Bug Lady Blog – Earthworms Enhanced

Greetings, BugFans,

They’re baaaccckkk!  April (and March) Showers bring more than May Flowers, they revive the earthworms that “slumbered” below the frost line all winter.  Here’s an enhanced version of a BOTW that appeared in 2009.  This episode demonstrates once again that the BOTW definition of “bug” is closer to that of a first grader than that of an entomologist.

In her childhood, the BugLady was deeply moved when a character in the play “Teahouse of the August Moon” said “When you kill a worm, you kill a friend.”  Worms, we are told, honeycomb the soil with their tunnels, letting in air and rain; their slime glues soil granules into clumps; and they build new soil with the deposits (worm castings) that mark their passing.  Soil engineers is what they are.  Happy earthworms make happy gardens!

So, in the ensuing decades, when it “rained worms” (rained so much that their tunnels flooded and worms were displaced to sidewalks and parking lots), she rescued these “road-worms” and deposited them in the moist grass instead of leaving them for hungry birds or for the noonday sun.  Why did the earthworm cross the road?  Several on-line sources are “rain-worm deniers,” stating that worms do not get tired of treading water, that they can breathe well enough in wet soil, that they’ve simply decided to broaden their horizons, or that they’re out because it’s cloudy, or that they’re taking advantage of the damp conditions to reproduce (they mate at the soil’s surface).

Then the BugLady learned that Wisconsin earthworms are aliens that damage forest ecology!  More about that in a sec.

There are about 180 species of earthworms (class Oligochaeta) in America, and 6,000 species in the world.  Of those, only about 150 species are widely distributed, “cosmopolitan” worms.

Earthworms live in U-shaped tunnels in the soil.  They stay indoors during the day and emerge at night because sunlight’s UV rays are toxic to them, paralyzing them, damaging muscle cells, and ultimately drying out the moist skin through which they breathe (there are studies indicating that earthworms may turn out to be useful as UV monitors for humans).  They’re at risk even on cloudy days.  They live in the dark.  Eyeless, they sense light with their skin, and earless, they are tuned in to vibrations.  They are, literally, front-loaded, with a brain, five “hearts,” and other organs at the head end.  Nerves stretch the length of their body, their sense of touch is well developed, and there are many chemoreceptors located near the mouth.

earthworm 4

An earthworm’s smooth, moist skin, with its rows of setae (bristles), overlays an impressive musculature.  Progress through the dirt (or resistance to potential predators) is achieved by extending their front end, swelling it so it fills/jams the circumference of their tunnel, allowing the bristles to grip, and then contracting its rear half.  And, yes, if enough of the front half of an earthworm is separated from its rear half by a shovel or a tug-of-war, the front can regenerate a portion of its “tail.”  The naturenorth.com website says that if you put a big night crawler on a piece of paper, you’ll hear its setae scratching as it locomotes.  The BugLady loves that warm evening in early spring when earthworms come up to rustle the dry leaves.

When a young worm’s fancy turns to love, any old earthworm will do.  Earthworms are hermaphrodites – male and female in a single body.  Each individual produces both eggs and sperm (they do not self-fertilize), and they clasp together to exchange sperm.  Research suggests that they may use pheromones to locate their significant other, down there in the dark.

Worms eat organic material, including dead leaves (they prefer leaves with a high sugar content like maple and birch), small soil invertebrates, bacteria, and fungi that they ingest as they move through/excavate their tunnels.  In loose soil, they are picky about what they eat; in compacted soil, the only way forward by ingesting everything.  As a result of its trip through an earthworm, material is broken down into smaller pieces and its pH is brought closer to neutral before the residue is deposited on the surface as a worm casting.  Check out Life in a Bucket of Soil, by Alvin and Virginia Silverstein and Discover Nature Close to Home by Elizabeth Lawlor for great descriptions of the earthworm lifestyle.

What eats worms?  What doesn’t?  They’re an important strand in the food web.  According to the naturenorth website “For animals that weren’t originally found in Manitoba, worms now fill a very important slot in the food web.”

Soils that the glaciers chewed up, froze, compacted, transported, and spit out ended up earthworm-free, and the plant communities that became established in those post-glacial soils evolved absent the considerable impact of worms.  The BugLady still hasn’t heard a satisfactory answer about Wisconsin’s Driftless Area, which, although it was untouched by the most recent glaciation, was still exposed to permafrost conditions caused by the flow of frigid air down the slopes of the ice sheet.

At its maximum, the most recent glacier extended into central Iowa, Illinois, and Ohio.  According to the people who measure these things, earthworms move at a top speed of about 5 ½ yards a year. They did the math and found that on their own, earthworms might have trekked north approximately 35 miles in the last 11,000 years.  So, earthworms must have gotten a little help.  Alien earthworms were introduced to the upper Midwest when settlers imported bulbs and fruit trees from the Old Country, from dirt that was used as ballast on ocean-going ships, and by people hoping to improve soil structure.  Worm introduction continues today through landscaping projects and recreation – fisherpeople, please take note – earthworm populations are densest near lake shores where bait buckets are emptied at the end of the day.

Earthworms live up to their reputations as soil builders, and although their contributions to farming and gardening are not argued, their impact on forests is profound and negative.  In forests that developed after the glacier withdrew, the decomposition rate of organic matter on the forest floor was slow, accomplished by bacteria and fungi and tiny critters that made big pieces into little ones, creating below the trees a soft, deep, nutrient-rich “duff” that sheltered small animals, acted as an insulator, and provided mulch for wildflowers and seedlings to grow in.

In contrast, earthworms compete with some important fungi (and eat others), and they mix, rearrange, compact and eat leaf litter, duff and top soil layers, making conditions hostile for seeds.  And in the process, the worms also eat seeds, depleting the seed bank and hampering future regeneration.  With the character of the forest floor altered and the nutrient cycle disturbed, ground-dwelling animals both tiny and large lose their homes and their food, setting the stage for additional alien “generalists” and lowering biodiversity.  As many as 80% of terrestrial plants are able to grow because they form mycorrhizal partnerships with fungi that enable them to take up nutrients and water.  Research is being done to explore the impact of alien earthworms on soil fungi and therefore on plant communities.

Fun Earthworm Facts:

  • Some species of tropical earthworms grow to be 10 feet long, but the World Record belongs to a 22 foot long South African earthworm.   
  • Charles Darwin was so taken with earthworms that he studied them for 39 years.
  • Earthworms produce a lubricant that seeps through their pores to moisten their skin.  A species of Australian worm can squirt this liquid through its pores to a distance of about 12 inches (the BugLady thinks maybe she saw the movie).
  • Worm farming is called Vermiculture, and it’s a huge business.  In 1980, Canada exported 370 million earthworms at a value of $13 million.  Vermicomposting of organic wastes is a potential green tool for businesses and municipalities.
  • Earthworm slime contains nitrogen, which benefits nearby plants.
  • Aristotle called earthworms “the intestines of the soil“.

So, what is a night crawler, exactly?  It’s a large (often 6” to 7”), reddish worm that goes by the name of Lumbricus terrestris.  Native to Europe, it has been introduced around the world.  Because they feed at the surface, night crawlers have a dark streak down their back that protects them (a little) from UV rays.  Except for the end of their tail, which is pink, because it remains anchored in the burrow as the worm feeds, Robin-ready.

See Project Worm Watch at http://www.nrri.umn.edu/worms/ for information about invasive worms.

As for you, little earthworms – you’re on your own as you cross the road, “bon chance.”  “Bon appetit” little birds.

Kate Redmond, The BugLady

Bug of the Week archives:

Become a Member

Take advantage of all the benefits of a Riveredge membership year round!

Learn More