Slug and Snail Bait
Have you ever had a problem with slugs or snails eating your garden plants? The majority of gardeners will answer yes. Have you used slug and snail bait to control the molluscs? Again the majority will answer yes. Slug and snail baits are effective in killing slugs and snails; they are generally granules of bait containing either metaldehyde, or iron EDTA, or iron phosphate to kill the molluscs. When baits are used gardeners are generally happy to find lots of dead and dying slugs and snails. However, bait needs to be used continuously because as soon as the bait is gone 'new' slugs and snails seem to appear quickly and plant damage occurs, perhaps, even worse than before!
So what is going on?
Methods that reduce slug and snail numbers suffer from a fundamental flaw in 'ignoring' their biology. Slugs and snails control their own population. They lay enormous quantities of eggs when hatched; most slugs and snails remain as relatively harmless juveniles. As slugs and snails move about, they leave behind trails of slime. Only when the density of slime trails falls will more juvenile slugs and snails develop to adults - which is exactly what happens as soon as a significant number of the creatures have been killed, so the numbers of large damaging gastropods are replenished rapidly and during growth, and development of the juveniles will cause more plant damage as they eat for growth. So the more slugs and snails you kill with slug and snail bait the more slug and snail bait you need to use!
Also, baits are only attractive to slugs and snails over a very short distance (c 5cm) i.e. baits must be spread and numerous so that slugs and snails will be likely to encounter them. And slugs and snails eating sub-lethal doses will become bait shy; stop feeding on the baits. Molluscs produce an alarm pheromone when distressed, e.g. when eating toxic bait. This will prevent other molluscs feeding in that local area, so there is no point having baits large enough to kill more than one mollusc.
What does this mean for slug and snail control?
Baits are a self-perpetuating control method; once you've started it is hard to stop. Prevention is better than cure so consider changing the conditions so that they are not suitable for slugs or snails and using repellents. Molluscs prefer damp conditions in and amongst leaf litter or dense vegetation so rake out leaf litter and keep clear open areas around susceptible plants.
Copper bands can be fitted around raised beds to keep slugs and snails away and crushed eggshells, diatomaceous earth, coffee grounds, wood ash and other natural substances have been shown to be repellent to slugs and snails.
Keep them out rather than kill them. Here are some ideas for controlling slugs and snails without trying to get rid of them.