Rain brings slug invasion

Wet weather during the key slug breeding season means growers should prepare for the autumn slug invasion. Farmers Weekly takes a look.

The wettest April to June period on record has resulted in slug populations being “off the scale”, prompting one expert to warn of a severe threat to crops this autumn.

David Glen, an independent consultant with Styloma Research & Consulting and former researcher at Long Ashton Research Station, says he has never experienced more widespread and intense slug activity in his 30-plus years investigating slug behaviour and control strategies.

“With slug populations off the scale, growers should be assessing the level of slug activity before harvest and in stubbles after harvest to determine the scale of the threat posed,” he says.

Most slug species have one breeding cycle a year with April to May being an important period when activity is at its highest. The grey field slug, the most threatening of the slug species to arable crops, has more than one generation a year and can breed at any time when conditions are suitable.

In favourable conditions each slug is capable of laying up to 500 eggs, so it is of little surprise that numbers are so visible. Slugs born in April will be laying their first clutch of eggs towards the end of summer so it is important to ensure timely and accurate control.

“The threshold for wheat crops is four slugs a trap in stubble. In stubble before oilseed rape, research has shown that just one slug a trap is enough to signal the need for action. Conventional practice has been to lay traps after cultivations but before emergence of the next crop, though this can give a false impression of slug numbers, according to Dr Glen.

“Research has shown that disturbing the soil profile disrupts slug behaviour to the extent that indications might suggest a low population burden when the opposite can be the reality.”

Cultivations are an important part of slug control though it will not remove the need for pellets in high risk situations. A single slug can kill up to 50 wheat seeds in the first week after drilling.

“Those following a direct drilling approach run a greater risk of sustaining slug damage. Cloddy seed-beds and wet weather around the time of drilling increase the damage risk whatever the method of cultivation. Ideally pellets should be applied just after the crop has been rolled,” he says.

“The potential for damage is so great that waiting for activity to become visible means it is too late,” he adds.

He advises caution when applying pellets with seed as it will reduce the opportunity for control. Broadcasting pellets to the soil surface is almost always preferable, he says.

Although important to apply pellets before signs of damage to the crop, growers should keep sufficient in reserve to allow for a second application if necessary while those using metaldehyde should not exceed the maximum single application of 210g/ha as prescribed by the Metaldehyde Stewardship Group (MSG).

It is important to use a high-quality pellet to ensure satisfactory durability and efficacy levels, says Simon McMunn of manufacturer De Sangosse.

“To be effective, slugs must ingest sufficient bait otherwise they can recover. Some pellets, such as TDS, contain an attractant and a feeding stimulant to ensure sufficient active is ingested to cause paralysis,” says Mr McMunn.

“Slugs are free-roaming creatures capable of moving up to 5m a night and able to detect food from 60cm away, so it is important to use a wet-process pellet with good ballistic properties to achieve an even spread across the soil surface,” he adds.

When to use methiocarb and ferric phosphate

The MSG advises growers not to apply metaldehyde within 6m of a watercourse to reduce the potential for pollution.

“Metaldehyde has quicker knockdown effects so will continue to be the basis of most programmes, but ferric phosphate and methiocarb are useful where water course contamination is a concern or ground beetles are active,” says independent agronomist Sean Sparling.

“Ferric phosphate is slower to take effect, but it is the only bait available that limits its affects to molluscs so can be used in the early season when ground roving beetles are active. Methiocarb is another useful alternative, but care should be taken to avoid applying pellets within 6m of a field boundary to protect beneficial insects such as carabid beetles which prey on slugs and aphids.

“I find metaldehyde best suited to early season use while other active ingredients can be utilised if metaldehyde limits are reached, watercourses are at risk, or ground beetles are active,” he says.

“It’s important that growers use metaldehyde sustainably if we are to preserve its future use by utilising the other forms of control when situations dictate,” he adds.

Stewardship under pressure

Producing consolidated seed-beds will be a challenge for many if the wet weather continues, says Paul Fogg of Makhteshim Agan UK.

“Soils are soaking wet and drains are flowing, so there are bound to be problems,” he says.

In this context, metaldehyde stewardship guidelines remain unchanged for 2012, he reports. “And one of the relevant factors with stewardship is that slug pellet applications should be delayed if drains are flowing.”

He believes this autumn will be the first true test of the stewardship guidelines. “We’ve had two or three low pressure years and, as a result, there’s been a declining trend in raw water exceedances. Complacency at this stage would be a mistake.”

Slug pellets are not all the same, says Dr Fogg. “The choice of pellet has to be part of the solution. There are differences in metaldehyde release rate, persistence and durability between different types. Wet process pellets should be used as a matter of course.”

Wessex Water – the way ahead?

Wessex Water has been compensating farmers for using ferric phosphate rather than metaldehyde for slug control, by refunding them the price difference.

In 2008, a large spike of metaldehyde caused the Durleigh reservoir – the main source of water supplies to Bridgwater – to be shut down. The reservoir then had to be drained and allowed to re-fill naturally.

Farmers in the catchment were contacted about the problem, with some of them voluntarily switching to ferric phosphate to try and alleviate the situation. In recognition of this action, and as a goodwill gesture, Wessex Water agreed to contribute financially to the farmers who switched to the higher cost method.

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