Environmentally friendly gardeners who attempt to deter slugs and snails from devouring their vegetables with eggshells or copper tape are wasting their time, according to a study by the Royal Horticultural Society.
Gastropods inflicted the same damage to lettuces protected with five natural methods – eggshells, copper tape, horticultural grit, pine bark mulch and wool pellets – as they did to lettuces left untreated.
These methods were tested on 108 lettuces sown in pots and raised beds at the RHS field research facility in Wisley. After six weeks, the leaves of each harvested lettuce were examined to calculate damage.
Lettuces planted in the ground were found to be more susceptible to slugs, with 5.7% of each eaten on average. This compares to just 0.2% of those in pots, which, despite this advantage, yielded less crop.
The natural methods were thought ineffective because slugs and snails’ thick mucus enables them to glide over these apparently sharp barriers.
But wool pellets and pine bark did have one clear benefit: by acting as a fertiliser and mulch, they helped lettuces yield a 50% bigger crop.
According to the RHS, some slug species have clear benefits for gardeners, such as the leopard slug, which eats fungi and rotting material, helping recycle garden nutrients.
But Dr Hayley Jones, entomologist at the RHS and lead researcher, said: “With the likes of eggshells, barks and mulch so far proving no discernible deterrent to slugs and snails, we would recommend using proven formulas like nematode biological control if the damage is just too much to bear.”
Nematodes are microscopic parasitic worms that feed upon molluscs but also risk killing non-pest garden species such as water snails.
Jones added: “Slug pellets do have their problems but they have been tested for safety and when used to their manufacturers’ instructions the risks are quite minimal. It becomes dangerous for pets and other wildlife when they are overapplied.”
Previous research revealed that gardeners who handpick slugs and snails from their vegetables must dispatch them more than 20 metres away, because otherwise a homing instinct can help them return. Jones said of this method: “It is time-consuming but it is definitely the most environmentally friendly way to go about it.”
One famous method not yet tested is beer traps. Jones said she would like to test this method because there is a risk such traps only lure more slugs on to gardeners’ vegetable patches.