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Slug control 4: Sustainable slug control

Course: Slug Control | Last Updates: 30th August 2017

Dr. David Glen
Biography >>

Slug pellets deliver exceptional levels of control when used appropriately, but where their performance falls short of expectations the cause is often a combination of inappropriate product choice, poor application and the pressure of maximising farm profitability at the expense of best practice.

Modern crop rotations make slug control a challenge. The increased frequency of oilseed rape, failure to tackle high volunteer populations between crops or address cloddy or loose seed-beds, and the adoption of non-inversion cultivation techniques have led to an increasing reliance on chemical control.

Active substance

Of the three actives used to control slugs – metaldehyde, methiocarb and ferric phosphate – it is the former that is recognised as delivering selective control at the most acceptable price. The issue with metaldehyde, however, is the EU Water Framework Directive which limits its concentration in drinking water to 0.1 part per billion (ppb).

To counter the threat of tighter regulation, the industry set up a body – the Metaldehyde Stewardship Group (MSG)- which advises applications do not exceed 210g of the active ingredient/ha between 31 August and 31 December and that it is not applied within 6m of a watercourse. There is a regulatory limit of 700g of the active/ha in a calendar year.

As a consequence of these restrictions, there may be instances where growers choose to supplement control through the use of a different active substance such as methiocarb or ferric phosphate.

Another crucial is the quality of the pellet itself used to deliver the active ingredient.

Manufacturing process

The manufacturing process employed is important because it largely determines two of the three attributes by which pellet quality is defined – persistence and spreadibility.

Persistence is the length of time the pellet will last in the environment before breaking down while spreadibility is the consistency and evenness of pellet spread behind the applicator.

Broadly speaking, the manufacturing process falls into two categories – dry and wet. There are, however, variations on this. Hybrid technology pellets incorporate elements of both processes while Technology De Sangosse (TDS) represents an advanced wet process formulation that produces outstanding scores for persistence and spreadibility.

Because conditions change and pellets need to persist until the plant is safely established it is always advisable to go for the pellet offering the longest protection. Wet process pellets of about 2.5mm fulfil this role because they spread evenly, they are readily found and eaten by slugs and they persist well in field conditions.

Quality of ingredients

The quality of the ingredients determines pellet palatability. Slugs are discriminate feeders and poor-quality pellets tend to have low palatability scores. These pellets can be a false economy as the slug will move on to find a more appetising meal.

Slugs that have taken in only a sub-lethal dose from a poor-quality pellet are likely to be bait-shy from then onwards and even more difficult to control. When you open a bag of slug pellets, if you find yourself wincing at the smell there is a good chance that you won’t be the only one to be put off.

Pellet size

A great deal has been made about the role of baiting points in achieving effective control, but higher baiting points come at a cost. While it might seem advantageous to have more pellets per square metre it means an increase in pellet surface area and a reduction in individual pellet mass. This does nothing to promote good environmental persistence or spreadibility scores.

Smaller pellets are also more likely to disappear down cracks in seed-beds, where they can be more difficult for slugs to find. Research has shown that effective control can be achieved with as few as 30 pellets/sq m.

The lesson of the very high slug pressure year of 2012 is clear: timely application just before the crop becomes vulnerable and pellet quality are the determining factors in achieving effective control, not the distance by which the slug has to travel to locate the bait.

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