One of the most problematical turf issues faced across western Europe is earthworm casting. STRI’s turfgrass agronomist Conor Nolan investigates their murky underground world for an insight into how to tackle the headache.
What causes the problem?
The three main species of earthworm responsible for casting in the UK and Ireland are Lumbricus terrestris (lob or common worm), Aporrectodea calignosa (grey worm) and Aporrectodea longa (black-headed worm). The lob worm is the most common and only comes out at night or during heavy rains.
Peak earthworm casting occurs during cool, damp weather and when low atmospheric pressure exists. However, they have a distinct dislike of windy conditions. Activity occurs mainly during the spring and autumn, but can be all year round. Earthworm populations overall are fewer in heavy, poorly drained clay soils or coarse, abrasive sandy soils. They thrive in rich, light and medium textured loams.
What damage can they cause?
Many people still say that the benefits far outweigh the harm that they do. They improve thatch breakdown, stimulate microbial activity and boost aeration through burrow formation. However, in the world of sports turf they are a major problem.
Often a structureless mushy paste is presented just beneath the surface where worms deposit their faecal matter of digested organic matter and soil. This paste can extend to 50-60mm from the surface, producing a less free draining layer and a softer surface.
Casting earthworms also take materials from the surface deeper in to the profile causing a mixing and turnover of soil, leading to the loss of materials such as sand previously applied to the surface. As the soil mixer and movers of the animal kingdom they can annually bring up to 40-50 tonnes of soil to the surface per hectare. Not what you want on your golf course.
In most soils, where costly engineered pipe and slit drainage systems have been installed, drainage rates can drop because of the soil mixing by earthworms.
On racecourses the combined action of earthworms and horses interacting with slit drains can lead to a more rapid deterioration in drainage than on pitches. A significant drop in drainage rates can be expected within five years, to the point where remedial action is necessary. Drainage rates will drop also within the gravels used in pipe and slit drainage systems over time. If the spaces between gravel particles are big enough earthworms will burrow through and leave finer soil particles, which clog the drainage pore space.
Casting can also dramatically reduce grass cover. So much so that cast numbers do more damage to the playing surface on winter games pitches than the matches themselves. Where casting density is high there will always be a higher daisy population, which together present poor quality golf fairway turf in particular.
How can I tackle them?
Outside of the UK, control of earthworms using carbendazim fungicide is prohibited. Unfortunately, products containing saponins, extracted from tea seed oil, cannot legally be used as an alternative control within the EU. While carbendazim brings control, it is a short lived measure. More longer term suppression of casting would be preferred even where carbendazim is available.
What about cultural controls?
Research to date recommends acidification of the soil, removing clippings (food source) and to sand top dress. However, these tests have produced mixed results.
Over a two-year trial researchers deduced that not all species are intolerant of acidic conditions (Backman et al. 2001). Aporrectodea calignosa and A. longa are more intolerant than Lumbricus terrestris, as the latter is more mobile through the profile. Over time the species can evolve and adapt to the soil conditions, so much so that populations can bounce back to previous levels.
Clipping removal in the same two-year study showed no change to casting number carried out by these same researchers. The researchers purport that there appeared to be enough other food sources available for the earthworms to remain active. STRI research over a four-year period in the late 1990s has, by contrast, shown a very positive reduction in casting with a 62% reduction by the end of the trial.
The notion that the creation of drier conditions would discourage earthworm casting is supported by a trial by Backman et al. (2002). They applied 3cm of sand annually for two years with significant reduction noted. Others have found that applications of angular sand top dressing over two years in a sustained manner suppressed casting to levels comparable with pesticides (Williamson 2004), most probably by irritating their skin. The degree of sand angularity was the determining factor in controlling casting, unlike particle size which had no influence.
My own experience has been that sand top dressing will over time see sandier casts produced, which are more easily dispersed, while the top of the profile will eventually become sandier despite sand being diluted and buried by earthworms initially.
One of the most striking observations that I have made over recent years has been the relationship between fine fescue grasses and earthworm casts, in terms of cast number and size. Patches of fescue in amongst bentgrass, annual meadowgrass or ryegrass swards contain both smaller and fewer casts.
There is hope
For those that are dependent upon cultural remedies to tackle the problems of casting, a two-pronged approach is necessary to make a change on golf courses in the shortest timeframe.
The first part is to promote the conditions required to encourage fescue on fairways. This will involve a good sand top dressing programme, sensible nitrogen levels and annual overseeding with fine fescue.
The second part involves clipping removal, which will also favour the spread of fescue. While we may not rid ourselves of earthworm casts, a substantial reduction is possible. Better playing quality will also be obtained as the fescue population increases and drier conditions develop.
With a reduction in recycling of nutrient as a consequence of clipping removal, the application of nitrogen and other elements will be necessary to counter thinning and potential moss spread. Only sufficient growth stimulation is necessary to provide good ball support and sward density. An additional element to this approach could be the inclusion of trinexepac-ethyl growth regulator to reduce clipping production, which helps reduce the amount of maintenance work required from the greenstaff.
For winter games pitches, reducing casting and turnover of soil are the greatest challenges alongside maintaining good drainage and retaining grass cover. The promotion of fescue is not feasible in most situations on winter games pitches because wear pressure will be too great for the grass to survive. Removal of clippings alone, though, will make a big difference. By reducing the casting activity through clipping removal, critical sand top dressing will have a more positive effect on performance in the early years on soil based pitches.
Where the removal of clippings or a sand top dressing programme is not feasible, other measures will be necessary to keep the pipe and slit drainage system working well. If the right sized drainage gravel has been chosen, lateral pipes should work well much longer than the slit drains. To help restore or rejuvenate the secondary slit drainage system because of ‘capping’ of the slits, either sand banding or gravel banding will be required, normally on a localised basis.
If you currently don’t have a chemical solution to controlling earthworms, it is unlikely that a new chemical will become available. For those that currently do have chemical control, how long that will continue to be the case remains to be seen. So you need to start taking cultural interventions more seriously. You must learn how to prevent the quality of surfaces being reduced to a very rudimentary level when you no longer have chemical control. Where promotion of fescue is possible it gives some added hope, but it will require serious investment.
Backman, P.A., Miltner, E.D., Stahnke, G.K., and Cook, T.W. (2001) Effects of cultural practices on earthworm casting on golf course fairways. International Turfgrass society Research Journal, 9, 825-827.
Backman, P.A., Miltner, E.D., Stahnke, G.K., and Cook, T.W. (2002) Worming your way out of a turfgrass situation. USGA Green Section Record, July-August, 7-8.
Williamson, R.C. (2004) Managing earthworm castings. Golf Course Management, May, 93-95.