Poison baits are helpful in controlling slugs and snails, but they will kill all slugs and snails, including those you may find interesting or pretty. Baits are always only part of snail and slug control. You can use them to get rid of as much as about 90% of an infestation, but they are never completely effective. You will always need some additional means of pest management.
Metaldhyde, also known as Meta, is the most commonly used slug and snail bait. About 85% of the snail and slug products sold contain this chemical.
Metaldehyde is the same chemical that the human body generates during a hangover (although humans take another step and transform it into acetaldehyde). Metaldehyde makes slugs and snails “drunk,” incapable of purposeful movement. Just as too much alcohol might make a human throw up, metaldehyde can make a slug or snail generate slime. If the weather is warm and dry, and the animal is poisoned when it is outside its hiding places, it may die of dehydration by sliming itself to death.
Metaldehyde slug and snail baits are about 95% cereal and 5% metaldehyde. The cereal is intended to entice the gastropods to take the bait. The problem with most metaldehyde formulations is that the 5% metaldehyde that kills about 90% of slugs and snails is also toxic to children and pets. Meta baits that contain 2% or 4% metaldehyde may only kill about 50% of snails and slugs, but they are much safer for pets and people.
Metaldehyde is also available without the usual bait ingredients. If you have pets or small children, or if you welcome wildlife in your garden, you should use non-bait granules rather than food-laced bait pellets. Food baits are attractive to pests, but they are also attractive to pets. There are many cases of poisoning of cats and dogs by slug and snail bait. Metaldehyde is also toxic to aquatic life. If you have a pond in your garden, you must not use metaldehyde anywhere on near it. The product kills fish and aquatic insects. In the USA, it’s against the law to use it near a body of water.
For this reason, metaldehyde is available without the bait component. Liquid metaldehyde drenches are a highly effective way to control slugs and snails in potted plants. Metaldehyde sprays a convenient way to protect flowering plants, but they are too toxic to be used on vegetables.
Regardless of whether you use pellets or liquids, you should only use metaldehyde under appropriate weather conditions. Meta kills slugs and snails by inducing them to secrete slime. They generate so much slime that they eventually die of dehydration. More slugs and snails will be killed if you put out metaldehyde in hot, dry weather rather than in moist, cool weather. In dry weather, Meta will kill a slug in just a day. If it rains, however, the slug may survive.
A less toxic approach to killing slugs and snails is to feed them iron phosphate, which is also known as ferric phosphate. This product is available as granules you can sprinkle over the ground, and as bait balls you can use to attract slugs and snails. Iron is nontoxic to plants. In fact, it may even prevent leaf yellowing (chlorosis). High doses of iron can be toxic to pets and humans, however, so if you are concerned about pets and children, you may want to sprinkle iron phosphate granules on the slime trails slugs and snails use to travel from their hiding places to their feeding grounds. Iron baits can be used safely around vegetables, berries, and fruit trees.
Chelated iron, also known as iron EDTA, is less toxic than ferric phosphate.
Generally speaking, Meta baits and other forms of metaldehyde kill fewer slugs and snails, but kill them more quickly, just in two or three days. Iron baits and other forms of iron kill more slugs and snails, but take about two weeks to do it. Meta is more effective in mating season (late spring or early summer in the UK, anytime the weather is humid and warm in the US) because it kills the pests before they can mate. Iron is OK for other times of year, when you are just trying to control your existing pest population, not to prevent reproduction.
Slugs and snails tend to travel to the same locations again and again, navigating along their slime trail. They often like to hang out around sprinkler heads, which are a prime location for placing baits. Because slugs and snails travel to the same locations repeatedly, you will get better results if you put out baits in the same locations repeatedly. Be sure you sprinkle baits rather than piling them. Piling baits makes them more attractive to pets and children, but doesn’t deliver a bigger dose of the poison to slugs or snails. “Bullet” baits like Deadline hold up better in parts of your garden that are frequently irrigated.
Metaldehyde baits tend to kill slugs and snails where they eat them. You may find slug carcasses or broken shells around your garden when you use metaldehyde. Iron baits, on the other hand, only discourage feeding. Slugs and snails will tend to retreat to moist, cool hiding places and die there rather than in the open.
Slug and snail baits are almost never effective when they are used as the only method of pest control. Frequently an application of metaldehyde or iron phosphate will only kill about half the slugs and snails in the treatment area. Some slugs and snails will be well fed on your flowers and veggies and ignore the bait, or they will not eat enough to be killed, and others will have been buried in the ground at the time of treatment, like the Keel Slug. Use baits to save you some of the hard work of pest control, but don’t expect them to do the whole job. An excellent technique to use in conjunction with bait, is to use copper repellents such as mesh and tape, which does a fantastic job, especially with protecting flowers in pots.