White Garden Snail
- Adult shell has 5 1/2 to 6 slightly convex whorls with shallow sutures
- First 1 ½ whorls are generally dark in color, ranging from tan to dark brown, and give the appearance of a dot on the apex of the shell
- Medium size, ranging from 12 to 15 mm (rarely to 25) in diameter and 9 to 12 mm (rarely to 20) in height
- Shell is opaque and moderately solid
- Well adapted to arid regions
- Leaves behind a trail of slime that may inhibit flower pollination and degrades the quality of fruits and vegetables
- Previously known as Helix pasana
- Snails live an average of 2-3 years, some having survived through 4 winters
The White Garden Snail, Theba pisana, is the worst potential agricultural pest of the helicid snails introduced to North America. It is the most frequently intercepted foreign land snail generally arriving in shipments from the Mediterranean countries. The White Garden Snail shows a strong proclivity for climbing up and into freight for aestivation and is difficult to detect.
This snail can survive long and arduous journeys because of its ability to form a wall of dried mucus, called an epiphragm, in the aperture of its shell which reduces water loss during dormancy. It is capable of explosive reproductive rates where it has been introduced, and can be found in densities of up to 3000 snails per tree after periods of less than 5 years. Once established, the White Garden Snail causes severe defoliation of a number of plants, including vegetables and citrus and ornamental plants. Also, White Garden Snail has been shown to assist in the dispersal of a fungus, Colletotrichum lagenarium, which is pathogenic to melons. The snail serves as an intermediate host for a lungworm and other nematodes that are parasites of sheep and cattle.
White Garden Snail is native in southwestern England and Wales, Ireland, western France, Switzerland, and the Mediterranean countries of Europe and Africa. Its habitat in Europe is near the coast. It has been introduced into the Atlantic islands, South Africa, Somaliland, and western Australia. This snail was first noticed in North America in La Jolla, San Diego County, California, in 1914. It soon spread to several locations in Orange and Los Angeles Counties, but apparently was eradicated there by 1940.
Mead reported a second infestation in Los Angeles County in 1966 which was declared eradicated in 1972. The snails were found and identified in August 1985 in San Diego, California, at several localities in about a 10 square mile area. Hanna (1996) stated that White Garden Snail had been introduced into several eastern localities in the United States. There are no published records for North American populations outside of California. White Garden Snail is also present in Bermuda but has never been recorded from Florida.
The ability of White Garden Snail to exist in desert environments indicates that it will colonize areas uninhabitable by Gray Garden Snail (Helix aspersa) and Milk Snail (Ortala lactea), which have been introduced to California as well. White Garden Snail can easily be distinguished from Gray Garden Snail and Milk Snail by its physical characteristics. On the White Garden Snail, the shell is subglobose with a moderately depressed spire. The umbilicus is narrow and partially to entirely covered by an expansion of the columella. The aperture of the shell is rounded, lunate and only slightly oblique. The lip of the aperture is sharp and unreflected, but some specimens show a thickening inside the lip. The juvenile shell has a sharp keel at the periphery, but in the adult shell the periphery is only slightly shouldered. The surface of the shell is not glossy, but is marked with many fine vertical striae.
The background color of the shell is nearly always ivory white (rarely pink), and there are often a variable number of narrow dark-brown spiral bands present. These bands may be solid, made up of dots and dashes, or absent. This difference in coloration does not have any systematic significance because it is apparently a polymorphic trait subject to differential selection pressures and it correlates with microhabitat.
During dry weather, most slugs and snails aestivate hidden under logs or stones or buried in the earth. However, White Garden Snail aestivates in the open on trees, fences, and other vertical surfaces. Like all helicid snails, White Garden Snail is a cross-fertilizing hermaphrodite. Pilsbry (1939) reported that this snail mates after rains during early November in California. The eggs are deposited several inches in the ground a few weeks after mating. The number of eggs per nest varies from 52-226, with an average of 120. Hatching occurs after a minimum of 20 days, but may occur later during dry weather. In the active season, this snail partially defoliates a variety of shrubs and trees, including citrus. The great density of the populations in California (up to 3000 snails per tree) and the rapid rate of reproduction are primary factors in making this snail a major pest.
Past infestations of White Garden Snail in California have been controlled and eventually eradicated by the combined use of poison sprays, poison baits, burning, and hand-picking. These campaigns were invariably long and costly due, in part, to the way in which this snail hides itself for long periods by climbing up, on, and into bushes, trees, and man-made objects. Any control program in Florida would require the use of metaldehyde baits or bran-calcium in conjunction with detailed surveys of the ground, plantings, and structures within the infected areas. The use of herbicides may be necessary in overgrown areas. Mead (1971) recommends rigid quarantine and fumigation of all suspect shipments.