The latest work builds on an AHDB-funded project led by Prof Keith Walters and Dr Emily Forbes at Harper Adams University, which found that grey field
slugs (deroceras reticulatum) occur in relatively stable patches within fields.
The aim is two-fold, explains Prof Walters.
“We want to develop a targeted molluscicide approach which maintains control while using less product; and we need to do this with farmers, consultants and other industry players to ensure commercial reality.”
The work addresses six key questions:
- Are slug patches stable in time and space?
Single hectare sampling grids, using surface refuge traps were deployed in each field to identify patches accurately.
“Overall, we have worked in a total of 49 crops, not counting those we are about to set up this autumn,” says Dr Forbes.
“A consistent picture has emerged over four seasons and in all regions. Patches are detected during favourable weather, but not during adverse conditions when slugs retreat into the soil.
“When favourable conditions return and slugs return to the surface the patches are again detected – and importantly they reappear in the same places.”
- Are slug patches large enough to target?
The work shows that although the size of patches varies, their scale and frequency allows them to be pinpointed and targeted by an automated system.
“Most farmers are aware of areas that are subject to increased damage by slugs and many already treat them preferentially,” says Prof Walters.
“But an important finding is that smaller patches which together can cause significant economic damage also occur but are often missed.”
- Can trapping grids be used for patch detection?
The simple answer is ‘no’, says Prof Walters.
“Scaling up the experimental system would be too expensive, so an alternative way of locating patches is needed.”
- Why are slug patches stable?
This question was addressed by using slugs implanted with radio frequency identification tags.
“The tags don’t affect their movement, survival or egg-laying,” stresses Dr Forbes.
Each tag has a unique identifying code which can be detected even below the soil surface, allowing individual slugs to be tracked.
After five weeks slugs were, on average, only 0.79m from their release point in spring and 1.02m in autumn.
“So, when forage is not limiting, slugs remain in highly localised areas for extended periods leading to patch cohesion.
“To be confident that patch cohesion is a feature of populations across the UK we still needed to know the precise behavioural mechanism underpinning this.”
Modelling slug movement in high and low densities with mathematicians at Leicester and Birmingham Universities has now confirmed the behavioural traits involved, says Prof Walters.
- Is patch treatment likely to be commercially viable?
How patch treatment works in conjunction with the current action threshold-based approach has been investigated in a theoretical exercise showing the two can be integrated, says Prof Walters.
“Other work has shown that savings of up to 50% in pesticide use can result from patch treatment while maintaining populations to below AHDB threshold levels.
“That makes the approach viable provided a cost-effective method of identifying patches can be ground-truthed.”
- How can patches be located with sufficient precision to ensure the overall approach is viable?
“The literature suggests factors such as moisture levels, soil organic matter, and others affect patch location,” says Dr Forbes.
“But when we tested them, no one alone could predict where patches would occur.
“Instead, our current research is investigating soil chemical and physical characteristics to find a combination that may predict them.
“To reduce potential commercial costs we are focusing on characteristics already assessed on farms for other agronomic proposes.”
Looking back, Prof Walters says the key but somewhat unexpected project finding has been that slugs tend to move within a very limited ‘home range’.
“The real ‘eureka moment’ was the subsequent finding that behavioural responses of individuals to others of the same species may cause this, as it gives us confidence that patch stability will occur in all commercial fields.
“Another surprise has been the interest shown in the research by scientists around the world.
“For example, work is planned in the USA to test our findings under their very different environmental conditions and cropping systems. AHDB funding has allowed the UK to be ahead of the game in this case.”