Night crawlers really get around: Nature’s Notes

Iowans working their gardens or strolling sidewalks on drizzly mornings are likely to encounter night crawlers. They are so common that most people would be astonished to learn that these normally appreciated animals came to North American from Europe in the 1700s.

Night crawlers are one of about 182 species of earthworms. Some are native but nearly all worms encountered in suburban and urban areas are imports.

Gardeners love earthworms for their ability to digest organic matter and excrete castings that are one of nature's best fertilizers. Their constant tunneling aerates and mixes soil, helping many garden crops thrive. And, no other bait entices as many fish species to bite an angler's hook. Like most invasive species exotic worms cause problems in some ecosystems, especially forests. Many northern trees need a thick layer of leaves and other organic matter on the soil surface. Introduced worms digest this organic material and stress entire forests.

Worms are sluggish and can't migrate far. Without the help of people, they would have trouble spreading. It's likely that worms reached North America in dirt used as ballast in sailing ships and in soil surrounding nursery stock roots. Once here they were transported whenever dirt was moved, and anglers helped them spread by dumping left over bait on the ground after a day of fishing. Today it's even possible to order live worms through the Internet, and many of these escape or are released.

On wet spring evenings, crawlers come to the surface to feed and mate. Each worm has both male and female organs and when two find each other they join together with a band of mucus that allows the swapping of sperm. Eventually each will lay eggs that create a new night crawler generation.

l Marion Patterson is an instructor at Kirkwood Community College. Rich Patterson is the former executive director of Indian Creek Nature Center in Cedar Rapids.