Good slug? Bad slug?

Slugs (and snails) are usually lumped together in one breath as “pests” and “gardeners’ enemies”. But are all slugs and snails really pesky pests? No.

A yellow slug (Limax flavus) on the lid of a compost bin - these slugs are common on compost and actually help with the decomposition process. A "good slug".
A yellow slug (Limax flavus) on the lid of a compost bin – these slugs are common around compost and actually help with the decomposition process. A “good slug”.

In fact, many of the most conspicuous slugs – the big slimey yellow ones and the big shiney black ones, for example – spend most of their lives cleaning up dead and decaying things.

At least one slug species in the UK actually eats the eggs of other slugs, and will even eat live slugs too – especially the naughty little ones that actually do most of the harm.

A netted slug on a strawberry. I suppose this is a "bad slug".
A netted slug on a strawberry. I suppose this is a “bad slug” because it does eat your fruit, veg and ornamental plants. It also tends to get in between the leaves of lettuces and cabbages giving a high “yuk” factor:)

The market for slug pellets is a big one and manufacturers are happy for us to consider all slugs as equally bad. And at the sight of a slug we’re encouraged to buy a tub of the little blue pellets and sprinkle them all around. Ironically, this means we kill the good guys as well as the bad. Possibly we kill even more of the good guys than the bad guys, because some of the worst culprits stay in the soil and have no interest in the slug pellets.

Domestic slug pellets usually contain metaldehyde as the active ingredient. This is mixed with meal and “flavoured” to actually attract slugs and snails. Surprisingly, given how many slug pellets are sold, the way the metaldehyde actually kills the slug is not fully understood. It used to be said it works through dehydrating the slug – which it does; but it also acts on the slugs’ nervous systems, paralysing them and helping to ensure they have a slow, lingering death.

There is also little robust information about the effects of the pellets on other animals that eat them (or the poisoned slugs) by accident. There are reports of cats being poisoned by metaldehyde slug pellets. Some also claim that the slugs and snails quickly learn to recognise that the pellets are bad news and they start avoiding them. This is particularly the case because people tend to use too many pellets and end up with a stinky pile of half dead slugs and snails which are off-putting to slugs and snails as well as people.

All in all, the utility of metaldehyde slug pellets is questionable. If you do use them, use them very sparingly; put them under something; and clear away the dead and dying slugs and snails as soon as possible. 

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3 thoughts on “Good slug? Bad slug?”

  1. Don’t forget that birds will eat slugs that have been poisoned and while it’s suggested that they need to eat a lot to sustain harm it can’t be good for them. Don’t forget that birds feed slugs to baby birds. If a food chain element is poisoned it’s hard to know the impact. Little blue pellets for the sake of our strawberries.

  2. I’m currently using manual picking as my slug control but I’m finding it hard to find information on the identity of carnivore slugs.
    Can anyone help

    1. I don’t know of any freely available information specifically about the carnivorous species, but the BBSRC slug watch guide is helpful and the Bedfordshire, Cambs & Northamptonshire Wildlife Trust have a very useful (and free) slug identification key (wildlifebcn.org) and lots of other sluggy information.

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