With the ongoing depletion of worm control chemicals, coupled with the ever milder and wetter weather conditions experienced in recent years, these 'pests' appear to be on the rise.
I think it is time, therefore, to remind ourselves of the problems they cause and ensure we are doing all we can to limit their progress.
There are 28 species of earthworm native to the UK, as well as a number of non-native species brought in by accident through the actions of humans - for example in the soils of imported exotic plants. Each species exhibits unique behavioural and physiological characteristics, their distribution being influenced by soil pH, soil texture, organic matter, mode of feeding, life style and their location within the soil profile.
Maybe surprisingly, only a small number of species actually produce the casts which are such a nuisance. The most abundant of the casting species is Lumbricus terrestris, which is also the UK's largest species at 90-170mm in length. This earthworm is nocturnal and forms a permanent vertical burrow opening onto the soil from which it forages for food.
The advantages and disadvantages of earthworms
There are many benefits from having earthworms in the soil. Their most important role is in the decomposition process within the soil, which increases nutrient availability and soil fertility, but other benefits include increased infiltration rates, the modification (improvement) of soil structure and soil aeration, thereby leading to improved rooting of plants.
Unfortunately, for those of us whose primary concern is the playing quality of our sports turf, the disadvantages brought about by casts are significant.
Casts are unsightly, but the main concern is the effect of the casts on the run of a ball across the surface. The relationship between the ball and playing surface is important in all ball games, but is particularly so in fine turf situations such as on a bowling green, golf green or cricket table, where ball roll or bounce is imperative. Anything that interferes with this relationship is, therefore, a big disadvantage.
Unhappily, that is not the end of the story, as there are further disadvantages to be aware of. These include the creation of muddy conditions brought about by casts being smeared on the playing surface. This can cause smothering of fine grasses as well as sealing of the surface to ultimately reduce surface drainage. In particular, worm casting can have a very significant impact on newly installed slit drains that can soon become capped over and ineffective if worm casting is not controlled.
Additionally, the production of casts can bring dormant weed seeds to the surface, and the casts themselves provide a perfect seedbed for germination and establishment, especially as they are rich in nutrients and have higher levels of nitrogen, phosphorus and potassium than the surrounding soil. Furthermore, earthworms may encourage other pests, as they are a rich food source for creatures such as moles.
The presence of casting worms should be minimised by cultural control methods, thus reducing the need for chemical control, especially as the range of available chemicals has slowly been eroded.
Aim to control the level of sub-surface thatch that provides a rich source of food to worms. Similarly, clippings should be boxed off where possible, as these provide a constant source of nutrient and could also increase thatch levels in the upper profile. Excessively organic topdressing materials should also be avoided, and the use of organic based fertilisers limited for the same reasons.
Worms do not like acid conditions and much prefer neutral or alkaline soils. This is most useful from a fine turf point of view as acidic soil conditions also favour the desirable bent and fescue species. Therefore, always try and use acidic-reacting materials as opposed to alkaline ones. For example, lime in any form is a definite no no - worms just love it! Always ensure that the material used contains no more than 0.5% lime - a fact that you should make your supplier aware of. Similarly, using limestone to backfill drain trenches is not a good idea, even if it is often cheaper than other materials.
Similarly, irrigation water should be tested for pH, alkalinity and salinity on a routine basis to ensure that it will not influence the alkalinity of the soil.
Modifying soil pH to create more acid conditions has become one of the more effective lines of control. Sulphur and sulphur-based products oxidise in moist soil to form sulphuric acid, which lowers the pH. Additional benefits of sulphur can include flocculation of the fine clay particles into larger ones, increasing pore spaces and improving soil structure and drainage.
The regular use of ammonium sulphate based fertilisers is recommended, especially as research has shown acidifying fertilisers have the added benefit of favouring the development of the finer grasses. Similarly, applications of iron sulphate will have a comparable acidifying effect and, in this instance, will also help control moss and harden the sward against disease.
Of course, this process should not be taken too far - if acidity levels were allowed to drop as low as pH 4.0, for example, then there would be very little earthworm activity, but the fine turf grasses would also be thin, weak and prone to moss invasion. For many fine turf situations the desired level should be a pH of about 5.0-5.5.
A pH of this level should have a significant impact on earthworm populations and is also perfect for the development of browntop bent grasses. With regard to cricket tables and tennis courts, where ryegrass can be a significant sward component, care should be taken when amending the pH as ryegrass does best in soil conditions around neutral pH.
On golf courses, a further technique that has been used successfully to reduce worm populations is to apply sand topdressing to an area, as the sand irritates the worm's skin. However, this again would not be possible on a cricket square (but perhaps the outfield), nor on tennis courts.