Research into grey field slug aims to reduce quantity of pesticides

Posted 4 June 2020

A research programme at Harper Adams University is investigating the movement and aggregation of the grey field slug, in the hope that the quantity of pesticides used to control it can be reduced.

Grey field slugs (Deroceras reticulatum) are a widespread and common pest among field crops, causing an enormous amount of damage. The AHDB has been funding work investigating a more efficient, sustainable way to tackle slugs in commercial crops.

Professor Keith Walters, who initiated and has led the programme since it started said: “Currently, thresholds are used to determine when a pesticide application to tackle slugs is needed. Thresholds relate to the number of slugs caught in surface refuge traps in a field and is meant to determine if it’s economically beneficial to apply pesticide or not.

“A problem with this method is that a variable proportion of the slug population is below the soil surface and the traps only monitor slug activity on the surface. For example, after a period of dry weather, more slugs will be below the surface than following a wet period, and this affects trap catches. This, in turn, is a problem when making accurate decisions on how to optimise pesticide use to prevent damage to the crops while protecting the environment.

"In addition, pesticides are applied to the whole field, but we know that slugs are unevenly distributed, with patches of higher numbers dispersed within areas of lower numbers.”

The research programme at Harper Adams has been underway for five years, throughout which Dr Emily Forbes has been at the core of the team, initially as a PhD student and then a staff member. Biological research has been combined with theoretical mathematical work through a collaboration with the Mathematics Departments at the Universities of Birmingham and Leicester. This multi-disciplinary approach has enabled rapid progress to be made to understanding the system they are working with.

The team questioned whether it would be possible to target pesticide application at the areas of a field where the higher population density of slugs occurs; the patches which were anecdotally talked about by farmers, leaving other areas untreated.

Keith said: “We initially showed that farmers are, of course, correct, patch treatment can work but in most fields there were more (and smaller) patches than was originally thought, which all need to be found and controlled.

“Next, we needed to know if these patches are stable in time and space. If patches always occur in the same place then we can target them more easily and effectively, if they move around the field then the method won’t work. At first sight this was a simple question, but how do we know where the part of the population which is underground is located?”

The team turned to RFID technology (the chips used by vets to identify lost dogs and cats); a technique which had first been utilised to identify individual slugs in the 1990s. Back then the chips were bigger, so were used on larger slug species, often in mark and recapture experiments. The Harper Adams researchers had a novel approach; to surgically insert the tag into the slug, allowing them to follow individuals in the field for long periods (five plus weeks), even when below the soil surface.

Emily said: “From this study we can say slugs move relatively short distances. Over a five-week period, on average, the slugs only moved about one and half metres. They will have moved further away to forage, but they always came back to the same area.”

This was true for 80 per cent of the slugs tracked. The other 20 per cent disappeared and Emily believes this may promote genetic mixing. The localised movement of the 80 per cent, however, helps patch cohesion and partly explains their stability, at least across a crop cycle.

The researchers then aimed to see if the size and distribution of the patches are suitable to act on or if they would require the pesticide applicator to be switched on and off too frequently as it moved over the field. From results from many commercial fields, from major crop growing regions of the UK, and in four different years, supported by detailed modelling work, the researchers believe that they are suitable.

The potential environmental and economic savings that can be accrued have been at the core of this programme. Based on empirical field data and modelling studies, the team have predicted, that even with safety margins to ensure the whole patch is treated, farmers could see an overall reduction of pesticide use for slug control of up to 40 to 50 per cent in many fields.

There is another big step before they can start testing protocols for the approach. Trapping and tracking slugs work well for research but are not viable methods for determining location of slug patches in practice. Automation is the key to economic viability of the approach and the team are looking into whether chemical or physical characteristics of the soil can inform a farmer where slugs are more likely to gather in their fields. They are also working with industry contacts to investigate potential synergies with soil analyses already taking place, for example, for fertiliser applications. Being able to amend or add to what is already conventional practice could lead to an increased value for money for the farmer.

Over the years, the team have worked with many farmers and farmers groups who’ve provided critical advice. As the team move towards a final output, they want to understand the range of concerns farmers have regarding current slug control approaches, and practical solutions. To provide thoughts and comments, please email Professor Keith Walters.