Know Your Friends

The Decollate Snail

The predatory decollate snail, Rumina decollata, is a small land snail of southern European, North African and western Asian origin from the areas around the Mediterranean Sea that attacks and kills slugs, common brown snails, and garden snailsIt is known as the decollate snail because it breaks off (decollates) the top 3 whorls or so of the shell. Supposedly this increases mobility, reduces shell weight, and improves resistance to desiccation.

The decollate snail was brought into the United States both accidentally and intentionally. It has been on the humid Atlantic seaboard for over 165 years and in the Gulf States for over 85 years. The decollate snail is now established in the southern areas of the country, and is widespread in southern California and Arizona, throughout much of Texas, along the Gulf Coast to Florida, and up to North Carolina. It also occurs in Bermuda, Cuba, and Mexico. 

The cone-shaped decollate snail feeding on a brown garden snail.The decollate snail, a mollusk in the family Subulinidae, has a black body, a dull olive-gray foot, and a brown, elongated spiral shell with 4 to 6 whorls, tapering to a blunt end. Large individuals have a purple cast at the mouth of the shell. [link to University of California photo.] In some populations the 3/4 inch long adults are pale yellow or whitish, while the young snails are light brown. 

The decollate snail is hermaphroditic, with both self-fertilization and cross-fertilization reported. They begin to produce eggs at approximately 10 months of age, and live from 1 to 1 years. The adult creates a shallow depression in the soil as a nest, and deposits the 2mm eggs individually, but close enough to form a cluster. Fifteen to 50 eggs are produced over a period of 3-5 days, with about 200 eggs produced per year. The eggs hatch in 10 days to 6 weeks. 

Decollate snails are quite adaptable. They can tolerate several months without water, and go dormant during periods of high temperatures and low relative humidity. During cold weather they burrow into the soil, often to considerable depths. They spend the winter burrowed into moist, well-drained soil, but will not survive in areas where temperatures remain below freezing for long.

Decollate snails are active mainly at night or after rainfall. During the day they are found in the upper inch of the soil, in leaf mulch, or under rocks. In the evening they come out to hunt. They track their prey by following the slime trails produced by brown garden snails or slugs. They feed only on small snails and slugs, not full-sized ones. They will attack adult garden snails, but rarely kill them because the snail can retreat into its large shell.

Although these snails prefer to feed on snails, slugs and organic matter, they will eat seedlings, small plants and flowers if preferred foods are not available. They can also be a nuisance when they cover patios, sidewalks or driveways after rain. And some homeowners have complained about high populations in lawns dulling lawn mower blades.

The decollate snail has been highly effective in controlling brown garden snail in relatively stable plantings such as freeway landscaping and established plantings around the home. In gardens and among seedlings there is the potential for plant damage, however. This species has been reported to feed on the leaves of Dichondra, violets, flower petals, strawberries, and germinating seeds. Most plant feeding follows damage from other sources, such as bruising from foot traffic, pruning, or bites by primary plant feeders perhaps in response to "stress ethylene" emitted by the plants. Physical or chemical barriers can prevent snail access to small areas to alleviate this problem. In most situations, however, the small amount of plant feeding by the decollate snail is insignificant compared to that caused by its prey. 

Citrus growers in the San Joaquin Valley use the decollate snail to control damaging brown garden snail populations, which can cause extensive damage in orchards by feeding on both the fruit and young leaves. Decollate snails thrive in citrus groves where low-volume irrigation systems create a moist environment. It takes 4 to 10 years to reduce brown garden snail populations to insignificant levels. However, in 10 years, there should be sufficient leaf mulch to preclude the brown garden snail problem. When 12 decollate snails per tree were released in an eight acre commercial grove, it took three years to develop an effective population. This original nursery block then provided snails for more than 150 acres over the next three years.

Little research has been conducted on the use of decollate snail for slug control in either outdoor or greenhouse settings. Presumably the snail will feed on slugs and have the potential to impact populations; one anecdotal mention indicated slug population reduction of 50% was all that could be expected from snail predation.

Decollate snails are commercially available from many suppliers. Because they are considered a threat to native mollusk species, it is not legal to sell this animal to residents in some areas. In California it cannot be released outside of Fresno, Imperial, Kern, Los Angeles, Madera, Orange, Riverside, Santa Barbara, San Bernardino, San Diego, Ventura, or Tulare counties. In other states decollate snails might be obtained with a letter from your county agricultural commissioner or similar local official stating that possession of the decollate in your county is not illegal and that interstate shipments of decollates are permissible. 

The snails are usually dormant when you receive them. If they are not moving around, place them in a pan or bucket and run cool water over them for 2-3 minutes. Drain the water and place them in a moist, shady area where brown snails or slugs are present. You have to be patient after releasing the snails, since it may take up to 2 years for snail problems to be eradicated. Slug populations are less likely to be controlled as well.

Decollate snails generally do not move very far from where they were hatched or released (unless that area is inhospitable), so place the snails in several locations where organic matter is plentiful. Recommended release rates are about 100 per normal size yard or 1000 per acre.

In areas where organic matter is not prevalent, population buildup will be enhanced by supplemental feeding. Pelletized alfalfa, a thin layer of compost, or similar materials can be used.

— Susan & Dan Mahr, University of Wisconsin

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