Beetles as a bio-control option for slug control in arable crops

Slug damage costs UK farming more than £40m per year, and growers have just one chemical tool in the armoury – ferric phosphate – to protect crops from the pest.

Finding innovative solutions to the problem is the focus of the SLIMERS (Strategies Leading to Improved Management and Enhanced Resilience Against Slugs) project, which will run for three years and involve more than 100 UK farms and six commercial and research partners.

One potential bio-control option for suppressing slug populations is to attract more natural enemies of slugs – such as carabid beetles – into arable fields.

“The hyenas of the beetle world”

According to Dr Kelly Jowett, an applied entomologist leading research into beetle monitoring at Rothamsted, carabids predate on a range of crop pests, and have even been found to consume up to 4,000 weed seeds/m² perday, helping to remove crop competition. 

In particular, Harpalus rufipes, Harpalus affinis, and Amara anenea can help reduce seed stocks of weed species by 65–90%.

They are also known to cause a dent in a number of pest species, such as the orange wheat blossom midge and cabbage root fly, the latter of which may be reduced by up to 90% while still in the egg and larvae phases.

“Carabids are the ‘hyenas’ of the beetle world – they’ll eat pretty much anything they come across; not just adult slugs but also eggs and baby slugs in the soil, so help with suppressing populations,” Dr Jowett explained.

What’s more, carabid larvae live in the soil and are more predatory than adults, as they require protein for growth – making them key players in slug control.

To boost beetle numbers in fields, Dr Jowett said farm environments should provide the resources they need to feed, breed and shelter.

“Carabids breed in the soil of the field, so things like low tillage are particularly important for supporting them, or tillage at certain times of the year.

“Beetles need areas to rest where there is not much disturbance, but they move into the crop for foraging and breeding,” she added.

“They also need refuge from pesticides; a lot of pesticides act by suppressing the insect’s ability to feed on crops and breed to ultimately suppress populations, therefore spraying can take out entire colonies.”

The effect of pesticides on beetle populations

Dr Jowett pointed out that pesticides are as harmful to beneficials as they are to pests, and providing refuge from chemical sprays will help minimise population losses and ensure beetles can recolonise fields.

“Unfortunately, pest species recover more quickly and can reproduce exponentially – meaning pesticide use must increase to ensure the same level of protection, causing further problems such as resistance.”

The harmful effect of spraying on beetle populations extends to all chemicals used on farms, even those not targeting pests such as herbicides and fungicides, Dr Jowett continued. The reason for this lies in the mode of action of biocides, which is similar across the different chemical compounds. 

Meanwhile, ferric phosphate – the only chemical means of slug control currently available to farmers – has been found to have one of the least direct mortality impact on beetles and least persistent in the environment.

While acknowledging that farmers rely on pesticides to maintain crop productivity, Dr Jowett noted a greater understanding of how to use these more specifically to minimise impact on beneficials would greatly improve the efficiency of these bio-control agents in suppressing pest populations. When it comes to spraying, her general advice to farmers who want to encourage carabid populations is to:

• Think about buffering resource areas
  • Use less
  • Use less toxic or persistent chemicals
  • Use more targeted programs.

The take-away message is that the more you reduce the slug population, the less frequently thresholds for pesticides are going to apply, so the less chemicals will need to be used – good for the farmer and the environment.

Carabid monitoring: The story so far

There are around 350 carabid species in the UK, 30 of which are common on farmland. All species have different attributes, ranging from around 4mm to 3cm in size. Some beetles fly; some crawl along the ground or under the surface; some are weed specialists; while others predate on slugs.

In her PhD research, conducted across the UK over several years, Dr Jowett found that contrary to the scientific literature, carabids increase in abundance towards the crop centre and are more sparsely distributed in field margins. 

When looking at the spillover effect from semi-natural habitats across a number of crops (wheat, barley, oats, OSR) and types of field margins at the 330ha Rothamsted Farm Site, Dr Jowett came to the following conclusions:

• Abundance and species richness differences evident by margin type
• Grass margins affect spillover zone – works as kind of a barrier
• Abundance in field centres was the same in all crops
• Wildflower margins clearly lower values in all crops

Essentially, she uncovered that there are differences between carabid species in terms of tolerance to cultivation practices and efficiency as slug predators. 

Moreover, field margins no longer seem as important to carabids as previously assumed, and farmers may need to think about their placement more to optimise conditions for populations to thrive. An example would be creating field margins between fields and urban areas that might have a disturbance impact, rather than putting them between fields, she explained.

Citizen science

Citizen science projects such as Slug Sleuths and Beetle Scouts were launched by BOFIN to encourage more farmers to get involved in monitoring and help advance science from the grassroots up. 

There are a limited number of spaces on each project, and participants are provided with all the equipment needed for capturing and monitoring slugs and beetles, along with detailed instructions and a guide to help farmers identify major species of interest.

More information about the project, including links to project partners and research initiatives that underpin the work can be found at www.slimers.co.uk