Earthworms are beneficial, and help aerate and drain the soil by tunnelling through it, however there is a species of earthworm (allolobophora) that deposit their muddy excrement on the lawn surface.
This can occur at any time of year, but more particularly in the wetter months from September to April.
These small piles of mud can spoil the appearance of lawns.
If they get smeared onto the lawn surface e.g. when walked on or during mowing, these deposits can create areas where moss and lawn weeds can establish.
Control;
There are no chemicals available to the amateur gardener for controlling earthworms.
The ‘casts’ should be brushed off the lawn with a stiff a broom or wire rake.
During the winter months the ‘casts’ might be quite wet, making it difficult to brush them off, so the best course of action is to keep off the turf to avoid trampling on them.