Work by scientists at Newcastle University, has looked at the impact slugs have on grassland biodiversity and how this might inform future restoration work.

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Although they are a major agricultural and horticultural pest, still relatively little is known about the effects slugs have on large scale grassland conservation projects. For the first time, researchers based in Newcastle revealed that some of the most ecologically important species present on grassland – such as the red clover – are threatened by slugs. The molluscs may be responsible for reducing the number of seedlings becoming established, affecting overall plant diversity.

The research team shed light on the impact of Deroceras reticulatum – otherwise known as the field slug – which, they said, may require management in order to help seedlings establish and contribute towards successful restoration.

However, a great deal of contemporaneous concern has been raised around the effects that metaldehyde slug pellets are having on the wider environment, meaning methods of slug control would have to be given serious consideration. Having been detected at record levels, exceeding EU thresholds by several times, in the wake of last year's washout summer, metaldehyde has again been discovered in the River Stour this year, this time in quantities up to 100 times the EU drinking water limit.   

The wet summer of 2012 led to an explosion of the slug population, but metaldehyde began showing up at worryingly high concentrations in drinking water shortly afterwards. This led green groups to state that the current voluntary approach to stewardship was clearly not working. Although industry groups pointed out that the concentrations being discovered do not threaten human health, concerns persist as the chemical cannot be removed from drinking water by current means.  

However, integrated pest management practices, including supporting populations of frogs, toads and birds that feed on slugs, could keep numbers down effectively, whilst providing other benefits for the ecosystem.

Dr Sarah Barlow, now based at the Royal Botanic Gardens at Kew, carried out the research on slugs and grassland while at Newcastle University. She said, "Herbivory is a fundamental driver of plant diversity. We know a lot about the benefits – and drawbacks – of grazing by larger vertebrates such as cows and sheep but we haven't studied in detail the impact slugs might have, particularly on very young plants in meadows that we are trying to restore.

"What these initial studies have shown is that [slugs] may actually be very damaging in this situation due to it finding some key species particularly tasty.  One of these, red clover, is particularly important due to the symbiotic bacteria which live in its roots and help to fix Nitrogen into the soil, benefitting not just the clover but all the meadow seedlings."

Only around 5 percent of the slug population is actually above ground at any one time. Slugs prefer to feed above ground and are particularly fond of seedlings, which is why their impact on conservation projects can be so great.

Dr Barlow's team looked at slug feeding damage to seedling monocultures of 23 meadow species. The severity and rate of damage suffered by each plant species was analysed and each species ranked for their vulnerability to slug attack in meadows.

Drawing up a 'hierarchy of acceptability', the team found the slugs' favourite five were: yarrow; Yorkshire fog; rough-stalked meadow grass, creeping red fescue and red clover. On the bright side, said Dr Barlow, the slugs did not like the seedlings of some of the desirable wildflowers such as wood cranesbill, rough hawkbit and greater burnet.

Newcastle University's Dr Gordon Port, a senior lecturer and an expert in pest management commented, "Just like us, slugs have their favourites and will eat some plants in preference to others. We have to cope with native slugs in our gardens and crops, but this research shows they can be equally damaging in natural systems. If invasive species, like the Spanish slug establish in the UK it will move the battle against slugs to a much higher level."