Too many farmers try to trap slugs at the wrong time and lose faith in this important approach to gauging the true need to apply slug pellets.
But with the pressure on the industry to cut metaldehyde residues in water supplies this autumn, or face severe restrictions on product use, the need to give trapping a fresh try is greater than ever.
A quarter of growers never use trapping, according to research conducted for the Metaldehyde Stewardship Group earlier this year. But 95% use visual assessment as a key factor when deciding whether to pellet. And when it comes to follow-up treatment, trapping’s role dwindles even further, with 43% never trapping or baiting to assess the need for such treatment.
“Visual assessment is valuable – if there are large numbers of slugs in a previous crop or if lots of slugs are active on the stubble, trapping is not needed to justify treatment,” says slug specialist David Glen of Styloma Research and Consulting.
“But if no slugs have been seen, you’ve got to get out in the field and trap slugs to justify treatment. This is especially so in fields where garden slugs and keeled slugs are known to occur.” Having spent 25 years researching and advising on slug control, he is well placed to comment.
The big mistake many growers make is setting traps at the wrong time. Trapping when soil conditions are too dry, or after cultivations when slug behaviour has been disrupted, gives artificially low results. That could explain why some growers feel trapping is less useful than it should be, he suggests.
“The ideal time to trap is when the soil is visibly moist. So, if that is when there is still a standing crop in the field, set the traps then, before harvest. It’s no good putting traps out in the dry and it won’t give an accurate picture if you try to trap straight after cultivations because they suppress slug movement.”
The value of trapping has been well demonstrated in trials over a number of years, including HGCA-funded trials. They show a link between the number of slugs trapped and the proportion of seedlings damaged, in both oilseed rape and wheat.
But with slug numbers potentially rising six-fold in August and early September the art is to trap when conditions are suitable and as close to drilling as possible. Where oilseed rape is being broadcast into a standing crop or cereal stubble that can mean trapping in the standing crop up to 10 days before harvest.
Traps must be baited with non-toxic bait, such as chicken layers mash, and left overnight. “Never use slug pellets as this is a risk to wildlife and pets and is illegal,” advises Dr Glen. Product labels specifically prohibit the use of pellets in traps and surface run-off or leaching could make such use of pellets a significant point source of contamination.
Dr Glen frowns on test pelleting too. “Some people like the idea of getting the pelleter out and doing a single pass to see what level of slug kill they achieve. But what is the threshold for treatment – no work has been done to assess that. Using pellets in this way is not environmentally friendly and is not recommended.”
When checking traps it is important to pinpoint which slugs are present. Grey field slugs (Deroceras reticulatum) do the most damage. They are usually quite small, breed throughout the year and are most active on the soil surface, so they inflict heavy losses on germinating and newly emerged crops. Each slug can kill up to 50 seeds in the first week after sowing.
With slugs looking to graze on the surface of the soil, pellets broadcast on the soil surface are encountered more readily than drilled pellets. Field studies show D reticulatum travels relatively shorter distances to begin feeding on broadcast pellets than drilled pellets.
Large, nasty-looking black slugs, often found on field margins, are not commercially damaging. The smaller garden slugs and keeled slugs, which are both juveniles in late summer, are less active on the soil surface, but they can cause severe damage to wheat seeds. Their presence in traps should be carefully noted.
Traps should be checked early in the morning. In fields destined for winter wheat, a catch of four or more slugs/trap indicates a possible risk if soil and weather conditions favour slug activity. For oilseed rape, the threshold is lower, at four slugs/trap if the traps are in preceding standing cereals and one slug/trap in cereal stubble.
If thresholds are not met, it is likely that cultural controls can keep slugs at bay. Avoiding cloddy seedbeds is crucial, as is rolling after drilling. “A well-timed roll is as effective as a dose of slug pellets,” Mr Glen insists.
But rolling must be thorough. Having a fine, level seedbed in the first place, to avoid localised patches of under-consolidation is important. A second pass with the rolls across the direction of drilling can ensure good consolidation across the entire field.
Rolling close behind the drill is key. And if trap thresholds have been met and conditions are conducive to slugs, broadcasting from a pelleter mounted on the roll – the pellets sit proud of the consolidated soil surface – can be ideal, he advises. “Not only is it putting the pellets where they need to be, at the right time, but it saves another operation and ensures a narrow spreading width, which reduces the risk of pellets getting into field margins.”
Shallow cultivation to incorporate crop residues reduces slug numbers and ploughing considerably reduces slug populations and the risk of damage, so it may be worthwhile in areas where severe slug pressure is anticipated, provided that a fine seedbed can be prepared after ploughing. In fine seedbeds, sowing at a depth of 3cm is sufficient to keep slugs away from seed, but in cloddier conditions move to 4-5cm.
If BYDV is a risk to the crop then using insecticidal seed treatment Deter can bring further benefits for slug control as the active ingredient suppresses slug attack.
If wet weather follows, drill trap again, Mr Glen says. “Crops should be monitored throughout early susceptible growth, especially up to first tillering (GS21) in wheat and the four-true-leaf stage in oilseed rape.”
- Trap pre-cultivations, when soil moist
- Use 20ml/2 heaped teaspoons of chicken layers mash under a 25cm cover
- Illegal to use slug pellets
- Nine traps/field (13 if 20ha+)
- W pattern + target vulnerable areas
- Thresholds: four slugs/trap for wheat; four/trap for broadcasting OSR into standing cereal; one/trap for OSR established into cereal stubble