The common garden snail, Cornu aspersum, is a frequently encountered mollusk across temperate climates, often recognized by its presence in gardens and agricultural fields. This species has spread far beyond its original territory, raising questions about its ecological role in new environments. Concerns often arise from its high reproductive capacity and the visible damage it causes to cultivated plants. To understand the impact of this mollusk, it is necessary to investigate its biological characteristics and its classification as an invasive species.
Defining the Common Garden Snail and Invasiveness
The organism commonly known as the garden snail is scientifically named Cornu aspersum. This land snail has a characteristic shell that is typically globular, brown or greyish-yellow, and marked with darker spiral bands or spots, usually reaching a diameter of 25 to 40 millimeters in adults. As a terrestrial air-breathing gastropod, it is a generalist herbivore, capable of feeding on nearly any plant matter.
The term “invasive species” carries a specific definition, distinguishing it from a merely non-native organism. An invasive species is non-native to the ecosystem under consideration and whose introduction causes, or is likely to cause, economic or environmental harm, or harm to human health. The invasive label is applied only when the species establishes itself and creates measurable negative impacts. The garden snail’s classification is therefore determined by its detrimental effects in its introduced range, not just by its origin.
Global and Local Invasive Status
The garden snail is native to Western Europe and the Mediterranean region. However, it has been intentionally and accidentally transported worldwide, often inadvertently through the movement of plants and soil. This dispersal has resulted in the species becoming an established pest in numerous areas globally, including North and South America, Australia, New Zealand, and South Africa.
In many of these regions, Cornu aspersum is legally considered a plant pest or a quarantine risk. The United States Department of Agriculture (USDA), for instance, considers it a plant pest, and several states have established quarantines to prevent its importation. While it is merely a garden nuisance in some areas, it is classified as a serious agricultural pest in others, notably in California citrus orchards and in South African vineyards.
The species’ invasive success is attributed to its high degree of phenotypic plasticity, allowing it to adapt to diverse climates. Its wide distribution in areas with Mediterranean, temperate, and even subtropical climates demonstrates its adaptability far beyond its native range. The snail’s introduction was often accidental, allowing it to rapidly colonize disturbed landscapes like agricultural and suburban areas after escaping from farms where it was being reared for food.
Economic and Ecological Damage
The primary damage caused by established garden snail populations is economic, stemming from their voracious and generalist feeding habits. Snails consume a wide range of plants, including ornamental crops, vegetable gardens, and commercial fruit trees. In agricultural settings, particularly citrus groves and vineyards, the snail can cause significant yield losses, with damage to fruit peel or leaves leading to crop rejection or fungal decay.
The damage is amplified by the snail’s reproductive biology. They are hermaphrodites capable of laying large clutches of eggs multiple times a year under favorable, moist conditions. An adult snail can lay approximately 30 to 120 eggs per oviposition. This high reproductive rate allows populations to quickly reach high densities, such as the documented cases of over a thousand snails per tree in California citrus orchards.
Ecologically, the presence of large non-native mollusk populations can lead to competition with native snail species for resources and habitat. The sheer volume of their population can alter grassland species composition by selectively feeding on certain native grasses. The garden snail is also a known host for various parasites, some of which are associated with diseases in mammals.
Effective Management Strategies
Management of Cornu aspersum populations often relies on an integrated approach that combines cultural, physical, and chemical controls.
Cultural Controls
A first step in control is habitat modification, which involves removing debris, boards, and other hiding places where snails aggregate during the day or prepare for hibernation. Reducing moisture, perhaps by altering irrigation schedules to avoid watering in the evening, makes the environment less favorable for snail activity and reproduction.
Physical Controls
Physical barriers offer a non-chemical method of protecting individual plants or trees. Copper strips placed around the base of trees or raised beds can deter snails, as the metal reacts with the snail’s mucus to deliver a minor electrical shock. For high-density infestations, hand-picking snails at night or early morning, when they are most active, can be an effective control measure. Snails can also be lured into traps, such as a board elevated on stones, which they will use for shelter and can then be collected.
Chemical Controls
Chemical control typically involves the use of molluscicides, with iron phosphate and metaldehyde baits being the most common options. Iron phosphate baits are generally considered less toxic to pets and wildlife than metaldehyde and are spread on the ground near affected plants. Applying the bait in small pellets and ensuring its placement draws the snails away from moist areas can maximize efficacy by promoting desiccation after consumption.