The following presentation is brought to you by the
University of Arkansas System Division of Agriculture.
Primary funding for this research was provided by the USGA.
Earthworms.
From a horticultural standpoint, earthworms are typically considered beneficial.
"They create macro-pores for air and water infiltration and root growth,
they can help kind of move nutrients around,
move soil around in the profile, and all of that is beneficial."
But when it comes to golf course management, uncontrolled earthworm activity
can wreak havoc.
"Casting on golf courses can cause issues with ball roll and playability
from a golfer standpoint, and then from a management standpoint,
you can get muddy surfaces, weed and disease pressure on your turf."
Earthworm casts also have the potential to interfere with
and even damage turf care equipment.
The primary earthworm control method utilized until the mid-20th century
was developed by a British greenskeeper named Peter W. Lees in the 1890s.
Lees discovered that application of mowrah meal, which contains a
class of natural surfactants called saponins, irritated earthworms
and caused them to rise to the turf's surface where they could be manually removed.
Around the 1950s, synthetic, worm-toxic pesticides entered the market
and led to a decrease in the application of Lees' removal methods.
Growing concerns for human health and the environment, however,
led the U.S. Environmental Protection Agency to discontinue these pesticides.
"There are no pesticides that are labeled for earthworm control,
so applying anything to kill earthworms on your turf would be illegal."
Other, cultural control methods, including application of acidifying fertilizers
and removal of clippings have given sometimes positive
but overall inconclusive results.
Chemical expellants similar to Lees' mowrah meal are also in development,
but are not yet labeled for use on earthworms.
Although studies have shown that these expellants can be effective,
long-term consequences of their use are unknown, not to mention
the immediate ecological dangers posed to wildlife,
especially to aquatic populations.
And finally, the harm done to otherwise beneficial earthworm populations is not ideal.
Recent research has demonstrated that sand topdressing can
deter earthworms and reduce earthworm casting activity on golf course surfaces.
However, the effects of topdressing have not always been consistent.
Building on this research, the University of Arkansas
Department of Horticulture undertook a study to determine the combined
effects of rootzone soil texture and long-term sand topdressing
application rates on earthworm casting activity.
This study, performed at the University of Arkansas Agricultural Research
and Extension Center in Fayetteville, AR, utilized Patriot Bermudagrass
maintained at a length of one-half inch.
These experimental plots were previously used in a study that
investigated four combinations of rootzone soil texture and
sand topdressing application rates over a period of seven years.
The four combinations include:
A Native Captina silt loam soil rootzone lightly topdressed
at a rate of 0.25 inches of sand per year.
The same native soil rootzone heavily topdressed
at a rate of 1 inch of sand per year.
A sand rootzone lightly topdressed at a rate of 0.25 inches of sand per year.
And a sand rootzone heavily topdressed at a rate of 1 inch of sand per year.
All four combinations were replicated four times for a total of 16 plots.
Each plot measured 5 feet wide by 16 feet long.
Over the course of 2 years from November 2015 through October 2017,
researchers measured volumetric soil moisture content
and soil temperature, and conducted cast counts at least twice per month
for each combination of soil rootzone texture and topdressing rate.
Volumetric soil moisture content was measured using a
Spectrum FieldScout TDR 300 Soil Moisture Meter at a depth of three inches,
while soil temperature was measured at a depth of about four inches.
In order to quantify cast counts, researchers used a standard
tally counter to record the total number of casts found in all 16 plots.
To avoid double-counting any casts, plots were subdivided while
collecting data using a five-by-five-foot frame, which was
further subdivided into one-by-one-foot squares.
After each data collection event, casts were cleared away either by mowing
or with a draggable Coco Mat to reset the experimental plots
for the next observation period.
The average weekly cast counts from each experimental block
were averaged by month, and comparisons were made between
each combination of soil rootzone texture and topdressing rate.
Cast counts were also analyzed relative to soil moisture and temperature
to evaluate any correlations between casting activity and
these environmental parameters.
In April of 2017, a ShopVac was used to collect earthworm casts from
experimental plots, which were dried, ground, sieved, and analyzed
to determine particle size distribution and percent organic matter.
Soil from each experimental plot was also collected for particle
size distribution analysis and percent organic matter determination.
Upon completion of this study, University of Arkansas researchers were
able to make several observations regarding earthworm activity on golf courses.
There was generally very little casting activity on the lightly
topdressed soil rootzone plots.
In the soil rootzone, heavy topdressing typically resulted
in significantly greater casting activity.
In the sand rootzone, topdressing rate did not impact casting activity,
except during the fall of 2017, when casting activity was significantly
greater under a light topdressing regimen.
A comparison of moisture content with cast counts demonstrated
that there was no statistically significant correlation between the two factors.
Soil temperature was found to be a significant predictor
of earthworm casting activity, explaining 10-34% of the observed
variation in cast counts.
The critical rootzone temperatures for peak casting activity were
determined to be between 11.1 °C and 14.5 °C,
across all four experimental variations.
And, contrary to hypothesis, this study's analysis of the collected soil samples
from each rootzone and topdressing combination suggests that
there is no correlation between rootzone organic matter content
and casting activity in 'Patriot' bermudagrass.
Another factor that plays a role in casting activity on golf courses
is the species of earthworms that reside in the affected turfgrass system.
To better understand the effect that earthworm species may
have on casting activity and control efforts, researchers collected
earthworm samples from the study location.
One-by-one-foot sections of turf were extracted from each
experimental plot to a depth of 8 inches.
Earthworm samples were also gathered during surveys of five
golf courses in the surrounding region.
Researchers separated the subsoil layer from the rootzone
and sorted through each sample by hand, collecting
any earthworms found for later identification.
Morphological identification was conducted on some earthworm samples,
but due to the small size or developmental immaturity of many
of the collected samples, species could typically not be identified
using taxonomic keys.
Therefore, a DNA testing methodology was devised.
Earthworm samples were cleaned and boiled to quickly kill the sample
while preserving the earthworm's tissue.
A small amount of DNA was then obtained using a proprietary DNA extraction kit.
And a polymerase chain reaction was used to multiply the DNA sample.
Each DNA sample was then sent to Eurofins Genomics for DNA sequencing.
Analysis of the resulting DNA sequences found that a majority
of the collected samples were from the Diplocardia genus,
which is native to North America.
The difference in earthworm abundance between the heavily
topdressed sand and soil rootzones was not statistically significant.
The light topdressing soil rootzone treatment was found to have
the greatest overall abundance of earthworms despite exhibiting
the least amount of casting activity.
Possible explanations for this discrepancy include differences in species abundance,
a higher percentage of juvenile earthworms compared to adults,
and the possibility that the more favorable soil rootzone environment
allowed earthworms to focus energy expenditures on reproduction
instead of foraging and biomass production.
Additional research on earthworm activity must be performed to
better understand how variations in turfgrass species,
maintenance practices, topdressing application rates,
and earthworm species will affect casting activity.
"Hopefully future research can kind of help nail down a specific recommendation.
You're kind of limited in what you can do to control earthworms right now,
and so you need to be patient with your superintendents as they figure out
what is going to work."
Golf course superintendents seeking to control excessive earthworm activity
should be aware that sand topdressing may impact surface casting,
but results from this study varied over time and may be further influenced by other factors.
"So it's hard to recommend one topdressing treatment over another
you know, heavy or light, because it depends on your rootzone,
it depends on the year, different years you could see different effects,
so it's hard to make one concrete recommendation."
This study suggests that light topdressing on a native soil rootzone
may reduce casting activity, while topdressing rate on a sand-capped
rootzone may not have significant effect on earthworm casting.
And overall, there was significantly greater casting activity on the
sand-capped rootzones compared with the lightly topdressed soil rootzone.
For more information about this study or The University of Arkansas
Department of Horticulture and their research program and projects,
please contact Dr. Wayne Mackay.
Primary funding for this research was provided by the USGA.
Additional funding for this research was provided by a gift from the
Arkansas Turfgrass Association and from the Arkansas Golf Course Superintendents Association.