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Stopping Slugs and Snails, Safe and effective methods of control
by Joel Grossman
Fine Gardening, November, 1991
Whenever I spot a trail of glistening, silvery slime leading across my garden beds to a plant with holes chewed out of its leaves, I know that a slug or snail has been feasting there. It never ceases to amaze me that these easy-to-catch creatures, both of which move at the proverbial snail's pace, can become such pests. It's not uncommon for slugs and snails to devour young seedlings overnight or decimate the buds or leaves of more mature broadleaf plants and trees. To these pests, which can be a problem in gardens and greenhouses throughout the United States, any plant is fair game. Fortunately, slugs and snails are similar enough in their lifestyles that you can use the same strategies to control both
What are slugs and snails?
Slugs and snails are mollusks, more closely related to seafood delicacies such as oysters, abalone, scallops and clams than they are to insects. Snails are the original inventors of the mobile home, lugging protective shells on their backs. They range in color from brown to rosy-pink to white, and adult garden snails measure more than 1 in. across. Most slugs are naked snails, with no visible outer shell. The typical pest slug is brown or grayish-drab, 1 in. to 2 in. long when full grown. Snails and slugs require a moist surface to crawl on. If none is available, they secrete a slimy mucus on which to slide, which then dries into a shiny, telltale trail behind them.
Slugs and snails feed on a wide variety of plants and are among the most common pests in the garden. The silvery slime trail left by the slug (above) is a sure sign of either pest. The holes in the young rhubarb leaves (below) resulted from slug feeding.
The brown garden snail (above), like the slug, favors moist environments.
One person's pest is another's escargot. Our major snail pests were originally brought over by European settlers with Old World gourmet visions. The snails that plague American gardeners are actually delicacies in France, where they can be collected only at certain times. In California, snails are collected from organic citrus orchards, where they can be a major pest, and confined to feedlots to be cleansed and flavored. The fattened mollusks are sold to local restaurateurs or deported to Europe at a profit. Slug pests also are imports, most likely hitching a ride as eggs or juveniles in the soil of nursery stock.
Native snails and slugs on the other hand, are shy creatures, seldom posing a problem. They usually stick to their native habitats, and they benefit the ecosystem as decomposers; some consume aphids and small caterpillars, as well.
Control strategies
If growing garden plants, rather than running a slug and snail ranch, is your goal, I recommend integrated pest management (IPM) strategies. In IPM, you first monitor the pest and then choose a tactic that is appropriate for the severity of the problem. This can include mechanical, biological or chemical controls.
Identify the pest and learn about its habits-It's hard to confuse the presence of snails and slugs with that of other pests. Their distinctive shiny or slimy trail is unmistakable, as is their damage-leaves with irregular holes in them. Some slugs feed on roots, as well.
Knowing where these pests live provides clues for effectively monitoring and controlling them. Moisture is vital to their existence, so snails and slugs are most active when the humidity is highest-at night, on cloudy days, and during periods of dew, light showers or overhead irrigation. On dry days. snails and slugs seek out moist, protected hiding places under mulch, plant debris, rocks, boards, weeds or groundcover.
Warm, wet winters favor snail and slug growth and are harbingers of spring outbreaks. Prolonged dry periods reduce mollusk activity, though, and drought triggers estivation, the summer equivalent of hibernation. Estivating snails withdraw into their shells, while slugs estivate underground.
Monitor -To observe snail and slug activity, go out into your garden at night with a flashlight. Follow their slime trails to nearby hiding places. To assess the slug or snail population, set out homemade or commercial traps. A wooden shingle nailed into the ground, smooth side down, will provide a habitat for slugs. A one-foot square board set on one-inch risers will attract snails. You can trap either pest under inverted grapefruit rinds or flower pots placed on the soil. Check beneath these traps early in the morning, before rising temperatures and evaporating moisture force the critters to hide elsewhere. Over several weeks, these monitoring devices can actually trap a large portion of the population. Monitoring once or twice a week in your garden should give you a fairly good picture of the snail and slug population trends. An increase in numbers, especially of young snails, may be a warning that tougher controls are necessary, while a decrease may signal that relief is on the way.
Predators such as this light-shelled decollate snail (above) and a snail eater (below) feed on brown garden snails.
Modify the physical environment - The simplest way to control uninvited mollusks is to make their environment less appealing, Start by eliminating the places where they like to hide, such as coarse mulches and weeds, and avoid over watering.
Snails shallowly bury clusters of their tiny, white, pearl-like eggs. Slugs prefer to lay eggs under boards or detritus, or occasionally on the soil surface. At the first sign of the eggs or of new populations rototill or cultivate where possible to destroy any unhatched eggs or soil habitats. (In established plantings, you'll need to handpick or trap them.)
Handpick -Though it's not for the squeamish, handpicking can make a sizable dent in the snail and slug population. It may provide only temporary relief, so combine this technique with other IPM methods.
Moist weather is the perfect time to hunt snails and slugs, Look for them under boards, flower pots, plant debris and in other hiding, you can pick them by hand or with cooking tongs or forceps. Kill them with a firm foot or the end of a 2x4. Alternatively, collect them into a jar filled with soapy water, which subdues them; then they drown. Or dispose of them in a tightly closed jar in the trash. Rubbing alcohol will remove the slime from your hands.
During drought, look for clusters of dormant snails under house trim or on walls, trees and fences, particularly in shaded and northern exposures. You can easily handpick or scrape them of f with a trowel and dispose of them. Or collect them on dewy nights or just after heavy rains, which cause dormant snails and slugs to come out and feed. When you are done mollusk picking, reward yourself and erect a barrier to keep snails and slugs out of the area you want to protect.
Create barriers - Like medieval moats that protected castles, barriers can resist breach by marauding snails and slugs. But be warned-barriers will also trap in any residents, along with their eggs, so you'll probably need other controls as well.
Barriers range from fence-like, metal enclosures to materials sprinkled on the ground around the area you want to protect. Solid copper makes the best barrier. Snails and slugs receive a very slight electric shock when their wet bodies contact the copper, and they are repelled by it.
Use a copper band at least three inch high. You can fasten it to the sides of planting beds, wrap it around tree trunks or pots, or attach it to a portable fence (see drawing on facing page). To protect your hands from the sharp edges of the copper, wear gloves when handling it. If you have small children, use it only where they won't run into it. Clear out any vegetation overhanging the protected area, since snails and slugs can use it as a bridge.
Paths of organic substances, such as diatomaceous earth (ground-up shells of ancient, microscopic organisms), shredded bark or wood ash are often recommended as barriers. Unfortunately, they work best during dry periods, when snails and slugs are least active. Even sand or sharp cinders fail to deter many slug species, which can glide by on their mucus secretion.
Set baited traps - if handpicking and. barriers don't adequately control your snails and slugs, trap them. You can use commercial traps or simple, homemade traps, such as those described for monitoring.
Commercial traps require some form of attractant. Both the "Slug Saloon," which traps only slugs, and the "Snailer," which traps both slugs and snails, come with a supply of nontoxic, grain-based bait. Mixed with water, the bait lures these pests into the traps where they drown. The traps are designed to keep rain out of the bait, and to keep the bait, albeit non-toxic, out of the reach of children and pets. Alternative baits that you can use in these traps include raw potato slices, lettuce, smashed snails, yeast or beer. You will have to clean out the traps periodically.
Sooner or later, every gardener hears about trapping snails and slugs with beer. These pests probably are attracted to the yeasty smell of the beer, and once they're in it, they drown. To make a beer trap, simply sink a shallow pan or bowl into the soil. Leave the lip of the container about one inch above ground level to avoid catching beneficial ground beetles crawling about in search of snails and slugs. Prop up a wider pan on sticks above the ground-level pan to keep rain from falling into the beer.
Personally, my idea of an IPM slugfest is drinking beer while gathering the pests, then pouring the dregs into the trap as the bait. (Slugweiser, anyone?) A light watering in the late afternoon entices snails and slugs out of hiding, thus enhancing the nighttime roundup by flashlight.
Poison bait can also be used to counter slugs and snails. The most common poison bait is metaldehyde, a chemical sold in pelleted, granular or liquid form, and sometimes combined with methiocarb, another poison. Metaldehyde is generally applied on the ground around the area you want to protect. Alone, it does kill snails and slugs, but depending on the formulation, it can also be harmful or fatal to children, dogs, other pets and wildlife if it's ingested. And over-reliance on this poison has produced snails and slugs that have grown resistant to it. Use it only as a last resort. Contain it within a trap, and store it in a securely locked area. Once the snail and slug populations are reduced, switch to other, less toxic methods of control.
Use biological control - Biocontrol is a long-term, ecological control option- it will not provide immediate relief. Only one major snail predator, the decollate snail, is commercially available. This snail moves very slowly, even for a snail, and it can take several years for it to control the pest snails. Decollates eat very young seedlings, though, so keep a watchful eye on these predators. If you do decide to purchase decollates, get approval first from your state Department of Agriculture or Fish and Game.
Fortunately, there are a number of naturally occurring beneficial creatures that eat snails and slugs. These include various beetles, firefly larvae, fly species and predatory snails, as well as toads, snakes, frogs, lizards and rabbits. Even ducks and chickens can be sent out on patrol to reduce snail and slug populations. Encourage these beneficials by providing them with a diverse habitat in and around your garden.
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