The Ecology of Garden Slugs

Slugs are, perhaps not surprisingly, very easy to kill. Like frogs or toads, they need to keep their skin moist at all times in order to breathe properly, and that makes them very susceptible to physical or chemical damage. They're also not very smart.

Thus, there are many easy and "natural" ways to kill your garden slugs, including beer, vinegar, cornmeal, and yeasty water traps; sprays of ammonia, caffeine, or nicotine; borders of salt, coffee grounds, eggshells, sand, diatomaceous earth, copper, sharp rocks, or pine needles; not to mention just picking them off plants and/or squishing them.

Problem solved right? Well, sortof, but not exactly. Getting rid of slugs this way is like taking NyQuil for a cold. It might help you sleep at night, but it doesn't actually mean you're not sick. In other words, it might get rid of your slugs for a few days or weeks, or even for the season, but chances are you're going to need to keep doing it to remain slug-free.

Slugs, like any animals, need air, water, food, and shelter. By taking away any one of those things, you make your yard much less hospitable to slugs, and much less likely to become infested. Because slugs need to be constantly moist, their air, water, and shelter needs are very closely intertwined. Without a shady, damp habitat, they would quickly dry out. Whether they'd technically be dying of suffocation, dehydration, or exposure is largely a semantic issue.

Because slugs prefer moist habitats, Denver is one of the last places you'd expect to find them. However, our lawns and gardens, being relatively damp, can attract slugs and other herbivores, especially with the bouquet of lush, delicious plants. You can reduce favorable slug conditions by raking and composting leaves in the spring to remove eggs and hiding places, not mulching plants more than 3 inches deep, not over-watering, watering in the morning (so gardens are dry at night, when slugs are active), and pruning or staking up plants so they don't drag on the ground.

You can also make your garden less favorable for slugs by making their food options less attractive. By planting slug-resistant plants in your shady spots or around and among your vegetables, you're making it a lot harder for them to find a good meal. In general, plants with stems and leaves that are waxy, hairy, red, bitter, or aromatic (i.e. smelly, like mint, rosemary, or marigolds) tend to be more well defended from herbivores of all kinds, not just slugs. By interspersing heavily defended plants like mints and marigolds among your poorly defended plants like lettuce and spinach, you can make your garden less palatable overall to herbivores.

Finally, you can help deny the slugs their shelter needs by promoting predators. Predators can not only reduce the numbers of slugs, but can reduce the damage they do by literally scaring them into eating less and hiding more. Although the biggest predators of slugs (ducks, turtles, snakes, and toads) also like very wet habitats, and are unlikely to be found in Denver, there are still a number of other predators we can attract. Beetles, fireflies, praying mantises, and songbirds all enjoy a tasty slug. Blue, purple, and yellow flowers tend to attract insects, while red and orange flowers tend to attract birds. Other amenities like bird feeders and birdbaths can help as well. If all else fails, you can even buy praying mantis eggs from your favorite garden store. However, if you want to attract natural predators, you should avoid using pesticides, or even herbicides, wherever possible. These chemicals negatively effect not only the herbivores, but the predators as well.

For more information, check out this site from the University of Minnesota Extension.