The White Garden Snail: A Serious Threat to Landscape Ornamentals?

 
The white garden snail (Theba pisana), sometimes known as the Italian white snail, can be a serious pest of landscape ornamentals, including trees, shrubs, groundcovers, and herbaceous plants (Figure 1). Although it has been established in California for at least 100 years, it has mostly been inactive but has recently become more active in southern California and in coastal San Mateo County. If this activity continues to increase, the white garden snail will become a major pest and do serious harm to landscape plants, especially in coastal California.

Damage and Signs

The white garden snail feeds on an unusually broad range of plants, including orchard trees, vegetables, and ornamentals. Feeding damage is typical  snail and slug damage. It makes irregular holes with smooth edges primarily on leaves or on the margins of leaves and on flowers (Figure 2). It also feeds on seedlings, ripening fruits, and young plant bark. Because this snail has an exceedingly high reproductive rate and thousands can gather on one tree, severe defoliation and eventual death are possible, making it a major pest. Pests such as earwigs, caterpillars, and other chewing insects can cause similar damage but will not produce the telltale silvery mucous trails of the white garden snail.

Identification and Biology

The shell of this snail is unusually variable in appearance, especially in the dark bands and other markings. The adult has a medium-sized shell about the size of a nickel or dime. The non-glossy shell is typically ivory white (rarely pink), but can be light beige with narrow, dark brown bands. A similar looking but much less damaging snail, the milk snail (Otala lactea), sometimes occurs with the white garden snail and can be confused with it. The milk snail tends to be larger, up to 1.2 inches in diameter, and the inside of the thick opening is dark.

Unlike most snails and slugs, the white garden snail climbs and rests in a dormant state (estivates) on the cooler and least wind-exposed sides of vertical surfaces like trees, shrubs, fences, posts, and walls during the hot, dry season (Figure 3). They can survive for long periods by forming a wall of dry mucus to seal the shell opening and reduce water loss. They typically congregate in great numbers in an exposed, conspicuous manner to “ride out” the hot, dry season until the return of more suitable conditions in the fall.

After the first rains of the season, usually in November in California, the white garden snail, which are cross-fertilizing hermaphrodites, become more active, mate, and descend to the ground from their estivation sites to lay eggs and forage. They deposit eggs just under the soil surface or in humus. Hatching usually occurs after 20 days. This snail has a relatively short life span of  1 to 2 years, breeds only over a single season, and produces a large number of eggs (over 4,500 eggs per pair).

Management

Control of the white garden snail can be time-consuming, difficult, and costly because they have a high reproductive rate, they climb high on objects, and they estivate for long periods. Effective management of this snail must rely on a combination of methods, including exclusion, early detection, and a variety of treatments.

Like most land snails, they move slowly so in order to reach new areas, they must be aided by people. To exclude the white garden snail from your area carefully check crates, boxes, and plants shipped from infested areas. To detect this snail, search plants, fences, posts, walls, and other vertical surfaces.

Measures used to manage other snails, such as sprays, baits, traps, and barriers, are only effective when the white garden snail is active and foraging on or near the ground. However, unlike other snails, this snail estivates in the open where they are visible and conspicuous, perhaps offering the best opportunity for their control; thus, hand-picking, knocking down, and then sweeping or vacuuming might be the best option, especially with limited infestations or in small landscapes. Because it can estivate in vacant fields or untended areas adjacent to landscape sites, these untended areas should be carefully checked and mowed.

For extensive details on the various management methods for the white garden snail, including habitat modification, biological control, hand-picking, and chemical control, see the full article at https://ucanr.edu/sites/HodelPalmsTrees/files/294710.pdf.

 

Word of Caution

Use rubber or latex gloves when picking or handling snails and vegetation with their slime trails, and wash hands thoroughly afterwards. Snails and slugs are intermediate hosts of rat lungworm disease, which is likely present but not yet officially detected in California. Rat lungworm disease is caused by a parasitic nematode that can attack the human brain and spinal cord if ingested.

 

[Original article published in the Summer 2020 issue of the Retail Newsletter]

 


By Donald R. Hodel
Author - Environmental Horticulture Advisor
By Elaine Lander
Posted by - Urban & Community IPM Educator