Worms in general are to be encouraged as their burrowing helps to aerate the soil. Worms feed on and recycle decaying vegetable matter. A few species deposit their muddy waste on the soil surface where in wet weather it can smear when in contact with your foot or mower. The smear marks are unsightly and can act as ideal spots for weeds to germinate.
Lawn Therapy has a range of products that discourages the earthworm from depositing casts on your lawn by encouraging them to dig down deeper into the soil. The treatment is economic and effective, keeping the lawn looking clean and preventing smears and weeds encroaching.