Auburn University plant pathologist Kathy Lawrence was a bit mystified a few years back when research in her lab revealed that soil- dwelling, crop-destroying nematodes spread significantly farther and deeper at a much faster rate than long had been assumed.
Now, five years later, another discovery by a graduate research assistant of hers is shedding new light on that earlier finding, offering the first scientific evidence that disease-causing nematodes can and do hitchhike to other parts of a field, not only on farm equipment but also on—or, rather, in — earthworms.
“What we wanted to know in this study was whether earthworms pick up nematodes as they move through the soil, whether plant-parasitic nematodes are present in earthworm castings in Alabama cotton fields and whether they’re present in the gut systems of earthworms,” says Ph.D. student David Bailey, who is working under the direction of Lawrence and entomology associate professor David Held.
And the answers were Yes, Yes and Yes.
The earthworm-nematode project was spawned by sheer curiosity on the part of Bailey, who earned his master’s in entomology from Auburn in December 2012 under Held’s guidance with research on mole crickets’ tunneling behaviors.
“I was in a cotton field one day and noticed there were a lot of earthworm castings on the surface, and I started wondering whether there were any nematodes in the casts,” says Bailey. “This whole side project has sprung from there.”
For the study, Bailey collected earthworms and earthworm castings — excrement the worms deposit on the surface — from nematode-infested cotton fields located at Auburn University’s E.V. Smith Research Center in east-central Alabama and the Tennessee Valley Research and Extension Center in Belle Mina.
One conventional, one no-till
The E.V. Smith fields are farmed using conventional-tilling practices, while those at the TVREC are no-till fields. The number of nematodes, both crop-damaging and beneficial, was significantly higher in the no-till samples.
Tiny mounds of earthworm castings abound on the surface of this turfgrass plot. Auburn researchers have found that crop- and turf-damaging nematodes ingested by earthworms as they travel through the soil survive passage through the worms’ digestive tracts and re-emerge, alive and active, in the earthworms’ excretions.
“The results we got suggest that nematodes and earthworms have a mutualistic relationship, one where both species benefit,” Bailey says. “As earthworms travel through the soil, they feed on organic matter and roots, and they consume plant-parasitic nematodes in the process. The nematodes pass through the earthworm’s gut system, uninjured, and are transported to new locations, emerging in the castings alive and active.”
The Auburn findings contradict results from a 2001 study in which nematodes were found in dissected earthworms’ pharynxes and esophagi, but not in the crops and gizzards. That led scientists to posit that nematodes could not survive passage through earthworms’ digestive tracts, and a later study indicating that high earthworm populations in peat meadows corresponded to low nematode numbers appeared to support that conclusion.
“What was most surprising in David’s work was that, in the earthworms
Further research also is needed to determine whether live nematodes that earthworms cast on the surface or in the soil proceed to infect cotton seedlings, Bailey says.
Reniform and root-knot nematodes are serious pests in cotton and other row crops, but they also can cause significant damage in turfgrass, and entomologist Held says he believes golf course superintendents will have a great deal of interest in Bailey’s findings.
“On golf courses, because the greens are mowed so closely, earthworm casts cause aesthetic problems, dull mower blades and gunk up the rollers,” Held says.
“David’s work showing that castings contain live nematodes could mean that if the mower isn’t cleaned before moving from one green to the next, it could be transplanting nematodes from an infested area to a nematode-free spot.”