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You must use the correct type of earthworm for the task that you wish it to perform. For example, if you wish to introduce earthworms to your compost heap you will require a different type of species than if you were adding some earthworms to the soil in your garden.

In compost heaps, the most common earthworm is Eisenia foetida, more commonly known as the tiger worm. It likes to eat lots of organic matter and survives very well in the compost heap where it has a rich, plentiful supply.

In most soils, however, there is not enough organic matter present for the tiger worm to survive. Instead we find that there are other types of earthworms that are specially adapted to living in the soil. The most common soil dwelling species is Aporrectodea caliginosa, more commonly known as the grey worm. This introduced Europoean species is now the most common of all the earthworm species in New Zealand and is usually the most dominant species in our gardens and agricultural land.

Different worms for different soils

Earthworms can be divided into three groups, each with different behaviour patterns:

• soil surface dwelling or compost preferring species
• topsoil dwelling species
• subsoil dwelling species
  

Soil surface dwelling or compost preferring species

Dung worm (Lumbricus rubellus)
These species tend not to burrow into the soil. They prefer to live at or near the soil surface or in compost heaps (see diagram), since they like to eat material that is high in organic matter, e.g. decaying plant roots and shoots, dung and leaves. Examples of such species found include the dung worm Lumbricus rubellus and the tiger worm Eisenia foetida.

Topsoil dwelling species
These species usually live in the top 20-30 cm depth of soil. They burrow through the soil, ingesting it as they go and thus mix the topsoil layer (see diagram). As they burrow they produce stable earthworm casts that help to improve the soil structure and increase the soil aeration by creating cracks and channels in the soil.

After soil has passed through the gut of an earthworm the nutrients in the soil can be changed to a form more accessible to plants. Therefore, the presence of earthworms usually makes the soil environment more favourable for plant growth. The most common earthworm found in New Zealand is a topsoil dwelling species called the grey worm, Aporrectodea caliginosa.

Subsoil dwelling species
These species tend to make permanent burrows in the soil that can be more than 3 m deep. They survive best in undisturbed areas where their burrows can remain intact. They tend to forage about on the soil surface for food and then drag food such as leaves down into their burrows for consumption.
They can be particularly useful species in situations where large amounts of organic matter are left on the soil surface. In orchards for example, they help in the removal of the large number of leaves that annually fall onto the soil surface. Examples of these species are the night crawler, Lumbricus terrestris, and Aporrectodea longa.

These species are more commonly found in the North Island of New Zealand. It is unclear at this stage whether the reason for their more common occurence in the North Island is as the result of more of them having been introduced there by chance or whether it is due to climatic limitations.

For more information contact:
Dr Trish Fraser

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