A recent study by AHDB concluded that the failure to control slugs could cost the UK agriculture industry more than £100 million annually, with similar figures reported around the world.
While slugs are mostly controlled using chemical molluscicide products, some actives have come under scrutiny due to their detrimental environmental effects and impact on non-target organisms.
This has resulted in the ban of actives such as methiocarb in the UK and the EU, and, more recently, the phasing out of metaldehyde in the UK.
So there is an urgent need to find alternative and effective non-toxic solutions in the interest of global food security.
A recently published paper in the journal Insects (A Literature Review of Biological and Biorational Control Strategies for Slugs: Current Research and Future Prospects) reviewed research into biological (biocontrol) and biorational slug control.
Biocontrol promotes the control of pests using their natural enemies, whereas biorational control uses products that have been derived from natural sources, such as plant extracts.
The paper also looks ahead to future research avenues.
Essential oils are a promising and easily commercially-available natural solution.
They can kill slugs and boost plants’ defence by enhancing their repellent, irritation and antifeedant properties against slugs.
Previous studies in the context of other pests, such as mites, beetles, mosquitoes and cockroaches, show plants treated with essential oils are able to demonstrate repellent, irritation and antifeedant effects towards these pests.
So essential oils might be able to offer all-round plant protection from slugs, which cannot be achieved using natural enemies, says the report.
While the toxicity of some essential oils has been confirmed against some slugs, their repellent, irritant and antifeedant properties against slugs have been little studied.
Overall, the research conducted on natural products addresses only fundamental questions and has mostly been carried out in laboratory conditions.
Pest management Further in-depth and field studies are needed to be able to recommend inclusion of these natural products in integrated pest management systems for slugs.
There is a requirement in the short term for more detailed guidelines to be provided regarding the use of natural products in-field, says the report.
Scientists must answer the following questions: How should the natural products be applied in the fields, on the plants or the ground? what concentration should they be applied? How frequently do the natural products need to be applied for effective slug control? Lastly, scientific evidence on these biocontrol/biorational agents of slugs (nematodes, sciomyzid flies, carabid beetles and essential oils) reveals that, although they possess slug controlling properties, they have individual limitations.
On their own, biocontrol/ biorational agents are currently unlikely to be capable of fully controlling slugs.
This concern makes a strong case for examining whether they can be used in combination to control slug populations more effectively.
Prior studies have shown that the combination of two or more control techniques may result in a ‘synergistic effect’ on pest organisms, i.e.,the combined use of several control agents can result in better control of pests than the sum of the individual effects.
To date, no research has examined the presence of any synergistic or antagonistic effect between nematodes, carabid beetles, sciomyzid flies and essential oils.
With few researchers covering slug control and a lack of funding, developing new strategies can be an uphill struggle, says Aberdeen University researcher Dr Jenna Ross.
A co-author of the paper, Dr Ross says there is a ‘massive opportunity to look at bacteria, fungi and other parasites of slugs apart from nematodes’.
When developing biologicals, several questions need to be answered, says Dr Ross.
“Does it have good efficacy, causing mortality in slugs? Can it be cultured, grown and multiplied? Can it be formulated? Is it applied using traditional methods or new equipment? We need something that works and is cost-effective at the end of it.”
Limitation The biggest limitation to research into biologicals and biorationals for slug control is lack of funding, she says.
“More investment is needed consistently – not just when it is a wet year – it can’t just be ad hoc.”
Lack of researchers with an interest in slug control is also a problem, she adds.
“We need to get the next generation interested. Very few people in the UK are working on it.”
Getting more collaboration with researchers overseas is key, she says.
In 2019, Dr Ross undertook a Nuffield Scholarship looking at slug control.
During the scholarship she encouraged overseas researchers to look for ‘new’ nematodes from the phasmarhabditis genus.
“Previously only three were known.
In the 15 years I’ve been working on them 15 species are now known.”
While this represents progress there is no ‘silver bullet’ and a range of practices – such as rotation, forecasting, physical barriers and biologicals – need exploring, she says, with chemistry as a last resort.