Last year’s weather was highly conducive to slugs. The wet and mild conditions saw the UK face a slug onslaught, and with a zero-tolerance principle widely adopted in the potato sector, growers are always predictably on high alert.
“At this early stage in the season, I would say that the risk from slugs is not quite as high as last year. But with the wet weather we’ve seen, particularly in East Anglia throughout February and the start of March, and the warmer than average winter temperatures, there is still a higher than average risk,” explains Mr Youngs.
“However, this could all change very quickly, as slugs are very responsive to changes in the weather, so any predictions can become outdated within a 10-day period.
“Different fields will also face varying degrees of risk, so with potatoes, you really need to plan ahead and carry out an individual risk assessment for each field.”
Field risk assessment
“As the season kicks off, it’s vital to build a complete picture of the field-by-field risk,” explains Mr Youngs.
“Firstly, you need to fully understand the field history as this can play a big part in the slug risk for the current crop. Where previous rotations include oilseed rape, or leafy vegetables, this dramatically increases the risk.
“Previous crop residues also play a role, as does whether manure has been used on the land, or if weeds are present on stubbles. All can act as a food source, maintaining populations of slugs over winter, which can prove extremely risky when it comes to planting new season crops,” he adds.
“Soil type is another key consideration, as it affects the ability of slugs to live and move within the soil profile. Generally, clay and silty soils will be more predisposed to slugs over mineral, sandy soils,” he adds.
“Varieties also make a big difference. Unfortunately, one of the main varieties grown in the UK is Maris Piper, which is extremely susceptible to slug damage, so this in itself adds another layer of risk.
“Finally, when assessing the field risk, the cultivation approach can make a big difference. Ploughing reduces the risk as opposed to non-inversion, as you are burying the trash and killing a larger population of over wintered slugs, by simply turning the soil over.
“The aim is to produce a firm, fine ridge of consolidated soil, as this makes it far harder for the slugs to move through the soil profile,” says Mr Youngs. “The added bonus is that it allows the tubers to get away well, as they have good soil contact.”
“Once the fields have been assessed, if I think I have a particularly high risk, I’ll advise trapping in the autumn, as this will showcase any slug activity on the surface,” says Mr Youngs.
"In this situation, if there is activity over the threshold, which for potatoes is one slug per trap, I would advise an autumn application of slug pellets to reduce the population going into the winter. This isn't the norm, but is warranted in extreme situations.
“Then trapping should commence again in the spring. Once there is enough moisture and the right ambient temperature to encourage activity, trapping provides a real-time picture of the pest levels you're dealing with,” he adds.
“It can also flag up what species of slug you're faced with, as this won’t be the same for every field. In potatoes, the damage done by slugs is all underground, so we’re particularly interested in keeled slugs and garden field slugs. This is because they are less surface active, and therefore more likely to feed underground, so pose a real threat for root crops.”
“Field assessments and trapping mean you're armed with all the knowledge to give you fair warning of the likely risk, and this can then influence treatment options and timings,” explains Mr Youngs.
“As it stands, we only have two slug pellet actives to choose from, ferric phosphate or metaldehyde, and they both work very differently.
“Potato crops are often at risk of surface run off, due to the nature of the ridges, and with 80 to 90% of the potatoes I look after being irrigated, the issues surrounding metaldehyde getting into water are a concern,” he adds.
“Other factors that affect product choice for potato crops, are treatment of field margins and harvest date. Sluxx HP (ferric phosphate) is my preferred pellet of choice for potatoes as it doesn’t carry the same limitations as metaldehyde on field boundaries, and there is a zero harvest interval, meaning you can treat right up to harvest, if required.
“This can be a ‘make or break’ factor in a crop’s success and prove invaluable in some high risk situations, where harvest is unavoidably delayed.
“Most growers I deal with have now moved over to ferric phosphate, and despite the different mode of action, growers have developed confidence in the product.
“In potatoes, because of the nature of the canopy, you don’t tend to see the slugs dying on the surface as you would in other arable crops, so the mode of action of Sluxx HP hasn’t been such a change of mindset for growers.
“There is a small window of opportunity to kill slugs in potato crops, as once they start feeding on the tubers, they are less inclined to come to the surface to eat the slug pellets. So, depending on the results of a field assessment and the trapping, the advice would be to get a treatment on, by around mid-July, once there is about 60% canopy,” he adds.
“The later you leave it, the more difficult it is to spread the pellets, and get them down onto the soil surface, where you see the most effective response. Follow on applications are then dependent on the season, and taken on a risk basis.
“Pest prediction is difficult at the best of times, but by assessing all of the known factors and understanding your treatment options and timings, it’s possible to stop slugs from eating into your profits.”