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White garden snail on iceplant. Photo credit: David Rosen, courtesy University of California Statewide IPM Program.

By Vincent Lazaneo.

Snails are found in irrigated gardens throughout the year, but their reproduction and growth accelerates after winter rains begin. The mollusks can severely damage plants when dense populations are allowed to develop.

For many years, the brown garden snail (Cornu aspersum) was the most common pest snail in California gardens. It was brought to the United States from France in 1853 to raise for food. The snail escaped and eventually became established in all areas where it was adapted, including a large part of California.

To combat the pest, a predatory species, the decollate snail (Rumina decollata) was introduced as a biological control agent for use in southern California. According to biological control specialist Erich Fred Legner, PhD, in the article Terrestrial Snails, as of 2014, a 98% decline of both brown garden snails and decollate snails was observed in many areas of southern California. It’s possible that the introduction of R. decollata and its interaction with other natural enemies was primarily responsible for the dramatic decline of the brown garden snail, but it is difficult to know how much of the decline was due to biological control and how much was due to other factors.

Another snail, the white garden snail (Theba pisana) has become a serious pest in parts of San Diego County. The snail, which is native to Morocco, has spread throughout the Mediterranean region and beyond. It was first detected in North America in La Jolla in 1914, and in 1985 was declared established in the county when it was found throughout an area of ten square miles in San Diego. The snail has since spread to other areas.

The white garden snail reproduces rapidly under favorable conditions. It is hermaphroditic and lays dozens of eggs in clusters in damp soil. The snail feeds on a variety of plants, including citrus, and densities of up to 3,000 white garden snails per tree have been reported.

In summer and fall when the weather is hot and dry, the snail stops feeding and seals the opening of its shell with a parchment-like layer to conserve moisture. Brown garden snails usually aestivate on objects near the ground. However, white garden snails aestivate in the open above the ground on trees, fences, buildings, and other vertical surfaces.

White garden snails aestivating on a branch. Photo credit: David Rosen, courtesy University of California Statewide IPM Program.

Aestivating snails are considered unsightly, especially when they are present in large numbers. While aestivating, the only way to get rid of the snails is by physically removing them. Poison baits are only taken when the snails are actively feeding, but baits scattered on the soil may not be very effective if the snails have climbed into shrubs and trees to feed on foliage. For more information on how to control snails, see the UC IPM Snails and Slugs pest note.

Vincent Lazaneo is UC Urban Horticulture Advisor Emeritus. He has a master’s degree in horticulture and a teaching credential in vocational agriculture from UC Davis. In 1983, Vince began the Master Gardener program in San Diego. Vince frequently contributes, or has contributed to, the San Diego Union-Tribune, Mira Mesa Living, and other publications, and he enjoys growing specialty plants in his home garden, reading, hiking, and fishing.

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