Nightcrawlers are a type of earthworm. They are highly beneficial because they aerate soil, allowing water and oxygen to penetrate more easily into the ground. Their feeding and excrement helps recycle nutrients and fertilize the soil. Nightcrawlers also feed on thatch, a layer of live and dead plant material that can accumulate at the soil surface and reduce the penetration of water and fertilizer.
However, as nightcrawlers feed on soil and plant material, they leave behind a waste product called castings. Nightcrawlers deposit castings in their burrows or at entrances at ground level forming conical mounds at the soil surface. These mounds can be seen in early to mid-spring when nightcrawlers are first active and again during fall. During late spring and summer, when weather becomes warmer, nightcrawlers move deeper into the soil and are not normally seen.
Although these mounds do not harm turf, they do make the ground rough and uneven, sometimes making it difficult to walk on the grass. In severe cases, nightcrawlers can reduce homeowners' enjoyment of their lawns.
Nightcrawlers can also be a nuisance when they occur in large numbers on sidewalks, driveways, patios, pools, and other places where they are not wanted. This usually occurs after a heavy or steady rainfall and soils are saturated. This can also occur when lawns are overwatered.
You can reduce the inconvenience the mounds cause by using a power rake (also called a vertical mower) to help knock them down. Early autumn is the best time to power rake, although you can power rake in spring once the ground is firm underfoot and before hot weather sets in. When you power rake in spring, it's also a good idea to apply a pre-emergent herbicide to prevent annual weeds from sprouting.
If you must power rake in summer, water thoroughly and frequently until the lawn has fully recovered from having slices cut into it. Power rakes may be rented from garden centers and rental companies. Do not use rollers to flatten nightcrawler mounds. This compacts the soil which adversely affects the turf.
Despite the nuisance of these mounds, you should tolerate nightcrawlers. Treating them with an insecticide is strongly discouraged. The value nightcrawlers have in keeping soil healthy far outweighs any problems their mounds create. If nightcrawler populations are low, turf will suffer In the long run.
When nightcrawlers are numerous on sidewalks, driveways, etc., just wash or sweep them off. If you ignore them, they will eventually move off on their own or dry up and die. Never use a pesticide on nightcrawlers under these circumstances.