FEATURES
Earthworms

by Marcia Passos Duffy

Friend or foe?

Earthworms are usually considered benign. Their feeding activities aerate soil, recycle nutrients and break down organic matter, they feed on thatch and improve turf health in many ways. For golf course superintendents, however, there is a dark side to earthworm activities.

Photo by Scott Bauer, courtesy of USDA.

The castings earthworms leave on top of the turf are the most troublesome. These mounds can interfere with play, are a nuisance to golfers and are aesthetically unpleasing.

In low-cut turf the castings can be seen on the surface where they can muddy equipment and dull mower blades, says David Shetlar, professor of urban landscape entomology at Ohio State University’s Extension and Agricultural Research & Development Center.

“Castings on putting greens also can interfere with ball roll, which is an unacceptable hazard on most golf courses,” says Shetlar. In higher-cut turf, especially on heavy clay soils, earthworm castings mixed with grass clippings that are allowed to dry result in lumpy surfaces, he says. “These can cause a rough ride in carts and even have been known to turn and injure ankles of walkers,” says Shetlar.

Are rough rides, muddy playing surfaces and even turned ankles enough to justify eradicating earthworms altogether? No, say the experts. Earthworms are classified as beneficial organisms and have no registered insecticides or fungicides to control them. Applying any substance to specifically kill them is illegal. The key is to find the right method to keep earthworm populations at a manageable level. The first step is to identify the type of earthworm that causes the most trouble on the course.

Worm species

Of the more than 24 species of earthworms in North America, only three are found in turf. Of those three, only two create surface castings. The earthworm that creates the most castings is the night crawler (Lumbricus terrestris), which is a European native introduced to the U.S. by way of horticultural imports.

These night crawler earthworm populations can be as high as several million under golf course fairways, and millions more in the roughs. They also live a surprisingly long time; they have average life spans of six to nine years and can live up to 20 years. One mature night crawler can produce several hundred offspring per year. Consistently fertile and moist conditions are perfect for these earthworms that feed on fairway clippings and organic matter in the soil.

Night crawler populations start to reach damaging levels on golf courses when fairways have high soil moisture and get regular nitrogen fertilizer application and consistent food supplies (clippings and other organic material). The earthworms cast on surfaces of the grass when performing “house cleaning” after heavy rains. The peak activity happens during the spring, fall and early winter. Earthworms are intolerant of drought and frost and will retreat deep in their burrows when the temperature is extreme or conditions are not optimal.

Earthworm management

Earthworms, in general, are beneficial to golf course turf and should not be treated as pests, but when they become a nuisance, it may be time for management.

“In my experience, earthworm activity can be managed through judicious water management. Keeping the turf just above the wilting point and applying only sufficient daily irrigation to maintain this point can dramatically decrease earthworm castings,” says Shetlar. While this type of management won’t kill off earthworms, it will reduce their surface activities.

Shetlar says that an application of carbaryl (at its full grub control rate) or thiophannate-methyl (at its full surface fungicide rate), applied the evening after a good rain or irrigation (the previous morning) can greatly reduce earthworm activity for long periods of time.

Studies have also found that earthworms do not tolerate soils with a pH of 5 or less, but thrive in soils with a pH between 6.5 and 7. “Reducing soil pH [to around 5] reduces earthworm activity,” agrees Alex Ellram, Ph.D., a professor in the department of plant science at the State University of New York College of Agriculture and Technology at Cobleskill.

“Sand topdressing and verticutting and dragging can level mounds and reduce the negative effects of earthworms,” Ellram suggests.

Other research has found that reducing the food supply on the surface (clippings) also helps reduce earthworm population.

Keith Kirchoffer, senior branch manager of golf for OneSource Landscape and Golf Services, Inc. based out of The Villages, Fla., takes a low-tech approach to ridding his clients’ greens and fairways of castings.

“We mow them down, pick them up, or use a bamboo whipping pole to spread out the castings ... there’s not much else you can do to get rid of them,” Kirchoffer says. In Florida, with its sandy soils, he says that castings are a minor problem and only when there are heavy rains.

Dr. Gwen Stahnke, extension turfgrass specialist for Washington State University, says the problem of earthworm castings is as old as the game of golf itself. “The more organic matter in the soil means more earthworms,” she says.

Her organization’s research has found the most effective way to limit the castings is through a sand topdressing, but even then, earthworms often burrow through that and deposit castings.

Tolerance and acceptance

“Unfortunately, we don’t have an answer to the problem,” says Stahnke. Except, perhaps, educating the golf course members about the benefits of earthworms and encouraging tolerance of a certain level of imperfection on the golf course.

While castings on the greens can be a nuisance, the biggest frustration is on the fairways, where there isn’t always a topdressing, Stahnke says. Golf course superintendents need to understand that in the spring and fall, when the earthworm activity is at its peak, ground crews will have to wash off mowers and sharpen blades more frequently because of the mud caused by the castings. The castings, coupled with insect and disease problems, can certainly add more stress to a superintendent’s already busy job.

The castings earthworms leave on top of the turf can interfere with play and are aesthetically unpleasing. Photo courtesy of OneSource Landscape & Golf Services, Inc.

Scott Thayer, superintendent at the Legends Club in Prior Lake, Minn., says that after heavy rains the course get castings on greens and bunkers, which are usually picked up by mowers. “In bunkers, they can get in the way of a shot,” he says, and adds that the club has not gotten any complaints yet about the castings.

Thayer has learned to live with the castings, and says that the positives of earthworms outweigh the negatives. “When we get a lot of casts it means the soils are rich in nutrients and organic matter, making it easy to remove them knowing our soils are happy,” Thayer says.

Perhaps what is needed is more tolerance and acceptance of the benefits of earthworms to the turf.

“Superintendents that have zero tolerance for earthworm castings really need to reconsider why they are so intolerant,” Shetlar says. “There are ways to deal with the castings, and not blame earthworms for other issues that may be more important on their putting greens and tees.”

The author is a freelance writer from Keene, N.H.