By Sjoerd Duiker, Extension Soil Management Specialist
This week is a great week to explore your earthworm populations — soils are moist and are warming up after the winter, and earthworm activity is high.
High moisture drives the earthworms to the soil surface. They don’t like sunlight but will stay near the surface on overcast days. Once the soils dry out in the summer earthworm activity will drop, after which we typically see an uptick in activity in the fall. Many earthworms die or hide at depth when frost season hits our state in December.
Earthworms are very important for maintenance of soil health in our agricultural systems. They consume organic matter and mix it with soil in their intestines. Casts are produced when they deposit their excrement at the soil surface or in their burrows.
There are different types of earthworms. Some of them live in permanent, vertical burrows (subsoil dwellers). The nightcrawler is the most prominent earthworm that has this life style. It is most active between 6 p.m. and 6 a.m. It comes to the surface at night, pulling straws, leaves, and sometimes even pebbles and small stones to the entrance of its burrow. After the crop residues partially rot the nightcrawler can consume them. Because it cannot easily use residues incorporated into the soil it is not common in clean tilled soil.
Other earthworms live in the topsoil. They make horizontal burrows and consume organic matter that is found in the soil. They are less sensitive to tillage, although they don’t become more numerous in tilled soil. They fill their burrows with their casts, so their burrows are not permanent like those of nightcrawlers.
Finally, there are earthworm living in organic matter or manure, like the red worms. These worms don’t thrive in soil. Some live in thatch or in leaf litter in the forest. Examples of these are red worms used to make vermicompost.
Earthworms are hermaphrodites; this means they are both male and female. They will mate by lying next to each other. They will release a large amount of mucus around their bodies while lying in this position. Copulation can last for an hour. Nightcrawlers copulate at the surface of the soil but other earthworms copulate in the soil.
After copulation, a hardened surface will form on the clitellum (the thickened part on the body of the earthworm, sometimes called ‘saddle’). A tube will separate from the clitellum and move over the head of the earthworm, picking up ova and sperm as it goes. After leaving the earthworm, the tube closes, forming a cocoon which contains from 1-20 fertilized eggs. Only a few (perhaps 3 or 4) will actually live to form young earthworms. Earthworms produce cocoons throughout the year when conditions are fit.
Here are some common earthworm species that can be found in our soils:
- Green worm (Allolobophora chlorotica). A topsoil dwelling species typically 2 inches long, moving mostly in horizontal burrows and coming rarely to the surface. Earthworm that has a clitellum (saddle) with three pairs of ‘sucker like’ disks on its underside. Has a yellow ring towards the head and often curls up in the hand. Greenish yellow form prefers very wet conditions, while the pink form prefers drier conditions. Can produce large amounts of yellow fluid from pores along its body when disturbed.
- Pink soil worm (Aporrectodea rosea). Topsoil dwelling earthworm, typically 1-2 inches long. Also called ‘rosy-tipped worm’, it has a rosy pink or pale head up to the male pores. Its clitellum is usually orange. It usually has two or more whitish raised pads before its male pores.
- Southern or purple worm (Aporrectodea trapezoides). A fairly large topsoil dwelling species (3-6” long). It is dark greyish brown in color. It has large pale swollen male pores on segment 15.
- Grey worm (Aporrectodea caliginosa). A 2-3 inch long, pale earthworm that is easy to identify due to the different shades of color along its body. It is a topsoil dweller, living in horizontal burrows in the topsoil that it fills with its casts.
- Red worm (Lumbricus rubellus). Earthworm lives in organic matter, like manure pads. 1-5 inch long, dark red colored earthworm with orange clitellum. Widespread but usually of low abundance in the soil. It may flatten its tale in a paddle shape. This species is used for vermiculture (worm composting).
- Nightcrawler (Lumbricus terrestris). A very common earthworm in no-till fields. It lives in 3-4 feet deep, permanent, vertical burrows, coming to the surface at night or during cloudy, rainy days, where it collects crop residue by pulling it into its burrow. It has a bright red head and a large orange clitellum. It has a grey, flat tail. Its length can be 10 inches. It deposits casts on the surface.