I've tried beer traps. They work great, but you must dump out the drowned slugs/snails every day and replenish the beer. I've tried copper borders. These work great, zapping the slugs/snails so they don't want to cross the copper - but they are only practical if you have raised beds or containers...and even then, only work so long as no leaves cross them and no dirt gets on them. I always hand pick and crush snails (sometimes putting boards down for them to hide under, so I know exactly where to find them), and I feed slugs to my chickens. (My current flock won't eat snails, whereas my last flock loved them.) But still, there are always more, more, more slimy creatures who think my garden is a smorgasbord.
So usually, I sprinkle Sluggo everywhere. This definitely works, and (unlike most similar products) it's safe for non-slimey critters. The trouble is, I need it most during our rainy springs - and it has to be re-applied after every rain. Which becomes expensive. Plus, when do slugs and snails love to come out? When it's raining! Some years, the rain has been so persistent, I've had to totally replant my vegetable garden because slugs/snails have completely destroyed my original crop.
So eggshells seem like a perfect answer. They are readily available - totally self-sustainable, since we have backyard chickens. And they don't become less effective due to rain. Theoretically, I should only have to apply them once - maybe twice - in the growing season, because they break down quite slowly. (Which is an added bonus: They feed nutrients to the soil, helping to fertilize next year's crop.)
But the question is: Do they really work? That's what I set to find out.
How to Use Eggshells to Deter Slugs and Snails
1. As you use eggs, hang on to the shells. I put mine in a plastic shopping bag that hung from a hook in my kitchen. I didn't bother to rinse the egg shells; I just plopped them into the bag after cracking the eggs. Odor wasn't a problem.
3. A little at a time, I put the eggs in my food processor and pulsed them. (I tried the coffee grinder first, since that's what I'd seen done on Pinterest. It didn't work at all. You might be able to use a blender, though I've not tried it. Either way, I think a food processor or blender is better, since they are easy to sanitize. If you could find a coffee grinder that works on eggshells, I'd recommend dedicating it just for that purpose, since coffee grinders are difficult to clean thoroughly.)
4. Finally, using a tablespoon, I liberally sprinkled the ground eggshells around my spring seedlings. The eggshells work because they hurt the slug/snail to cross, so don't be stingy with them, and make sure you get them all the way around your plants.
5. Then I waited.
The Good News:
It rained lightly. I watered several times. And slugs and snails did not eat my seedlings! And I sure love the cost of this organic pest control.
The Bad News:
When I watered, the eggshells did jump around a little, and some got covered by soil. I imagine a hard rain would knock them around more. So I will have to reapply more often than I initially thought.
Also, grinding the eggshells in my food processor scratched the plastic cup badly. I'm going to have to reserve that cup just for processing eggshells. In the future, I may experiment with crushing the shells with a rolling pin or something similar. (But if you try this, know that the eggshells must be ground pretty finely or they won't deter slugs and snails.)
If you have leftovers, you can either store them in an air tight container for a later applications, or you can offer them to your hens in place of oyster shell. Laying hens need plenty of calcium or they'll have health problems. Their own eggshells provide it nicely. (It's important,however, to crush the eggshells; not only does this make them easier for chickens to consume, but it prevents hens from identifying food with their eggs. Trust me, you don't want chickens that eat eggs from their nesting box!)