Snails and Slugs

Snails and slugs are mollusks with similar anatomy and biology, except slugs lack an external shell. As they feed and move, slugs and snails secrete mucus, which dries to form a silvery to whitish trail that indicates these pests' presence. Note that fungus gnat larvae also leave silvery trails on growing media, which when abundant can resemble trails of slugs and snails.

Snails and slugs are active mostly at night and on cool, cloudy, damp, and foggy days. On sunny days, they hide from the heat and sunlight. The only daytime clues to their presence may be chewing damage to plants and silvery to whitish trails. In mild winter areas of Southern California and coastal locations, snails and slugs are active throughout the year. During cold weather, snails and slugs hibernate in the topsoil. During hot, dry periods, snails seal their shell opening with a parchment-like membrane, often attached to containers, fences, trunks, or walls.

Snails and slugs are hermaphrodites; they have both female and male reproductive parts. But generally they must mate with another mollusk of the same species in order to lay viable eggs. Brown garden snails mature in about 2 years, whereas some slugs reach reproductive maturity in about 3 to 6 months.

Brown garden snail is the most common snail pest. It has a spiraling shell up to 1-1/4 inches in diameter. The shell is patterned brown, gray, tan, and yellow in bands, flecks, and swirls. Where calcium is abundant in their food or soil, the shells will be hard. However, where calcium is low, shells may be quite fragile. Adult brown garden snails can lay about 80 eggs per month in shallow depressions in growing media and topsoil, but clutches (groups of eggs) may be smaller, often 10 to 20 eggs. The spherical to teardrop-shaped eggs are 1/8 inch in diameter and initially white, but become browner as they develop. The eggs may be mistaken for certain slow-release fertilizers.

Amber snails have an elongate shell up to 4/5 inch long that is translucent and mostly brown or yellowish-orange(amber colored). These semiaquatic snails occur mostly in the wetter portions of growing areas. Several Novisuccinea and Succinea species can be present and they are not reliably distinguishable to species. These snails generally feed on algae on the bench or pot, but will also feed on tender leaves and petals.

Cellar glass-snail has a shiny, pale brown, translucent shell up to 1/2 inch wide. The body on top is pale blue. Cellar glass-snail is an omnivore; its food includes dead and living plants, earthworms, fungi, other snails and slugs and their eggs, and sowbugs.

White garden snail has coloration and a shell shaped similar to brown garden snail, but overallit is more pale and smaller at maturity. White garden snails are about 1/2 to 3/5 inch in diameter at maturity, and rarely grow up to 1 inch. In California this introduced pest is established in San Diego County and has been found in Los Angeles and Orange counties.

Slugs of various species can be pests in greenhouses and nurseries. These include, garden slug, gray garden slug (gray field slug, or milky slug), greenhouse slug, threebanded garden slug, and tramp slug. For more information consult California Pest Snails and Slugs, Slugs: A Guide to the Invasive and Native Fauna of California, and Terrestrial Mollusc Tool.

Snails and slugs chew and feed on algae, decaying organic matter, and various living plants. They chew irregular holes in leaves or entirely clip off succulent plant parts. They sometimes chew flowers, fruit, roots, and tender bark. They make shiny to whitish trails on plants and other surfaces where their dark to pale feces may also be found. Because they prefer humid or moist conditions and prefer to feed on succulent plant parts, they are primarily pests of herbaceous and low-growing plants and seedlings. Quarantines prohibit the sale and shipment among counties and states of plants infested with the introduced brown garden snail, white garden snail, other exotic snails or slugs, and commonly all mollusks.

Employ a combination of methods for effective snail and slug management:

• Eliminate where feasible the places where snails and slugs can hide during the day. Boards, debris, stones, leafy branches growing close to the ground, weedy areas, and dense ground covers such as ivy are ideal sheltering spots. Reducing hiding places allows fewer snails and slugs to survive.
• Inspect regularly the remaining places where slugs and snails seek shelter. The survivors will congregate there for spot control, such as by handpicking. Brown garden snails are often found on nursery containers. Slugs may be found under containers.
• Switch from sprinkler to drip irrigation to reduce humidity and surface moisture, making the habitat less favorable for mollusks.
• Where possible, do not grow plants on wooden benches and pallets. These stay moist for a long time, tend to be good sites for growth of algae on which mollusks feed, and provide snails and slugs places to hide between the bench or pallet and containers.

Snails and slugs have many natural enemies, including parasitic flies in the families Sciomyzidae and Phoridae, mites, nematodes, pathogens, Scaphinotus spp. and other predaceous ground beetles, staphylinidbeetles such as devil's coach horse (Ocypus olens), and small vertebrates such as birds, snakes, toads, and turtles. But natural enemies do not provide satisfactory control of mollusks in production nurseries.

The omnivorous decollate snail (Rumina decollata) is commercially available for biological control of pest snails in Fresno, Imperial, Kern, Los Angeles, Madera, Orange, Riverside, Santa Barbara, San Bernardino, San Diego, Ventura, and Tulare counties. Because of the potential impact of the decollate snail on certain endangered and native mollusks, it cannot be released outside of the above counties.

Decollate snail release in commercial nurseries is not recommended because all plants must be snail-free. In addition to feeding on other snails, decollate snails feed on some seedlings and succulent, small plants. Before shipping plants, any decollate and other snails must be removed.

Carefully inspect new plants and propagation stock to ensure they are pest free. Reduce hiding places that allow snails and slugs to survive and thrive. Use good sanitation and keep growing areas free of clutter and weeds. Grow plants off the ground and on metal benches and not on anything made of wood. Switch from overhead irrigation to drip to reduce the wetness of growing areas that favors pest mollusks. Increase the frequency between irrigations to the extent compatible with good plant growth. Irrigate early in the day so surfaces are drier by evening when mollusks become active. Apply copper bands or screens on benches to inhibit the movement of snails and slugs from the ground to plants on benches.

Handpicking

Handpicking can be very effective if done thoroughly on a regular basis and in combination with reducing the places where mollusks can hide. Snails and slugs can be controlled and monitored using trap boards positioned throughout the nursery as discussed below. Crush and dispose of mollusks in covered containers away from crops. Snails and slugs may also be killed before disposal by dropping in a bucket with soapy water or diluted ammonia (5 to 10% solution). Do not apply salt to destroy snails and slugs as salt can increase soil salinity.

Biological and cultural controls are organically acceptable management methods. Bordeaux mixture, copper sulfate, and iron phosphate are organically acceptable molluscicides.

Baits are effective when used properly in combination with cultural management practices. Note that baits will kill predatory decollate snails if they are present. The active ingredient of most baits is ferric sodium EDTA, iron phosphate, or metaldehyde. Growers report that iron phosphate appears to be more effective and persistent where humidity is commonly high, plants are watered overhead, or rainfall is common. Monitor to determine where to apply the bait. In commercial nurseries to be most effective bait is scattered throughout planting beds and places where snails hide. The timing of any baiting is critical; baiting is less effective during cold, dry, and hot times of the year because snails and slugs are less active then. Because snails and slugs are more likely to travel and encounter any bait when soil is moist, apply bait after an irrigation.

Bordeaux mixture. Properly mixed copper sulfate and hydrated lime mixture (Bordeaux mixture) can be brushed or sprayed on trunks or bench legs to repel snails and slugs. One treatment should last about a year. Adding a commercial spreader-sticker may increase the persistence of Bordeaux mixture through two seasons.

Copper barriers. Slugs and snails will not cross copper unless the surface is fouled with debris or becomes heavily oxidized. Copper flashing or vertical copper screens can be erected around planting beds to prevent in-migration of mollusks. The screen should be 6 inches tall and buried several inches below the soil to prevent slugs from crawling underneath. Copper screens can be placed between the bench-support legs and the bench to prevent snails and slugs under the bench from reaching the top where the pots are located. Copper foil or screen wrapped around planting boxes and bench legs can repel snails for months or longer.

Regularly inspect growing areas for slugs and snails around chewed plants on cloudy, cool, damp days or around sunrise or sunset. During the day, scout beneath dense foliage, under benches and under and on pots, in nearby weeds, under edges of nursery mats, and other protected locations that are dark, moist, and near growing media, organic litter, or the ground. Slugs commonly are found under pots and in growing media. Snails are commonly found attached to the less-exposed sides of plant containers.

Amber snails tend to remain on the surface of planting medium. Look for these on pot surfaces and where moist planting medium and dry areas (e.g., the container) meet, generally around the inner lip or head space of containers.

Snails and slugs seek cover during daylight and can be trapped under slightly raised boards positioned throughout the nursery. Traps can be made of lumber about of 12 x 15 inches or any convenient size or similar material. Raise traps off the ground with 1-inch runners to facilitate pest movement to the shaded underside. After counting and recording the number of mollusks trapped, scrape off snails and slugs daily and dispose of them by crushing or killing in soapy water or dilute ammonia solution as described above.

A method of both control and monitoring that is effective for the brown garden snail is to place an attractive bait (e.g., containing metaldehyde) in small piles strategically around the growing areas. Nearby snails will be drawn to the bait overnight and will quickly die. The next morning, the number of dead snails can be counted to help identify locations where snails are most abundant. As with the trap-board method above, repeated over time the use of bait piles allows comparisons of whether brown garden snail numbers are increasing or decreasing. If dogs wander the nursery, they may feed on and be poisoned by exposed metaldehyde bait; place the metaldehyde in a way that it is accessible to the snails but not the dogs, such as in a commercial bait trap.