Answer: Earthworms are so beneficial that there are few remedies on the market for an overpopulation. On golf courses, where the casting are difficult to walk on and to putt through, Diatomaceous earth is sprinkled on the grass which takes care of the worms. I would suggest you try to encourage birds to visit your garden. They'll take care of the worms for you. You might also try rolling your lawn (you can rent rollers at Home Depot). This won't get rid of the worms but will level out your lawn and hopefully they will move on to easier digging. As I said before, there are no pesticides registered for use against earthworms but turf pesticides labeled for other lawn pests (Sevin) and (Merit) are very toxic to earthworms.
Best wishes with your lawn!
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