As featured in Arable Farming Magazine
Future solutions for slug control
by Arable Farming
Marianne Curtis reports on the latest research on how growers could use biological and biorational products to control slugs.
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.
Sciomyzid flies and carabid beetles
Although the slug control potential of sciomyzid flies and carabid beetles has been known for decades, there still exist a number of crucial research gaps.
For instance, research examining the slug control efficiency of tetanocera elata (field buff snailkiller), which is the most studied sciomyzid species concerning slugs, has been performed mainly in laboratory conditions.
More field-based observations are needed to see if T.elata can be an effective addition to practical biocontrol regimes for slugs.
As for nematodes, the authors of the paper also suggest that it is important to look beyond T.elata and examine the biocontrol potential of other sciomyzid species and their habitat preferences.
Although the predatory potential of carabid beetles has been mostly assessed in the context of adult feeding behaviour, the larval stages are also significant predators of slugs.
A few studies have reported that carabid beetles also predate on slug eggs.
Despite a significant amount of scientific interest in identifying effective biological measures for controlling slugs, there are only two bio-molluscicides commercially available in the form of nematode products.
Only phasmarhabditis hermaphrodita has so far been formulated as a commercial biocontrol agent with the trade names Nemaslug (BASF) and Slugtech (Dudutech).
Further research is needed to establish whether other nematode species which possess slug control potential can be developed into commercial biocontrol agents.
Studies have cast doubts over the pathogenicity of the strain used in Nemaslug as the same strain has been cultured over a long period of time, highlighting the need for the identification of new strains of P.hermaphrodita.
Little research has been carried out to identify novel (or wild) strains of P.hermaphrodita and to test their potential to control slugs, says the report.
Pathogenicity The pathogenicity of nine wild strains of P.hermaphrodita found in the UK against the slug deroceras invadens (Chestnut slug) was recently compared against that of the commercial strain of Nemaslug.
The study found the wild strains were causing rapid mortality and feeding inhibition in D.invadens compared to the commercial strain.
So the isolation of new strains from different regions provides an avenue for further research, the report suggests.