Iron phosphate:
The first honestly effective snail & slug bait
"Speak not of grief, young stranger, or cold snails
Will slime the rose to night."
-John Keats
(1795-1821)
(1795-1821)
Slug & snail baits with Metaldehyde (such as Ortho) are not very effective. They are also sufficiently toxic that such baits are not recommended for use around edible vegetables, & can be harmful to dogs, cats, & fish.
Iron phosphate is more effective than Metaldehyde because Metaldehyde ceases to be functional when it gets rained on or if you water the garden, whereas Iron phosphate remains active even with repeat wettings, easily up to two weeks.
Iron phosphate is completely safe for pets or beneficial insects, & is safe to use around vegetables. Sluggo & Escar-Go brands are two named brands. It's not as cheap as Metaldehyde, but then it's not money wasted as it is with other slug baits, & it's a garden nutrient rather than a toxic chemical, so worth a little more. Because Iron phosphate remains active longer, it requires less to kill more slugs, so in fact it is more cost-effective.
Because Metaldehyde is such a crappy slug bait that doesn't really work, some products like Ortho Plus include carbaryl to increase its toxicity. Carbaryl kills beneficial insects & thereby destroys the natural balance of the garden.
Snails frequently recover from Metaldehyde poisoning if there is rainfall or access to wet locations where they will not fully dehydrate & die. A slug can lose half its body weight & shrink to a third its size from Metaldehyde poisoning or by covering it in salt, but if it can get itself to wet soil fast enough, or if it rains, it will recover. By contrast they never recover from Iron phosphate.
Metaldehyde is known to be fatally poisonous to dogs & wildlife even in the weak (4%) mixes sold as slug poison. Plus it is sufficiently toxic to humans that if it gets on vegetables, it would be illegal to sell them.
By contrast, Iron phosphate is non-toxic to dogs or wild animals, & can be used legally even around commercial crops. EPA's Office of Pesticide Program states that iron phosphate "has no unreasonable adverse effects to human health [nor] toxicity to birds, fish, insects." If a garter snake ate a slimy slug that was dying from injesting Iron phosphate, the garter snake would not be harmed, but if the same garter snake, or a dog, ate a dehydrating slug that had been exposed to Metaldehyde, it could be bad news for the health of an unhappy diner.
Metaldehyde products after being dampened no longer work, but they do mold & invite plant pathogens. Iron phosphate not only remains active after damped, but any that remains uneaten by slugs just breaks down into iron & phosphate which the plants take in like any other fertilizer.
The baits work differently. Metaldehyde poison dehydrates the snail or slug rather rapidly if it eats the poison on a warm day (though of course snails & slugs aren't usually about on warm dry days which is one reason it has limited effect; on a chill or a wet day slugs will not dehydrate, but will often recover from the toxin). Iron phosphate on the other hand induces extreme mucus production in molluscs, so that the snail or slug is no longer able to eat while more slowly its cells break down; it works at any temperature & the slugs will not recover due to rainfall.
In summation, you've choices. You can go with a Metaldehyde bait poisonous to cats, dogs, wild animals, people, that cannot be used around vegetables, & which doesn't work if the weather is cold or wet, which may need reapplication as often as daily to have noticeable effect, & with only small effect even under ideal conditions. Or you can have a non-toxic bait that is a plant nutrient harmless to animals & people, safe on vegetables, with longlasting effectivity unaffected by temperature or wetness -- i.e., any Iron phosphate product.
No other method works as well. Beer traps do attract slugs but don't reliably kill them unless the trap is deep enough slugs can't reach over the top to get out (so saucers don't work, slugs climb right off the saucers, but Yoplait yogurt plastic cups are just barely too deep for a slug to climb out of, so it drowns in the beer). Such a trap would also endanger frogs & small lizards & beneficial insects that per chance fell in, without really resulting in a dramatic lessening of the slug population.
The alleged method of slug control using spent coffeegrounds amounts to gardening folklore & doesn't work at all. See the article on the folklore vs the reality of Coffegrounds & Slugs.
Diatomaceous Earth, when assessed in controlled studies, killed so few slugs that it has to be judged as worthless, though it does kill beneficial insects. Because Diatomaceous Earth, which is almost pure silaca, is "organic," many organic gardeners try it, but it is the perfect example of why "organic" is not a magical synonym for "harmless." It is pretty much the same as dumping ground glass all over your garden.
For cold-frames or raised beds, it is possible to attach copper flashing to the frames. Slugs do not like to cross copper because it causes them to experience a minute electrical discharge (or such is the prevailing theory). It works only if the copper strip is wide enough they can't raise their bodies over it. The majority of copper stripping sold in garden shops for this purpose is not wide enough to create an effective barrier, which would need to be six inches wide, or the largest most destructive slugs will hump right over it without touching the copper.
Copper-barriering an entire garden is not going to be practical, but it is an excellent method for protecting very sensitive seedling beds or small containers. Copper screen or copper flashing can surround a raised bed, or for small planters, copper foil (such as Snail-Barr) can be used to wrap the entire container. Shrubs can have a detached band of copper around the lower trunk.
Copper needs to be cleaned periodically with vinegar or will tarnish & no longer have the desired effect. Personally I cannot imagine going about making rings of copper for all the shrubs or encircling gardens with copper mesh, risking my hands or the feet of animals that can be cut on the edges of copper, then remembering to periodically polish the copper scattered about the garden. But at least this is a system that can work, unlike so many folklorish methods & worthless toxins that desperate gardeners try.
The only other method that can honestly be argued to have merit is to go out at night with a flashlight & gather slugs by hand to drop in soapy water in a container they can't climb out of. If you gather up all the adults before they reproduce, things will get dramatically better slug-wise, the tiny ones you miss don't do the most damage.
The only thing beneficial that would be harmed by Iron phosphate would be predacious snail-eating decollate snails. If a garden does have a perfect balance of snail-preditors (frogs, toads, garter snakes, ground beetles, ducks, & snail-eating snails) the balance might be thrown off by killing the predatory snails. It's the rarest of gardens where this kind of balance occurs; these snails are semi-tropical & don't thrive in temperate or cold gardens; & in some places where they would thrive, they are banned as potentially invasive species. But they are physically attractive snails, & if they do the work effectively for some gardeners, they should be encouraged to keep doing so, avoiding baits of any kind.
I don't want to be a shill for Sluggo but it's what I use, & it works so well that when I've killed off the adult slug population early in the year before they lay their eggs, I'm close to slug-free & snail-free the rest of the year without further applications. No bait other than Iron phosphate works alone as all others require one or more additonal methods to have any impact on a slug & snail population. My suspicion is that all products with ingredients other than Iron phosphate will over time vanish from the marketplace as people figure out Iron phoshate is actually effective, without sharing the harmful potential of less effective slug baits.
The best time for long-term control is to treat the whole garden in the dampness of autumn. That way, come spring, there will be very few adult slugs to lay their eggs. Another treatment might be useful in late winter or early spring. I've found that twice a year does the trick. And of all the things I ever tried previously, none of them achieved the level of effectiveness I now take for granted.