By Ben McGraw, PhD

It is difficult to tell whether Aristotle was speaking favorably of earthworms when he dubbed them “the intestines of the earth.” What we can be sure of is that he never managed fine turfgrass in the presence of heavy earthworm populations. In most crop systems, earthworms are considered beneficial organisms since they aerate the soil, bust thatch layers, decompose organic matter and eject nutrient-rich fecal matter (“casts”) to the surface when tunneling. However, the casts can be the bane of many turfgrass managers’ existence—they muddy the surface, damage mower reels and cause general thinning of the turf. Damage can be especially severe in spring and fall in areas where turf is grown on native soils. Managing these invasive organisms is made difficult by the lack of control options and earthworms’ ability to quickly reproduce.

Unfortunately, there are no pesticides registered in the United States and Canada for controlling earthworms. Some turf managers seek to indirectly reduce castings when targeting other pests by selecting pesticides that negatively affect earthworms. The fungicide thiophanate-methyl (e.g., Cleary’s 3336) and the insecticide carbaryl (e.g., Sevin) are most commonly used in this manner.

Several studies have investigated the effects of cultural practices on reducing castings in turfgrass settings. Generally, earthworms prefer moist, cool conditions, with near-neutral pH. Frequent sand topdressing, removing clippings and applying acidifying fertilizers have been investigated in several university studies. Sand topdressing has produced the most promising results of the three, although soil modification requires frequent or consistent applications and is cost prohibitive to many operations to perform on large areas.

Saponin-based fertilizers

Turfgrass managers have recently turned to using saponin-based fertilizers to culturally control earthworm populations. Saponins are naturally occurring compounds found in a wide variety of plant species. Materials possessing saponins readily form a soapy foam when added to water and shaken. Applying saponin-based materials to soils after rain or irrigation expels earthworms from the soil, causing them to desiccate and die.

Using saponins to control earthworms is not a new idea, as it was once a popular means of suppressing casts on putting surfaces in the United Kingdom in the early 1900s. The proliferation of golf courses in the interior of the UK began with the expansion of the British railway system. Courses moved from traditional “links” land with sandy soils to areas with heavy soils or greater percentages of fines. These soils were dominated by earthworms, and managing them on greens was a challenge. An ingenious superintendent named Peter Lees was the first to concoct his own saponin product (ground mowrah meal) to control earthworms and apply it to his putting surfaces. Saponins replaced harsh mercury-based products and were regularly used for several decades.

Control with saponins fell out of favor, however, in the 1950s when many chemical pesticides became publicly available. Many of these pesticides, including DDT and chlordane, were broad-spectrum in activity, and it is reported that they provided earthworm suppression for several years! It was not until after these products were phased out that earthworm populations rebounded in many turfgrass sites and alternatives were sought.

The first modern saponin-fertilizer product (Early Bird by Ocean Organics) was developed in the mid-2000s. Early Bird, a by-product of tea manufacturing, has been shown to effectively expel earthworms from the soil and reduce castings over several weeks. Recently, another saponin-based fertilizer, RhizoAide (Grigg Brothers, owned by Brandt Consolidated) has come onto the market and is available for use in turf. Our laboratory has been assessing the efficacy of these products and attempting to find optimal field rates.

Our research

Earthworms are especially challenging to control due to their biology. They are hermaphrodites (possessing both male and female sexual organs), although they require a mate to produce eggs. Eggs are deposited in pearl-shaped capsules or “cocoons” throughout the year. This stage is relatively impervious to chemical or saponin control. Therefore, applying a short-residual product may kill only adults and juveniles but leaves a portion of the population unaffected.

We have observed how populations are able to rebound quickly even with effective products. In 2015, single applications of RhizoAide (4, 6 or 8 lbs./1,000 ft2) were compared to the granular formulation of Early Bird (6 lbs./1,000 ft2). All rates and products provided ~ 70% or higher control compared to the untreated checks 7 days after treatment (DAT). Two rates of RhizoAide provided 50% control of castings after 21 DAT, which suggests limited residual activity.

In spring 2016, we sought to determine if sequential applications of RhizoAide could provide more lasting suppression. RhizoAide (4 or 6 lbs./1,000 ft2) was applied monthly to plots on a golf course fairway in either 2 or 3 sequential applications. All treatments produced strong statistical differences by 28 DAT. Treatments receiving 6 lbs./1,000 ft2 reduced castings by 49% to 70% (average 61%), which was similar to Early Bird (60% reductions). After the second month, castings in the 6 + 6 lbs./1,000 ft2 RhizoAide treatments had been further reduced (80%), although this was not significantly different from treatments receiving 6 + 4 lbs./1,000 ft2. The single application of Early Bird at 0 DAT provided 80% cast reductions at 55 DAT.

Finally, by the end of the trial (90 DAT), three applications of RhizoAide at 6 lbs./1,000 ft2 provided the greatest numerical reductions compared to the controls (90%). No differences were detected between the Early Bird treatment (84%) and RhizoAide applications. RhizoAide applications that received 6 lbs./1,000 ft2 at the start of the trial had generally the highest numerical reductions (> 80% control).

Although more testing is needed, it appears that the first application’s rate may be the most important in “culling” reproducing adults. It is possible that rates of subsequent applications may be reduced to clean up the newly formed adults that were not initially controlled when in the juvenile state or were present in cocoons at the time of first application.

Controlling earthworms, much like insects, requires a thorough understanding of their behavior and biology. We will continue to look for solutions for turfgrass managers. Currently, we are conducting trials to determine if late fall applications provide greater control of populations than with the traditional spring applications.

Ben McGraw, PhD, is an associate professor of turfgrass entomology Penn State University, University Park, PA.