If you have mounds of soil in your lawn, nightcrawlers may be the cause. These bumps are randomly spaced rather than a “run” that is characteristic of moles.
Nightcrawlers are large worms — usually 4 to 8 inches or more.
They belong to a group of earthworms known as deep-burrowers. The deep-burrowers build large, vertical, permanent burrows that may reach as deep as 5 or 6 feet. Nightcrawlers pull plant material down into their burrows to feed on later. The bumps you see on top of the ground are called “middens” and are a mixture of plant residues and castings (worm feces). These middens may be used for protection and food reserves. The burrows can have a significant positive effect on soil by opening up channels for water and air to penetrate. Roots also like these channels because they ease root penetration and supply nutrients from the casting material lining the burrow. The middens, however, are a source of aggravation for homeowners. They can make it hard to mow or even walk on a lawn because they become very hard when they dry.
Getting rid of the middens is difficult. Rolling the lawn while the middens are soft may help temporarily, but mounds will be rebuilt when nightcrawlers become active again. Also, there is nothing labeled for nightcrawler control.
Some gardeners want to protect the nightcrawlers because of their positive effect on soils.
Pesticides that may be used for other pests vary widely in their toxic effects on earthworms. One that has no effect is Dylox (Proxol).
Malathion may be slightly toxic while Sevin and copper are extremely toxic. Avoid using the latter group while nightcrawlers are active.