Regular and even abundant rainfall in the spring and early summer has encouraged high populations of snails and slugs this year. Snails and slugs belong to the mollusk family along with oysters. They crawl along on a single “foot” over a thin layer of slime they produce to ease their way. Snails carry a shell on their backs into which they can retreat when threatened; slugs do not have shells.
In our mild climate, snails and slugs are active and cause damage virtually year-round. They will feed on many of the bedding plants and vegetables in our gardens this time of year. Trees, shrubs, most ground covers and vines are rarely badly damaged by snails and slugs, however.
Snails and slugs are most active at night or on cloudy days during moist or wet weather and in beds that are regularly irrigated. The damage they cause is obvious but can be misdiagnosed. They eat holes in leaves and flowers, especially favoring low-growing, succulent growth.
Caterpillars also chew holes in leaves. Because the methods and pesticides for controlling them are different than for snails and slugs, it’s important to know which pest is causing the damage. The presence of slime trails, which look like meandering, reflective silver lines, indicate snails and slugs are the culprits. These may occur on concrete, pot sides or plant foliage. Caterpillars may leave behind dark pellet-like droppings, which would confirm their activity.
Controlling snails and slugs requires perseverance, and it’s best to use several techniques. The goal is to keep the population low enough to prevent an unacceptable amount of damage – not totally eliminate them. The main strategies for their control involve baits, traps, hand picking and encouraging predators that eat them.
Baits are generally available in the form of pellets, meal or liquid slurries. The snails and slugs must eat the bait for the active ingredient to work, so apply them in such a way that they are likely to be eaten. Baits are best used when the snails and slugs are most active. So water the area during the day to provide a suitable environment, and then apply the bait at dusk so that it will be fresh at night when the critters are active. Follow label directions carefully.
The chemicals in the baits are toxic to snails and slugs and will kill them if they eat the bait. Baits should be used regularly until new damage is reduced to tolerable levels. Products containing iron phosphate are the safest options and are recommended.
The most popular type of trap for catching snails and slugs is baited with, of all things, beer. All snails and slugs are very attracted to the yeasty smell of beer and will gladly crawl into it if given a chance.
To make a beer trap, sink a small plastic bowl up to its rim in the ground and fill it half full of beer. Any kind will do, but it should be fresh. Set the traps in the early evening in the beds where snails and slugs have been causing damage. Empty and replenish the traps daily until you stop catching many snails or slugs. Traps are also a great way to monitor population levels. If you see holes in leaves and put out beer traps in the area but don’t catch any snails or slugs, the damage is more likely due to caterpillars.
Hand picking is done at night with a flashlight and is not for the squeamish. It helps to wear latex gloves or use kitchen tongs to pick up the slimy creatures. Put them in a plastic bag and throw them away.
Avoid using salt to control snails and slugs. Using salt around your plants can damage them. Salts in the soil can burn plant roots.
Several types of barriers will keep snails and slugs out of planting beds. The easiest to maintain are those made with copper flashing and screen. It is believed that copper barriers are effective because the copper reacts with the slime that snails and slugs secrete, causing a disruption in their nervous system similar to an electric shock. When erecting vertical copper screens, it is best to use ones that are at least 4 inches tall, so you can bury a portion of it an inch or two below the soil to prevent slugs from crawling beneath the barrier. Make sure an area is free of snails and slugs before you enclose it inside a copper barrier.
Finally, predators can help control snail and slug populations. The predatory decollate snail has a distinctive cone-shaped shell. Do not kill these – they eat other snails.
Toads are excellent predators of snails and slugs and can consume large quantities of them. Yes, they’re good guys in the garden, and you should get over any squeamishness you have toward them. You can attract toads and keep them in your garden by providing a water source such as a small pond, pool or water garden and cover for them to hide under during the day. I build small enclosures or “toad houses” with pieces of brick or stones. Gardens with active populations of hungry toads rarely have major problems with snails and slugs.
Dan Gill is a horticulturist with the LSU AgCenter. He can be reached at DGill@agcenter.lsu.edu.