The use of chemicals for worm cast control is subject of much controversy. Some people believe that we should leave worms alone, whilst among those who do wish to deal with the problem, there is confusion as to how to use carbendazim effectively.
This article is intended to help clarify some of the issues involved in the use of carbendazim as a worm killer.
Many years ago I worked for May & Baker, developing new products for the amenity market. Amongst the projects I was involved with was a worm control product called ‘Castaway’, developed to replace worm killers based on chlordane that were seen as harmful to the environment. These ‘old style’ worm killers worked by skin contact and had a long persistence in the soil. The result was that they could eradicate all worms from the soil with just one application. Although very effective at dealing with worm casts, chlordane had a broad spectrum of activity against many soil living creatures and micro-organisms – so it had to go.
‘Castaway’ was originally formulated from thiophanate methyl, which is a precursor of carbendazim, also known as MBC.
In use the product was rapidly transformed into carbendazim, so essentially we were applying the same material as we do today.
The Chemical Regulation Directorate’s pesticide database currently lists nine formulations containing carbendazim that are approved for worm cast control in managed amenity turf.
These are sold under a variety of brand names e.g. ‘Caste Off’
In contrast with chlordane the uptake of carbendazim is by ingestion rather than skin contact and persistence of the chemical is much shorter.
Therefore, in order to get the best control from these products, we have to understand a little of the biology of the earthworm.
Earthworms play an important role in the development and maintenance of soil structures. They live in burrow in the soil , some of which can be several feet deep, which can provide useful localised aeration. They need a habitat that provides loose, moist soil so they can move about freely underground. Most importantly, they consume large quantities of soil and decaying plant material from which they obtain all of their nutrition. The waste material, a well mixed, highly fertile compost, is excreted by worms in the form of 'convuted tubes' which we refer to as worm casts. Not all worms leave their casts above ground, of the 28 species found in UK turf, only 3 or 4 are known to deposit surface casts. The remaining species release their casts underground, so avoiding all the mess and misery experienced by Groundsmen.
Although most earthworms feed on decaying plant roots and organic matter found below the soil surface, the surface casting species will also collect leaf maetrial from above ground and take it down into the burrows where it is eaten. It was this phenomenon that first led us to the wormkilling properties of MBC. In the 1970's one of the largest selling fungicides used in apple orchards was 'Mildothane' as it was effective against powdery mildew, apple scab and also controlled red spider mite. It is a well known fact that earthworms will completely clear the fallen leaves from an orchard floor during the winter months. However, after ‘Mildothane’ had been used for two or three seasons in apple crops we began to notice that the leaves were not disappearing from orchards as usual. Investigation revealed that the surface feeding species were absent from soils of treated orchards and it was concluded that they had been killed by the residues of thiophanate methyl left on the leaves from repeated sprays throughout the growing season.
Some people believe that MBC (carbendazim) doesn’t kill worms but just suppresses casting for a few weeks. In fact evidence shows that MBC does kill the worms that feed on the surface and that the surface feeders are responsible for casts deposited above ground. In our field trials with ‘Castaway’ we extracted all of the worms from treated and untreated plots and then counted the number of each species present. Extraction was achieved by pouring diluted formaldehyde solution onto the soil. This irritates the worms and they rapidly emerge from their burrows. The surface feeding/surface casting species were absent in the treated plots but were found in the unsprayed plots. We also counted worm casts in the sprayed and unsprayed plots and were able to demonstrate that casts were absent in the treated areas. Please note I would strongly advise not to use formaldehyde on worm infested turf as it kills the grass and is extremely damaging to the ecology of the soil.
Why then do we have such difficulty controlling worm casting? The answer can be found in the life cycle of the earthworm. A worm colonised soil will have at any one time; eggs, juveniles and adult worms. Under normal conditions, only the adult worms of these three or four species will come to the surface to feed. Juveniles generally remain in the burrows and feed below ground. When they mature the adults come to the surface to feed, deposit casts and to mate with another worm. As soon as they have mated, the eggs are released, fertilised with sperm from the other worm and sealed in a sack which is deposited at the bottom of the burrow. As you are probably aware, earthworms are hermaphrodites (they are both male and female) and therefore both individuals will produce eggs after copulation, doubling their reproductive capacity. When we use MBC based wormkillers we can only control those individuals that are adults at the time of treatment. A few weeks after spraying some juveniles will start to develop into new adults and we begin to see a few casts appearing on the surface. By the time the problem has got bad enough to spray again, a whole new batch of eggs have been laid and so the worm colony survives.
Having understood a little of the biology and behaviour of earthworms, we can use this knowledge to our advantage to get more reliable worm cast control. We can take an integrated approach to dealing with worm casting using cultural methods designed to discourage earthworms combined with more effective use of carbendazim, to achieve a more sustainable result.
Starting with cultural practices, the first thing that comes to mind is to remove the source of freely available food and that will involve collecting grass clippings when mowing and removing fallen leaves from surrounding trees in the autumn. This is not always feasible on large areas such as sports pitches and golf course fairways but if it is possible to ‘box’ the clippings and clear away fallen leaves, this will help reduce the severity of the problem in the long term.
Next, consider improving the drainage in badly infested areas. Worms need plenty of moisture to move around in so drying the ground will help to slow them up. Improving drainage is going to be a costly operation but it would bring benefits to the quality of the playing surface as well as discouraging worm casts.
Another consideration is the soil pH (acidity) – earthworms prefer a neutral or slightly alkaline soil, so in some circumstances we can discourage them by lowering the pH with careful use of acidifiers such as sulphur. This is by no means an easy task and you would need to start off by having a soil test done and then taking professional advice from an agronomist, as the pH of the soil will also have a profound effect on the health of the turf. Even if we chose not to manipulate the soil pH with acidifiers, we should avoid using lime or calcified seaweed on areas that have a worm casting problem. In turf that is irrigated on a regular basis, it is worth having the pH of the water tested as this might be adding to the problem. Tap water can often have a pH in the high 7’s or low 8’s indicating a fair degree of alkalinity. Bore hole irrigation can have a high pH as well, particularly if the hole is drawing from chalky or limestone soils.
The second part of our integrated approach is to optimise the effectiveness of the worm killer. When we spray for earthworms, much of the chemical will remain on the leaf. A typical carbendazim label recommends a spray volume of 500 to 1000L of water per hectare. A light shower of rain that deposits 1mm of rain hardly penetrates more than 2 to 3 mm of the soil profile. This 1mm of rain is the same as spraying at 10,000L water per hectare, so unless we water the chemical in thoroughly we are not going to get the product very far into the soil. This is fine because to encourage the surface casting worms to ingest sufficient carbendazim to kill them, the best plan is to serve it up as a dressing on a freshly mown grass salad! The surface feeding, surface casting worms can then be dealt with leaving the remaining worms in the soil unharmed. The method is simple; apply the chemical and when dry mow the turf leaving the clippings on the surface. Leave the grass box off for the next two to three cuts. On fine turf areas such as golf greens or on bowling greens and cricket tables during the playing season this is not always possible due to the demands of play. For bowling and cricket then, the best time to tackle the problem is in the early autumn and early spring when these areas are not in play.
When spraying a wormkiller we should also consider adding an appropriate adjuvant to the spray tank to improve the efficacy the product. 'Aqua Tick’
is a water conditioner that creates the ideal pH environment in the spray tank, buffering it to a value of around 5.0 and preventing alkaline hydrolysis of the chemical.Research has shown that carbendazim breaks down rapidly at pH 9.0 having a half-life of just 12 minutes. This means that at this extreme pH, 50% of the carbendazim added to the tank will be ineffective within 12 minutes of adding it to the tank. In contrast, at a pH of 5.0 carbendazim has a half-life of 30 hours.Alkaline hydrolysis only occurs in dilute solution so once the chemical has dried on the plant leaf it will not be rapidly broken down and so will remain available to deal with the casting worms.
Article updated 29/09/09