, why can nightcrawlers be available?
That’s correct, as the last ice age some 12,000 years ago would have killed all of the native worms in northern Minnesota and technically they don’t move very far over the years. The abundance of night crawlers is such that it’s too late to avoid the spread. Minnesota Conservation Magazine did an article on the Leech Lake area, the introduction of nightcrawlers to the forests by fisherman, and damage to the leaf duff on the forest floor. When I moved to my current property I planted nightcrawlers in my garden, now I can collect my own, just water the garden and collect at night. Even after 25 years it’s surprising how contained they are in a certain area, maybe because it’s surrounded by sandy soils underneath.