Worm Control

Graham Paul, of Sherriff Amenity, offers some advice on how to get the best from Worm Killers.

There is some controversy regarding the worm control products that are used in the turf industry today. This article is intended to help clarify some of the issues involved in the use of these chemicals.

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'. This was originally formulated from thiophanate ethyl, which is a precursor of the worm control products we use today (carbendazim - also known as MBC). Thiophanate methyl and thiophanate ethyl break down rapidly to form MBC and it is MBC that gives the product fungicidal and worm control properties.

To get the best control from products based on MBC such as 'Caste Off' or its precursors based on thiophanate methyl ('Mildothane Turf' and 'Snare') 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 burrows that can be several feet deep with some species and this can provide useful aeration of the soil. As they play out their lives, for the most part hidden away from us, 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 in the form of 'convoluted tubes' which we refer to as worm casts. Not all worms excrete their casts on the surface. Of the 25 species found in UK turf, only 3 are known to deposit surface casts. The remaining species release their casts underground, so avoiding all of the mess and misery experienced by the groundsman and greenkeeper!

Although most earthworms feed on decaying plant roots and organic matter found below the soil surface, the surface casting species will also collect leaf material from above ground and take it down into the burrows where it is eaten. It was this phenomenon that first led us to the worm killing properties of MBC. In the 1970's one of the biggest selling fungicides for use 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 but, after 'Mildothane' had been used for two or three seasons in apple crops, we began to notice that the leaf litter was 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 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 readers 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 lies in the soil! A worm-colonised soil will have, at any one, time; eggs, juveniles and adult worms. Under normal conditions, only the adult worms 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 have the reproductive organs of both sexes) 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, juveniles 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 and get more reliable results in dealing with the problem of worms. We can take two approaches to this strategy; firstly we should consider discouraging worms by cultural practices and secondly we can use wormkillers more effectively.

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, this will help reduce the severity of the problem. Secondly, consider improving the drainage of badly infested areas.

Worms need plenty of moisture to move around in, so drying the ground will help to slow them up. The third 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 areas that are 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. Borehole irrigation can have a high pH as well, particularly if the hole is drawing from chalky or limestone soils.

The second approach to dealing with earthworm problems is to try to get the best results from using a wormkiller. When we spray for earthworms, much of the chemical will remain on the leaf and because carbendazim (MBC) has root uptake systemic properties, some of the spray that ends up in the top part of the soil profile will be moved internally into the grass leaf, providing that the plant is actively growing.

So, if we want to control the surface feeding worms (the ones that are responsible for casting), we need to leave the grass clippings on the surface for as long as possible after spraying so that they can feast! This is not always possible on fine turf areas such as golf greens or on bowling greens and cricket tables during the playing season. 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 played on.

When spraying a wormkiller we should also consider adding an appropriate adjuvant to the spray tank to improve the efficacy of 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. In contrast, at a pH of 5.0 carbendazim has a half-life of 30 hours. Once the chemical has dried on the plant leaf it will not be rapidly broken down by hydrolysis and so will remain available to deal with the casting worms.

Weed Free's Mike Seaton offers further info on effect, control and legislation

The subject of worms, or in fact their casts, is very timely. The increase in rainfall over the past month has seen a dramatic increase in worm activity. This is because of the change in the soil/water/air ratio. Worm activity is increasing and we will have them on our playing surfaces until the spring of next year.
As soil technicians we are all aware that the activity of worms can be very beneficial to the overall texture of the soil.

The downside is that in clearing their burrows at certain times of the year, (mainly October to late February dependant upon soil moisture levels) they leave a cast - a small mound of soil on the playing surface or lawn. The casts play havoc with mowing practices, making playing surfaces and lawns messy and ultimately encourage weeds and weed grasses. Annual Meadow Grass in particular, as it sets seeds all year round.

The subject of worms and their control has always been a very emotive one. The humble worm plays an important part in the decaying processes within the soil, thereby speeding up the recycling of thatch and organic materials, making them available for further use by turf grass plants and other essential soil micro organisms. The aim should be to control the population of earthworms, keeping them at a level where they cease to be a pest. This does not mean annihilating every single one.

The advantages of having worm activity in the soil are many as they:

• break down organic matter to humus, so improve the soil fertility
• Improve the crumb structure
• aerate the soil - only deeper burrowing types cast
• only cast for around 6 months of the year - Early October to March
• reduce compaction
• eat decaying leaf litter like grass clippings that would otherwise turn into thatch
• improve surface drainage
• reduce toxic gases in the soil
• increase the activity of soil bacteria

The disadvantages of having worm activity in the soil -bearing in mind that it is not the worms that are the problem, merely the effects of their casts on the lawn or playing surface, is that casts:

• are unsightly and numerous
• get stuck onto the soles of shoes and are slimy and sticky
• are a problem for machinery as they damage the bottom blades of mowers and can alter the height of cut as they attach easily to mower front rollers
• can smother finer leafed grasses
• can encourage weed seeds and germination
• are expensive to control
• can produce an uneven playing surface and distort the run of the ball (e.g. hockey)
• will, in time, ruin playing surfaces

CONTROL

Where cultural methods of prevention, elimination or eradication control have been exhausted, in practice the best chemical control is applying a control product based on the active ingredients of Carbendazim or Thiophanate-methyl from early October through to late February when the worms are visibly active and becoming a bit of a pest. The chemical will prevent worms from casting after an initial period of reduced cast production. The casts already on the surface of the turf just prior to application will need to be harrowed or brushed off. The chemical control will last between 2 - 3 months dependant upon rain fall, soil type and worm population density.

There are approvals to tank mix worm cast control products with Leatherjacket controls based on the active ingredient Chlorpyrifos. This tank mix is excellent. Because of the added wetting agent in the Chlorpyrifos, the worm control active ingredient gets into the soil quicker with less chance of the turf grass plant actually taking up the Carbendazim as a fungicide.

If your site is on a really heavy soil such as clay, you may need to plan to spray twice, once in October and again after Christmas to gain a 'clean' surface. It is important that you ensure even coverage of the control product and follow the instructions on the label. If you find yourself only spraying playing surfaces and not the surrounds, golf fairways or green approaches, you will be making it difficult for players to get to your 'worm cast free' oasis without wading through a worm cast infested area first. It has been frequently documented that really alkaline water supplies can reduce the effect of the active ingredient.

LEGISLATION

Now the legislation bit - over the past few years, companies selling pesticides, which include worm cast control products, have been scaremongering turf managers saying that the worm cast active ingredients are under scrutiny by other EC Countries. This may be true but a withdrawal of the active ingredient is many years off, if at all.

The web based information service, Pesticides News (http://www.pan-uk.org/pestnews/Actives/Carbenda.htm) declares that Carbendazim, which has a worldwide registration as an active ingredient, has extensive applications worldwide, with the global market worth over £116 million at user level, equivalent to over 12,000 tonnes of active ingredient.

For further information I suggest you visit the site, meanwhile I think I'll go and eat worms.

Mike Seaton is a fellow of the IOG and was a practising groundsman for six years before founding the contract spraying company Weed Free in 1991.