Why Do Earthworms Come Out When it Rains?

I’ve always wondered why earthworms come out of the ground when it rains.  I don’t know about you but after a rain I can always find them on driveways and roads.  Sometimes I’ll see thousands of them on a short stretch of road as if they are having some kind of mass annelid exodus from a field on one side of the road to the other.  So what gives?

Earthworm in my driveway

At times I’ve wondered if a hard surface like concrete or asphalt provides a source of relative warmth over wet soil.  As it turns out the answer is pretty simple.  They can’t breath underground when the soil is saturated.  The air pockets in the soil fill up with water and worms basically breath through their skin.  Teri Balser is an associate professor of soil and ecosystem ecology at UW-Madison and she says, “Oxygen from air or water passes directly from their outer cuticle into their blood vessels. Normally, soil has a mix of air and water — about 50 percent of the pore space in soil is air, the rest is water. Oxygen diffuses easily through air, and the soil stays aerobic because oxygen comes in from the surface.”

So that’s all there is to it, and now I know better.  Rather than suffocate the worms move elsewhere.  I’d probably do the same thing.  Hopefully I won’t lay in the middle of a road and get squished though.

BUT!!

An big, fat earthworm crawling through our cereal rye cover crop

Another article claims they may reveal themselves in such great numbers following a rain for a number of other reasons.  Stemming from the possibility that “they can even survive several days fully submerged in water” some experts believe pounding rain may mimic the vibrations of worm eating predators causing the worms to surface.  Another thought is they may be using the wet conditions to travel longer distances overland than they could underground.  Since the surface is wet they don’t have to worry about drying out in the hot sun.

What’s your worm experience?  Why do you think the come out on a rainy day?

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