Earthworm Swarming

April rains bring in May flowers is a Canadian saying. After a heavy downpour last evening, I had to fling a bunch of worms off our driveway and put them back in the flowerbed. Earthworms lying on sidewalks or streets after a heavy rain is a common sight in Canada on the onset of spring. They move out of their burrows and swarm the roads, sidewalks, and driveways.

Why do they do this?

Scientific name for earthworms is Lumbricus Terrestris Lumbricus in Latin means a worm and Terrestris means of the earth.They belong to Oligochaeata class meaning few bristles. The bristles help the worms to stay anchored in the soil as they move. Oligochaetes are distinguished from other worms by their lack of legs. Instead, Oligochaetes move by contracting and relaxing their longitudinal muscles, which run along the length of their body.

Researchers hypothesise several reasons why heavy rain bring these worms out of their soil homes.  Many scientists opine that they come out to the soil surface after a good rain to prevent drowning in their water-filled burrows.

Earthworms do not have lungs.  They breath through their skins. They need moisture-enriched soil with a certain oxygen content to survive. They are least likely to drown and can survive several days fully submerged in water if oxygen levels are right.

Some experts believe that the earthworms surface during rains for migration purposes as it gives them an opportunity to slither through greater distances across the wet soil surface or grass surface than they could through soil.

Certain species of earthworms surface to mate, but only a few of the 4,400 existing species, making it unlikely that mating is a primary reason for this surfacing. Earthworms are hermaphrodites, meaning an individual worm has both male and female reproductive organs. Earthworm mating typically occurs after it has rained and the ground is wet.

Being hermaphrodites, they can fertilise themselves (parthenogenesis,) but is rare. They emerge from the soil and jut out their anterior end. They wait for another earthworm to point in the opposite direction. The two worms join together and a mucus is secreted so that each worm is enclosed in a tube of slime. The eggs of both mates become fertilised with the sperms of the other.

Another explanation involves rain drop vibrations on the soil surface sounding like predator vibrations, like that of moles or birds. Earthworms often come to the surface to avoid falling prey. According to Professor Josef Gorres of the University of Vermont’s Department of Plant and Soil Science, “Rain can set up vibrations on top of the soil like mole vibrations. Similar to how earthworms move upwards and out of the way when predator vibrations are felt, they could move in a similar way for rain vibrations.”

What happens when drought conditions prevail?

During the peak of summer or during draught conditions, earthworms burrow deeper. They may either die or revert to a hibernation called diapause. In a diapause state, they coil up in knots in a little hole with a slimy substance to avoid moisture loss. Eggs in cocoons survive prolonged drought, allowing earthworm populations to survive drought periods.

According to Mary Ann Bruns, Associate Professor of Agronomy/Soil Microbiology in the Department of Crop and Soil Sciences at Penn University, “Earthworms dig deeper into the soil where it is moister when conditions are dry. They will do all they can to avoid extreme temperature fluctuations.

Anglers looking for earthworms as baits create vibrations to coax worms from their burrows.  They run a piece of steel or a hand saw across the top of a stake, which causes a rubbing sound to occur as the stake vibrates. These vibrations cause the earthworms to move to the surface. 

Earthworms are the true friend of a gardener or a farmer. Presence of earthworms is a sign of healthy soil. The burrowing and feeding activity of earthworms has a positive effect on soil quality. The burrows help in water infiltration, soil aeration and improve soil porosity. Earthworm are known to easily consume two ton of dry matter per acre per year, partly digesting and mixing it with soil.  Earthworm casts improve soil fertility as they have higher nitrogen, phosphorus, potassium, and calcium contents. 

In the Netherlands, some soils reclaimed from the sea at first did not have any earthworms. In these soils the formation of topsoil with reasonable organic matter content did not take place, resulting in poor crop growth. Once the cause was established, the government of the Netherlands started a campaign to introduce earthworms. After the introduction of the earthworms, a dark topsoil layer was formed, and crop growth increased substantially.

In the spring mornings, armed with a dustpan and a brush, I collect all the earthworms on our driveway and deposit them in the garden soil.

Tips to encourage or sustain a healthy population of worms in your garden are:-

  • Reduce tilling your soil.
  • Leave organic matter on the surface.
  • Add organic manure and compost.
  • Avoid chemicals – fertilisers, insecticides, pesticides, weedicides, etc.
  • Use an organic mulch to keep soil moist and cool.

Natural science has not come out with any conclusive reasons as to why earthworms surface after a heavy rain in spring, but they continue to surface every spring.