Metaldehyde residues put slug pellets in the firing line

Metaldehyde-based slug pellets could be restricted unless they are used more carefully.

Two years ago nobody imagined metaldehyde slug pellets could be in the firing line. Two years ago the water companies didn't consider that residues in river water of this popular and effective pesticide would pose anything like the problem caused by other pesticides.

But a revised desktop study identified the 6000-10,000t of pellets applied to farmland each year posed a much greater risk to water supplies than previously imagined, so further tests were initiated. What emerged surprised the water companies, pellet manufacturers and the whole industry.

"It was a shock," admits Bob Breach, a water industry expert with extensive knowledge of farming's role in water quality. "Nobody imagined there would be a problem, far less that it would be such a significant problem."

The agricultural sector needs to respond immediately, to cut risks, he warns. "There is a very limited window of opportunity to show government and regulators that voluntary stewardship can work. A dramatic reduction in river water levels of metaldehyde must be secured by autumn 2009 or we could face regulatory action, which could be much more draconian – involving compulsory buffer strips and severe limits on product use."

A ban on metaldehdye slug pellets, which account for 80% of all pellets used, could slash oilseed rape and wheat yields. Initial figures from HGCA/Potato Council work suggest damage from slugs could cost UK growers ￡200m.

"Don't underestimate the urgency of the problem," warns Dr Breach. "Yes, the voluntary initiative has shown we can change behaviour on pesticide use, but only after a period of several years. We only have months to achieve the same result with metaldehyde."

Growers should get involved and believe their actions can make a difference. Had people heeded the warnings about isoproturon and followed advice for using it more carefully, the industry view is that it would not have been lost.

When the water companies initiated more intensive monitoring for metaldehyde in 2007 they used an improved, super-sensitive method to check river water. Subsequent tests have shown that most lowland rivers contain significant quantities of metaldehyde, routinely ranging from 0.2ppb to 2ppb (parts per billion) in raw water, double to twentyfold the EU limit.

Significantly, the EU pesticide limit, although a strict legal requirement, is not derived from any risk to human health or the environment. Humans could be exposed to more than 3000 times the statutory limit of metaldehyde before suffering any health risk. And metaldehyde has a very benign environmental profile.

But the EU Drinking Water Directive limits were set at the limit of detection, so 1 part in 10bn is too much. That represents just one drop of active ingredient in a swimming pool of water. Or 0.5kg of grain out of the UK's total wheat harvest.

Worse still, it is almost impossible to remove metaldehyde by water treatment methods, which, although very costly, are effective for other pesticides. With no scope to address the problem through treatment, the only alternative is reducing the amount of metaldehyde getting into water in the first place.

As the frequency and magnitude of metaldehyde detection increased last autumn, farming wasin the firing line. Residue spikes tied in with on-farm use, particularly in the autumn, but also for summer potato protection. There was no link with the domestic use of pellets in gardens in spring.

"The water companies need to consistently achieve 0.1ppb at all intakes, which requires a significant reduction from current raw water levels, by a factor of at least 20-30 times. That is a very major challenge," stresses Dr Breach.

He anticipates little leniency. "Regulators require urgent action by the agricultural sector.

In the first instance they have been supportive of voluntary industry action through the Metaldehyde Stewardship Group. But the Pesticides Safety Directorate and the Environment Agency will have no choice but to consider regulatory options, if voluntary action fails to deliver enough of a reduction in the desired timescale."

So what sort of timescale is involved? "It's uncertain, but if the industry can't demonstrate a big change in the coming autumn, not just a 10% reduction, but a radical improvement, there will be more pressure to regulate. And don't expect exemptions if the weather is bad."

To drive industry efforts the Metaldehyde Stewardship Group was formed last year, backed by all suppliers of metaldehyde products. It takes the issue seriously. "We invested ￡100,000 in stewardship last year and will spend closer to ￡300,000 this year, plus an equivalent amount inkind, which all adds up to close to ￡1m. It is serious money," says MSG chairman David Cameron.

Where is the contamination coming from? As with isoproturon, three main routes to water are implicated: Direct, such as spreading pellets into ditches and ponds; point source, such as spills; and diffuse, such as field run-off (see panel).

Peaks in levels are often linked to heavy rain, with field run-off strongly implicated. To pinpoint the importance of different sources, the MSG is undertaking studies in the Cherwell Valley catchment area in Oxfordshire. In the meantime a baseline survey of operator practice is providing some useful pointers.

One of the most startling findings was that almost half of users (43%) felt field margins were a source of slug problems and needed treating.

That just isn't true, says East Anglian agronomist and MSG spokesman Colin Myram. A few black slugs may emerge from there, but the slugs growers need to control are the small, grey field slugs, which live in fields, usually well away from compacted headlands.

Given the extremely high risk of such applications getting into water courses, either directly or from run-off, they must stop, he says. "A single pellet containing 5% metaldehyde would be enough to push 10,000 litres of water over the 0.1ppb residue limit.

That is the equivalent of one pellet in a 33m run of a typical farm ditch.

"If two or three pellets get into a ditch, that will supply enough active ingredient to take the residue level in a stretch of water to 2-3ppb, or almost 30 times the EU limit."

The survey suggested that only 3% of applicators were not cleaned. But techniques varied widely, with 26% saying the water used to wash an applicator went into surface drains.

"Think of the residue left on a typical applicator at the end of a day's work," says Mr Myram. "We found up to 100 or so pellets, and dust, on the machine of a best-practice operator. That is enough to take millions of litres of water above the legal limit if it is cleaned off in the yard and gets into the drains."

Protective clothing was also implicated. Just 26% of respondents used disposable overalls. That meant residues were being washed into water supplies when normal clothing/overalls (51%) or personal protective equipment (32%) were washed. Almost 40% of such clothing was put in the washing machine.

Dr Breach urges the industry to take the threat seriously. "All my experience says that this is a big challenge. Plan A, through voluntary action, needs to work, because Plan B involving potential regulation is something the agricultural sector simply does not need. It is essential that everybody understands this and the need to change their behaviour."

Sources of contamination
1 Point source 
- Washed into drains from concrete yards 
- Mud from vehicles falling onto road/yard
- Applicator cleaning 
- Spills in yards/field
- Washing contaminated overalls

2 Direct application
- Spreading pellets into surface water 
- Spreading pellets into ditches

3 Diffuse
- Leaching through soil into field drains after heavy rain
- Surface run-off during heavy rain

Best practice advice
- Slug pellets are pesticides – treat them as such 
- Do not apply within 5m of a water course or ditch 
- Treat headland last to avoid carrying mud and pellets out of field 
- Calibrate application rate before use 
- Calibrate spread distance to side and rear 
- Fill and clean applicator in field away from ditch and road – not in yard 
- Do not apply pellets if heavy rain forecast 
- Do not apply if leaching and/or run-off likely, especially on heavy clay soils. 
- Minimise rates of ai/ha to give economic control