What is causing the small mounds seen in the picture at right? Is it moles? Mole crickets? Or something else? The little mounds are actually earthworm castings. Recent rains have been helped plants stressed by drought, but more soil moisture and cool temperatures increased earthworm activity.
At first glance, many people believe these small mounds indicate insect damage to the lawn. Mole crickets, which are a major pest of turfgrass, tunnel near the soil surface and feed on roots and foliage. They also can make mounds that look like those created by earthworms. The absence of tunnels is a key difference in discerning between the two.
A “soap flush” can be done if not satisfied with a visual check. A solution of one to two tablespoons of dishwashing liquid mixed in three to five gallons of water can be poured over the mounds, bringing the creature below the surface up to be seen.
The good news is that earthworms are not pests of turfgrass and do not feed on turf. They burrow through the soil to feed on microorganisms and partially decomposed organic matter. This activity helps improve aeration and the movement of water and fertilizer through the soil.
After earthworms feed, they move to the soil surface to deposit the castings, which are rich in nutrients and organic matter and are actually beneficial to the turf. However, when casting piles become large or numerous, they can be considered unsightly and over time can make the lawn lumpy.
When earthworm populations increase, there are a number of steps that can be used to reduce the mounds they create.
First, smooth the soil surface by raking the castings. Next, irrigate lawns with an inch of water once a week to encourage deep root development. Deeper roots allow turf to go longer between watering; extended periods between watering permits the soil surface to dry before the next watering. Earthworms are less likely to come to the surface when the surface is dry.
Lastly, mow the grass at the highest recommended height to help hide castings. For Bermuda, zoysia and centipede, the recommended mowing height is 1½ inches. Mowing height for St. Augustine is 2½ inches in full sun to 4 inches in shade.
Moles are enemies of earthworms but are not a friend of turf. There are a few other natural enemies of earthworms that can aid in the control of their populations. These include ants, centipedes, birds, snakes, toads, carabid beetles and nematodes. These predators don’t cause problems in the lawn the way moles do.
Some people would like to control earthworms with pesticides. While some pesticides and fertilizers are known to have an impact on earthworms, none can be recommended as controls. Today’s pesticides have relatively little impact on earthworm populations, in part because of the registration process that examines adverse environmental effects such as mortality to beneficial, non-target organisms. Although earthworm mounds can be annoying, the homeowner should consider the benefits provided to the lawn’s health from these beneficial organisms.