Worm activity in lawns is a common problem particularly in autumn and winter when the soil is wet... although their presence does also indicate that your soil is in quite a healthy condition! The casts that are produced from worm activity are the main problem as they look unsightly and can cause muddy conditions when they are stepped on. A high level of worm activity can also attract moles which feed on worms.
There are a few things that you can do to discourage them. Worms prefer a neutral to alkaline soil pH. The pH can be lowered by adding fertilisers which contains sulphate of iron. Sulphate of iron will make the lawn surface more acidic and discourage worms.
Worms enjoy feeding on organic matter in lawns, often referred to as ‘thatch’, which is can be caused by organic matter such as moss and old grass clippings. Make sure that you remove clippings when you cut the grass and rake up fallen leaves in autumn to reduce the accumulation of thatch. Scarifiying your lawn will also help to reduce thatch.
Casts can removed by sweeping them off the lawn, but make sure that they are dry before you start this task or otherwise they will just smear across the grass.