It’s probably imagination, but it seems that most snail or slug attacks occur after a recalcitrant seed has finally germinated or a weak plant is showing signs of vigor. Everyone who’s ever had a garden is well aware of the damage snails and slugs wreak. They can be incredibly destructive, decimating rows of seedlings, disfiguring perennials, and chewing fruit. They have a major impact on commercial agriculture and nursery plants too.
Taxonomically, snails and slugs are mollusks, as are other invertebrate animals such as clams, scallops, and oysters; they are placed in the class Gastropoda, which is a very large group with some 400 families and as many as 80,000 species. From the Greek roots, gastropods are literally “footed stomachs”. And eat they do, some consuming several times their body weight in a day. In addition to garden plants, various gastropod species eat animal waste, carrion, algae, small arthropods, fungi, lichens, worms, and others of their own kind.
Snails and slugs are essentially similar organisms. There are marine and terrestrial examples of both; the terrestrial ones have lungs rather than gills. Snails, of course, have shells and must occupy an ecological range where calcium for shell building is available, and because of their shells, they are less compressible than slugs. Slugs are descended from snails, though, and most slug species retain vestigial shells, albeit reduced ones, that are mostly internal, but some have no shell at all.
Terrestrial gastropods are adapted to most environments, but they require some moisture for survival and will thrive in moist, cool climates like the Pacific Northwest. They do perfectly well even in the desert southwest, though, as long as there is a source of moisture, such as irrigation. They are mostly active at night; they avoid the sun and hide during the day in soil or under cover. If there is a period of drought or cold weather, gastropods can enter a period of low metabolic activity. Snails will seal themselves in their shells with a layer of slime, and slugs will move underground. Gastropods overwinter as eggs, juveniles, or adults.
Gastropods are hermaphroditic; though most begin life as males, they later develop female genitalia as well (that doesn’t mean though that they can breed with themselves). As soon as gastropods hatch, they begin feeding. They can reach sexual maturity in 3 to 5 months and begin reproduction themselves. Terrestrial gastropods lay egg masses in soil or on objects such as plants. They lay several “clutches” of eggs per year. The eggs are resistant to heat, cold, and drying, and under favorable conditions, hatch in about 1 month. The cosmopolitan brown garden snail (pictured), Cornu aspersum, lays as many as 100 eggs in each of one or two clutches per year in cooler climates and up to six in warmer climates.
The destructive snails and slugs seen most often in gardens and agricultural fields and greenhouses are largely nonindigenous to North America (most of our native gastropods are not particularly harmful to plants). Many were intentionally imported. The brown garden snail is thought to have been brought to San Francisco from France in the 1850s as a culinary delight, but it quickly escaped to feed in neighboring gardens and from there colonized most of the rest of the continent. Giant African snails, Achatina fulica and A. achatina, which are illegal for individuals to possess in the USA and may transmit human pathogens, have been imported for their shells and as pets and classroom science projects. There are currently no naturalized populations in the continental USA, but these snails are a significant threat and are known to eat as many as 500 different plant species. Other gastropods were introduced accidentally with agricultural shipments or as simple stowaways. Imported household tiles, presumably because of their calcium content, are frequently accompanied by exotic snails.
Managing gastropods in the garden is best accomplished by a variety of tactics. Any combination of tactics will depend on a number of variables including and not limited to the size of your garden and its climatic zone. Simple hand picking may be enough to curb them. Because snails and slugs are nocturnal and move slowly, they can be found and captured at night by means of a flashlight. All terrestrial gastropods produce “slime”, which they use for mobility and to maintain body moisture. Slime trails can be seen and followed in daylight on the ground and on plants themselves (pictured); these slime trails appear silvery in light. These can tell you where snails and slugs have been and give you some clues where to look for them. During daylight, they can be found resting under leaves, debris, clay pots, and other available objects. Limiting their likely hiding places and reducing a garden’s moisture by changing irrigation practices will ultimately reduce their numbers.
Snails and slugs have lots of natural enemies including some arthropods, reptiles, and birds, but these alone will seldom solve a snail or slug problem. Domestic fowl (chickens, ducks, and geese) will happily feed on snails and slugs, but they will also eat seeds and seedlings. The decollate snail (Rumina decollata), a predatory snail native to the Mediterranean area, has been introduced to Arizona and some other areas as a predator of the brown garden snail. Its effectiveness is mixed and its introduction controversial; it is illegal in parts of California because it will eat native as well as pest gastropods. It will also feed on plants.
Trapping can be effective. Traps can be homemade or purchased; they may be passive or baited. Clay flower pot shards or an elevated piece of wood (such as roof shake) may be an attractive hiding place for snails and slugs. Check for them every day or so. An example of a homemade baited trap is beer or sugar and yeast mixed in water in a cup buried to its rim. Traps should be easy for the snail or slug to crawl into but difficult to escape. Check with garden stores for commercial trap recommendations; Amazon sells a number of “slug traps”.
Commercial baits generally contain two types of active agents—iron phosphate and metaldehyde. Both are nonspecific and will kill their intended targets as well as native gastropods (and the decollate snail) and other soil and debris fauna, many of which are beneficial in the garden. The health risk of iron phosphate is low, but metaldehyde is toxic to dogs, cats, children, and other wildlife. Consult garden stores or county agricultural agents regarding commercial baits.
Barriers composed of copper strips or screens can be placed around planting boxes or tree trunks and will keep gastropods at bay. Copper reacts with gastropod slime by creating an electrical current; this type of barrier functions much as does an electric fence. Copper barrier tape with an adhesive back is commercially available. Also, slurry made of copper sulfate and hydrated lime can be painted onto surfaces for the same effect as the other copper barriers. This slurry, parenthetically, is Bordeaux mixture, so called because it was originally used along roadways at the edges of vineyards in Bordeaux to impart to the grapes a sickening green-blue color intended to deter passersby from sampling; only later was its inhibitory effect on downy mildew recognized.
Another form of barrier is to spread wood chips, fireplace ash, gritty sand, or diatomaceous earth. These are supposed to be too rough for snails and slugs to move on. The effectiveness of ash will clearly be short lived in areas with lots of rain, though it may enrich the soil somewhat.
However you decide to deal with snails and slugs in your garden, be vigilant and systematic. Learn which snails and slugs are which; there is lots of information with descriptions and pictures on the internet. By and large, the exotic ones will be the most damaging. Snails and slugs provide another example of how much more destructive exotic pests tend to be than their local cousins.