Cornu aspersum
(common garden snail)

C. aspersum, the common garden snail, is represented by several forms that are highly differentiated genetically. Only one lineage, the western one, is considered to be invasive in regions where it has been introduced recently (since the sixteenth century) either accidentally or intentionally (e.g. North and South America, South Africa, Oceania). It was in California, USA, where it was introduced in the 1850s, that it was first treated (1931) as a regulated pest. Its success in colonizing new areas after introduction and establishment may be due to: (i) large phenotypic variation in combinations of life-history traits, especially reflecting a high degree of plasticity (e.g. trade-off of egg weight/egg number), and (ii) great resistance against natural enemies. Also, genetic data indicate that C. aspersum is capable of establishing even after a severe genetic bottleneck.

C. aspersum has long been the subject of extensive studies that have led to the recognition of several endemic forms (Taylor, 1914). Whilst the origin of the one form considered distinct, i.e. the farm reared 'gros-gris' or form maxima, is doubtful because its range is currently unknown, the other common form, aspersa sensu stricto, is probably native to North Africa (Taylor, 1914) where genetic discontinuities indicate differentiation of well-defined eastern and western lineages (Guiller et al., 1994, 1998, 2001, 2006). The western lineage would have then expanded from north Africa to Europe via both the Tyrrhenian route and the Straits of Gibraltar (Guiller and Madec, 2010). Historical events involving vicariant and dispersal processes would explain the 'east-west' genetic split and the northward expansion of the western clade. The distribution essentially reflects both Pliocene/Pleistocene climatic changes and Tertiary geomorphological events.

Nowadays, the western lineage of C. aspersum has become a typically anthropochorous form widespread throughout the world in many regions having Mediterranean, temperate and even subtropical climates. Its presence is now reported in North and South America and Africa, as well as in the Mascarene Islands, Oceania and Asia. Much more recent events related to human activities are undoubtedly responsible for such longer distance dispersal. The subject of farming since the Roman age, the western form has successfully colonized a large range of landscapes disturbed by humans (agricultural, urban and suburban areas), and it is considered an important agricultural and garden pest in lands where it has been recently introduced and naturalized (Barker, 2002).

In Canada, C. aspersum was recorded in the 1850s in Nova Scotia (Taylor, 1914) but has not been found there since (Grimm et al., 2009). It was recorded in the 1970s in Newfoundland but whether it persists there is not known (Grimm et al., 2009). There have been sporadic records around Vancouver and on Vancouver Island, and at present in Canada C. aspersum can only be considered to occur, but not to flourish, around Victoria (Grimm et al., 2009).

C. aspersum is farm-raised in several countries where it has been introduced intentionally (eastern Europe, South America, Asia, etc.) as a source of food and skin care products. Countries, especially east European, south American and Asian ones, importing snails either live, fresh, frozen or canned, propose now to augment their economies through development of snail farming and the promotion of processed snail meat products on the world market. The promise that specific ingredients of the snail secretion give a rejuvenating appearance to the skin have led to increased deliberate introductions as some specific creams marketed since 1995 in Chile are now commercialized in 35 countries. C. aspersum is a quarantine plant pest in various states in the USA (Arizona, California, Louisiana, Oregon, South Carolina and Washington) and in Canada.

C. aspersum is a polyphagous grazer with a large diet spectrum. In its natural habitat, it feeds on wild plants such as Urtica dioica or Hedera helix, which are also used for shelter. In human-disturbed habitats, a wide range of crops and ornamental plants are reported as hosts: these include vegetables, cereals, flowers and shrubs (Godan, 1983; Dekle and Fasulo, 2001). In particular, it causes serious damage in citrus groves and vineyards. It will feed on both living and dead or senescent plant material. The Host Plants/Plants Affected table does not cover all plants that C. aspersum will feed on, as the list is so extensive but aims to provide an insight to the well-known species affected. The categorization as 'Main', 'Other' or 'Wild host' is also subjective and should not be considered definitive.

Genetics

Extensive work has been carried out on the population genetics of C. aspersum in its native range. There is a clear pattern in geographical structure in North Africa with a west versus east pattern of variation for all the polymorphisms considered (Guiller et al., 1994, 1996, 1998, 2006; Madec and Guiller, 1994; Madec et al., 1996, 2003; Guiller and Madec, 2010). This divergence becomes still more apparent when European populations are considered because of their clustering with those of western North Africa. Moreover, the great genetic diversity in North Africa contrasts with the smaller genetic divergence between populations in Europe.

A study using mtDNA and microsatellite markers (Hiraux, 2009) showed that invasive populations (from California and New Zealand) contained mixtures of haplotypes from different European source populations (indicating multiple introductions) and, as a result, exhibited increased genetic diversity.

As noted above, C. aspersum is a highly variable species. Two forms (considered by some authors to be valid subspecies), i.e. C. aspersum sensu stricto and the form maxima, are relatively well known and both are reared in French snail farms. Molecular markers showed strong genetic divergence between the two forms (Guiller et al., 2001) and the consideration of the standard anatomical criteria used for diagnosis of pulmonate species (Solem, 1978) suggested that the variation in distal genitalia could lead to precopulatory reproductive isolation (Madec and Guiller, 1993, 1994). However, successful experimental interbreeding showed that egg development and hybrid fertility seem not to be altered (Chevallier, 1980). Moreover, some breeders also market giant varieties they have selected for shell ground colour (e.g. ‘blond des Flandres’ with a yellow shell without any bands).

The number of chromosomes and chiasma frequencies seem variable. Karyotype analysis gave counts of 48, 50 or 52 chromosomes (Rainer, 1968). However, no differences between the two forms were observed by Madec (1989a) who reported a haploid number of 27 and the same high chiasma frequencies.

In relation to snail farming performance, experiments including quantitative genetics of life history traits of C. aspersum (form aspersa) led to estimates of genetic parameters for adult weight, growth and reproductive traits (Dupont-Nivet et al., 1997, 1998). Metabolic scaling in fast- versus slow-growth phases of non-selected lines versus lines artificially selected for increased size has also been investigated (Czarnoleski et al., 2008).

Reproductive Biology

Hermaphroditism is universal among pulmonate gastropods. The reproductive biology of C. aspersum, with special reference to mating behaviour and physiology, has been studied in detail by Giusti and Lepri (1980), Tompa (1984), Chung (1987) and Adamo and Chase (1988, 1990). C. aspersum is an obligate out-crossing species. A mature snail mates from 2 to 6 times in a single season and mating requires 4 to 12 hours. Both members of the pair act simultaneously as male and female. Thus, both animals receive sperm in very large numbers (each ejaculate contains about 5.5 x 10⁶) in every mating. A spermatophore is used to package the sperm and thereby protect them during transfer. The reproductive behaviour of C. aspersum exhibits several features, including multiple mating, long-term sperm storage, allosperm digestion, internal fertilization and dart shooting, that may promote sperm competition. The role of dart shooting and the related mucus gland in sperm competition has been extensively studied in C. aspersum (Rogers and Chase, 2001; Chase and Blanchard, 2006; Chase and Vaga, 2006; Chase and Darbison, 2008).

C. aspersum is oviparous, meaning that fertilized eggs are encapsulated in a partially calcareous shell and are laid with little or no embryonic development. The snails deposit white spherical or oval eggs about 3-5 mm in diameter in a cavity 4-7 cm deep that the snail has dug out of moist, loose soil with its foot. The egg mass is concealed by a mixture of soil and secreted mucus followed by a faecal ribbon in contact with the clutch and the nest is covered with soil. Oviposition occurs about 2 weeks after fertilization and its duration is from 10 to 35 hours. Snails from natural populations lay eggs two or three times per season for at least 2 years (C. aspersum is iteroparous), with egg numbers per clutch ranging from 70-80 (dwarfs from sub-tropical islands) to 200-220 for the large form maxima, averaging 100-120 (Madec and Daguzan, 1993; Madec et al., 1998, 2000). According to the pioneering work of Basinger (1931) on a population in California, each individual is capable of laying about 86 eggs once every 6 weeks from February to October, so that approximately five ovipositions are made each year and 430 eggs laid. Numerous data showing the variation in fecundity and reproductive output under artificial conditions are also available because the species is the principal subject of heliciculture (Daguzan, 1982, 1985; Stephanou, 1986; Elmslie, 1989).

Mating and oviposition frequencies are influenced by abiotic (temperature, photoperiod, humidity, soil conditions) and biotic (crowding) pressures (Dan and Bailey, 1982; Lucarz and Gomot, 1985). Low temperature and low humidity inhibit the activity of the snails, and dry soil is unsuitable for ovipositing. During warm damp weather, oviposition may be as frequent as once a month.

During the summer, the eggs hatch in about two weeks but the time to hatching varies from 14 to 40 days according to the environmental relative humidity (Guéméné and Daguzan, 1983). Egg cannibalism by hatchlings is frequent: 70% of new-born snails, which remain in the soil cavity for several days, ingested one sibling egg during their first 4 days of life, facilitated by high hatching asynchrony within the clutch (Elmslie, 1988).

Maturity requires about 2 years in natural populations, with the snails attaining a diameter of 16-20 mm within the first year, and 26-33 mm by the second year in North Africa; 3 or more years are needed for the larger form maxima. In many human-modified areas where snail activity is not interrupted by aestivation or hibernation (parts of South Africa, New Zealand, Mediterranean Europe, California), the snails take about 10-12 months to become mature, producing one generation a year (Potts, 1972; Albuquerque de Matos and Corto-Real, 1994; Sakovich, 2002). In artificial conditions optimized for growth, the recurved lip often appears after three months, but this does not coincide with reproductive maturity.

As for reproductive traits, many environmental factors, abiotic (temperature, humidity, photoperiod) and biotic (density, maternal effects), influence the growth rate and the trade-off of age versus size at maturity. Thus, long days are an ultimate environmental cue stimulating growth and egg-laying whereas short days inhibit them (Bailey, 1981; Enée et al., 1982; Le Guhennec, 1986). In artificial conditions, growth rate is influenced by both food availability and conspecific crowding (Cowie and Cain, 1983).

Physiology and Phenology

Being an ectothermic animal, C. aspersum is active only when external conditions are favourable. Its wide distribution leads to geographic variation in annual activity rhythms, with differences related to latitude and microclimate (Madec and Daguzan, 1993; Iglesias et al., 1996). This species is characterized by considerable plasticity in its responses to environmental differences. Thus, the breeding season is restricted to spring and summer in northern localities, to autumn or even winter in the Mediterranean area (Chevallier, 1983). Sacchi (1971) suggested that reproduction is potentially continuous and might occur during all sufficiently wet and warm periods of the year, as supposed for populations in La Réunion (Madec and Daguzan, 1993). Periods of activity are followed in higher latitudes by hibernation, which has a diapause value (Bailey, 1983; Lorvelec and Daguzan, 1990), and in lower latitudes by aestivation, which in some cases is only a warm torpor. Overwintering in C. aspersum lasts more than 7 months in Scotland (Crook, 1980), 6 months in Wales (Bailey, 1981), approximately 5 months in northwestern France (Lorvelec and Daguzan, 1990) and 4 months in northwestern Spain (Iglesias et al., 1996), while some Mediterranean populations do not hibernate (Madec, 1989a). Cessation of activity is influenced by photoperiod, temperature and humidity, the relative importance of which depends on latitude and local conditions (Jeppesen and Nygard, 1976; Jeppesen, 1977; Bailey, 1981; Bailey and Lazaridou-Dimitriadou, 1986; Lazaridou-Dimitriadou and Saunders, 1986; Aupinel, 1987).

During dormancy, snails bury in the soil or crawl into crevices among rocks, retire into their shells and produce a calcareous and mucous epiphragm sealing the shell aperture (Barnhart, 1983). Their metabolic rate is depressed to 5-30% of its normal value (Herreid, 1977; Barnhart and MacMahon, 1987; Storey and Storey, 1990; Brooks and Storey, 1997): oxygen consumption, water exchange, heart rate, neural activity and patterns of protein synthesis and activation are modified (Machin, 1966, 1972; Riddle, 1983; Barnhart and MacMahon, 1987; Bailey and Lazaridou-Dimitriadou, 1991; Biannic et al., 1994; Brooks and Storey, 1995, 1997; Feneglio et al.,1997; Pakay et al., 2002). A moderately enhanced cold hardiness is related to hibernation, allowing the snails to bear temperatures as low as c. –5°C (Ansart et al., 2001). However, young individuals seem unable to really hibernate and thus exhibit higher mortality during overwintering (Charrier, 1980; Biannic, 1995).

During active periods, activity occurs mainly at night, when temperature declines and humidity is enhanced. Bailey (1975) and Lorvelec et al. (1991) described two peaks of activity, approximately 2 and 6 hours after sunset. During the day, locomotor activity is restricted to periods of rainfall (Biannic et al., 1995).

Longevity

Precise estimates of the longevity of C. aspersum are not available. Observations in natural conditions indicate a life span of 3-5 years but the species is known to live up to 10 years in artificial conditions (Taylor, 1914; Comfort, 1957). Indirect observations (growth breaks and inner layers of shell peristome) show that giant individuals collected from natural populations in North Africa have a longer life expectancy (>10 years).

Nutrition

Having rasping mouthparts (radula), C. aspersum is a polyphagous grazer with a large diet spectrum. Its enzymatic complement allows degradation of cellulose, hemicellulose and xylane, and its gut microflora enhances digestive abilities (Charrier and Rouland, 1992; Charrier et al., 1998, 2000). It preferentially selects green living plants but is also reported to graze on flowers and fruits. It can also eat dead animal tissue and paper products.

Feeding activity is essentially nocturnal and occurs only when relative humidity is sufficient, around 80% (Gallois, 1983). In the laboratory, feeding behaviour is very plastic and sensitive to conditioning (Desbuquois and Daguzan, 1995). Although field observations are scarce, the diversity of the snails’ diet varies quantitatively and qualitatively with the season and availability of plants, and the snails do not eat at random (Iglesias and Castillejo, 1999). Pulmonate snails use distance chemoreception and taste to realise their choices, feeding preferences being influenced by the biochemical composition of the plants and especially by secondary metabolites (Chevalier et al., 2000). C. aspersum will preferentially eat plants rich in calcium such as Urtica dioica (Iglesias and Castillejo, 1999; Chevalier et al., 2003) and reject plants rich in metals, such as zinc and nickel (Boyd et al., 2002; Chevalier et al., 2003). Humus ingestion has also been reported as important in the diet of C. aspersum, improving digestibility of food and enhancing growth of juveniles (Perea et al., 2008).

Diet varies over the life cycle: in hatchling snails, oophagy during the hatching period inside the hollow in which the clutch was laid has been reported, significantly enhancing growth and survivorship (Desbuquois and Madec, 1998); after a few days, this behaviour is lost. Juveniles feed more often on fresh plant material than do adults (Iglesias and Castillejo, 1999).

In snail farms, a dry food (powder or pellets) is often used, composed of cereal flour (essentially soya, corn, wheat, sunflower, rye, oats) and enriched in vitamins and calcium. In the laboratory a mix of powdered calcium carbonate, dried milk powder and instant breakfast cereal has been used successfully to rear snails (Cowie and Cain, 1983).

Environmental Requirements

C. aspersum is a generalist species, found in a large range of habitats and climates, from Mediterranean to temperate, oceanic and tropical (Chevallier, 1977).

Activity necessitates a temperature of between 7 and 28°C and an elevated humidity of 75-90% (Daguzan, 1980). If conditions are unfavourable, adult snails are able to remain dormant for several months. However, they cannot withstand long periods of frost (Ansart et al., 2002). Eggs are particularly sensitive to dehydration (Machin, 1975; Riddle, 1983) and cold temperatures (Le Calvé, 1995; Ansart et al., 2007), such that in temperate regions egg deposition occurs only in spring and autumn (Madec and Daguzan, 1993).

Habitats are also highly variable, but the snails preferentially choose microhabitats with greater light intensity and structural complexity, offering more dormancy retreats and better protection against predators, and reject dimly lit microhabitats with smooth substrates (Perea et al., 2007). As the availability of calcium is critical for shell construction, richness in calcium is often suggested as an important criterion for microhabitat selection (e.g. Crowell, 1973).

In culture, crowding rapidly and persistently affects growth of juveniles, with accumulation of slime and faecal matter suggested as having a negative effect on individuals (Cowie and Cain, 1983; Lucarz and Gomot, 1985; Jess and Marks, 1995).

To date, no natural enemy specific to C. aspersum is known. Terrestrial snails are a food source for many animals, including mammals (rodents, hedgehogs, shrews, badgers, wild boars, mustelids), many bird species (magpies, thrushes, blackbirds, ducks, owls), reptiles (lizards, turtles, snakes), amphibians (frogs, salamanders, newts), myriapods, insects (some Diptera, Carabidae, Staphylinidae, Lampyridae, Silphidae), planarians, spiders (Porrhothele antipodiana) and predatory terrestrial snails (e.g. Euglandina rosea, Rumina decollata, used for biological control) (for complete information, see Barker, 2004). In certain regions, human predation for consumption purposes can also be important.

Some ectoparasite species have also been described, such as the hematophageous mite Riccardoella limacum, living in the lung cavity of terrestrial gastropods. Flechtmann and Baggio (1985) studied the effects of R. limacum on C. aspersum and reported that when the mite population was sufficiently high, there was high mortality among snails. It has also been shown to influence life history (decreased activity, reproductive output and winter survival) in a related species (Schüpbach and Baur, 2008).

Endoparasitic nematodes (Alloionema appendiculatum, Nemhelix bakeri, Phasmarhabditis hermaphrodita, Rhabditis maupasi, Angiostoma aspersae) can also affect reproduction or cause mortality, particularly in rearing farms (Morand et al., 2004).

Epizootic diseases, regularly appearing during the dry season in C. aspersum rearing farms, have been related to pathogenic strains of the bacterium Aeromonas hydrophila (Kiebre-Toe et al., 2005) and yellow fluorescence leading to death could be caused by pigment-forming bacteria of the genus Pseudomonas (Raut, 2004).

Eggs of C. aspersum can be invaded by microbes, notably fungi. The most frequently described fungus is a Fusarium species, responsible for 'pink clutches', triggering egg degradation (Meynadier et al., 1979).

General

• Laboratory use
• Pet/aquarium trade
• Research model
• Sociocultural value

Human food and beverage

• Eggs
• Meat/fat/offal/blood/bone (whole, cut, fresh, frozen, canned, cured, processed or smoked)

Materials

• Cosmetics

Medicinal, pharmaceutical

• Source of medicine/pharmaceutical

The following information is from the Canadian Food Inspection Agency Cornu aspersum fact sheet (CFIA, 2014).

Indications of an attack by C. aspersum are ragged holes chewed in leaves, with large veins usually remaining; holes in fruit; and slime trails and excrement on plant material.

Adults and larger juveniles are likely to be visible among the host material or attached to the transporting containers. They may also be hidden in protected locations, sealed into their shells to avoid desiccation. Check the undersides of containers and their rims. Small snails and eggs in soil could be difficult to find. C. aspersum hides in crevices and will overwinter in stony ground.

Inspections are best carried out under wet, warm and dark conditions. Under bright, dry conditions it is necessary to thoroughly search dark, sheltered areas where the humidity is elevated, such as under low-growing plants or debris. The snails may bury themselves in loose soil or other matter, so the only way to be reasonably sure an area is not infested is to make repeated surveys over a long period of time.

In North Africa, native populations of C. aspersum exhibit conspicuous shell variation that led to the description of many subspecies or varieties on the basis of shell shape and colour (e.g. Germain, 1908; Taylor, 1914; Chevallier, 1977). However, distinguishing discrete forms is difficult because of the high number of loci involved in the colour variation (Albuquerque de Matos, 1984) as well as in shell size and shape. Three main forms, namely aspersa, maxima and conoidea, are easily recognized and frequently encountered in the Maghreb. The giant form maxima (D > 40 mm) can be distinguished by its pigmentation (dark colour of the mantle edge) and by a very large shell aperture (megalostoma). The form conoidea is characterized by a ratio of width to height > 1. Only the form aspersa (the common garden snail) is widespread around the world, in regions with Mediterranean, temperate and even subtropical climates. The forms aspersa and maxima are the two forms that are commonly reared.

Confusion with other species is highly improbable. In the escargotières of the Maghreb a common mistake is to misidentify Helix melanostoma as C. aspersum, but live specimens of H. melanostoma have a thicker and paler shell that has a dark peristome without a reflected lip.

In other countries, i.e. in northwest Europe and other more recently colonized regions, there is no risk of confusion.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

SPS Measures

The Canadian Food Inspection Agency (CFIA, 2014) considers C. aspersum to be a plant pest and have quarantines established for preventing the importation of the snail in plant and soil matter. The United States Department of Agriculture requires a permit to import snails (or slugs) into the USA and between states within the USA from APHIS, Animal Plant Health and Inspection Service (APHIS, 2002).

Public Awareness

APHIS (2009) encourages people using C. aspersum in classrooms, nature facilities, or keeping them as pets to turn them in voluntarily. Some US states have created an ‘invasive species hotline' (e.g. Oregon, Hawaii, Michigan) for early detection of potential pest invaders. Residents of the state of Hawaii are asked to be on the lookout for this pest and to call a resident agricultural specialist on their respective island if they see this snail. It is currently recorded on only two of the Hawaiian Islands (Cowie, 1997; Cowie et al., 2008)

Containment/Zoning

In the USA, APHIS has produced containment guidelines to assist a researcher, educator, or commercial entity to design, build, maintain and operate a facility for rearing nonindigenous snails in the USA, including C. aspersum (APHIS, 2002), the non-respect of which is subject to civil and/or criminal penalties and loss of permits.

Control

Physical/Mechanical and Cultural Control

Handpicking with subsequent destruction of animals is the oldest method of control of pest molluscs. Even though it is time intensive, it can be effective as C. aspersum is gregarious and can be found in huge numbers under snail traps. Other places around susceptible plants where snails can hide during daily or seasonal dormancy, such as boards, debris and dense cover such as ivy, branches growing near the ground, stones, must be removed (Barker and Watts, 2002; Dreistadt et al., 2004).

Physical barriers such as continuous lines of wood ash, sand or diatomaceous earth can be effective for short-term control but their effectiveness is drastically reduced once they become wet (Davis et al., 2004; Dreistadt et al., 2004). Lines of lime and copper sulphate or copper screen are repellent to snails and can be used to prevent movement into an area. Various other organic and inorganic substances have been used as barriers to prevent snails getting access to plants. These barriers have proved to be moderately successful, and their effectiveness is greatly improved as part of an IPM approach along with pit-fall traps and hand collecting.

Containers filled with beer attract slugs and snails, but beer must be replaced regularly to be effective.

Bordeaux mixture (a copper sulphate and hydrated lime mixture) can be brushed on tree trunks to repel snails, one treatment should last about a year (Dreistadt et al., 2004). Thin copper sheets can also be wrapped around tree trunks to prevent snails from climbing into the canopy (Davis et al., 2004; Dreistadt et al., 2004). Du Toit and Brink (1993) tested treatments for the control of C. aspersum in citrus trees in South Africa, including chemical control using copper sulphate, metaldehyde and copper oxychloride, but the only treatment that reduced snails to an average of less than two per tree after 29 weeks was a copper strip around the stem preventing snails from climbing up it. The strips were 0.127 mm thick and 50 mm wide, and were fastened 500 mm from the ground with a paper-clip. They were still effective 12 months after application, and are recommended as a long-term control method. Removal of the lower branches of trees by pruning can minimize the contact of the foliage with the ground and help to reduce the number of C. aspersum on the trees. In California, skirt-pruning associated with copper sheets on the trunk and handpicking of snails stopped by this barrier can be very effective (Sakovich, 2002); and although no longer practised, running cultivation equipment through an orchard several times a year contributed to control of C. aspersum through destruction of buried egg clutches and snails buried in the ground (Sakovich, 2002).

Biological Control

As terrestrial molluscs have many natural enemies, there has been strong interest in the biological control of C. aspersum using other, predatory snails (e.g. Fisher and Orth, 1985). However, as most of these predatory snails are not host-specific, they are not appropriate to use in control programmes in which effects on non-target species are of concern (Cowie, 2001: Barker and Watts, 2002).

There have been several attempts to develop biological control of C. aspersum in California, South Africa and New Zealand, which began with the introduction of predaceous snails (Euglandina rosea, Gonaxis sp.) and beetles during the 1950s and early 1960s (for more information see Fisher and Orth, 1985; Barker and Efford, 2004). These efforts were largely unsuccessful, although one staphylinid beetle (Staphylinus (Ocypus) olens) showed potential; however, the use of this species as a biological control in orchards has not been actively pursued (Sakovich, 2002). In 1966, however, another (opportunistic) predaceous snail, the decollate snail Rumina decollata (of European origin) was found to have invaded California (see Pictures). Experimental releases of R. decollata in southern California citrus orchards were begun in 1975 and, in most cases, resulted in complete control (displacement) of C. aspersum (Fisher and Orth, 1985). Rumina decollata is now used to control C. aspersum in some 20,000 ha of citrus in southern California, but is currently permitted only in certain Californian counties (Dreistadt et al., 2004). As this predatory snail consumes young to half-grown snails, control is achieved only in 4-6 years. Sakovich (2002) recommended first using molluscicidal baits to reduce the population, and then combining skirt-pruning and copper barriers with introduction of R. decollata. Once control by R. decollata is achieved, maintenance of copper barriers can cease, R. decollata can be harvested and transferred to new areas. However, Cowie (2001) expressed concern regarding both the effectiveness of R. decollata in control of C. aspersum, its potential impacts on native (even endangered) species and its potential as a garden plant pest.

A study by Altieri et al. (1982) was carried out in a daisy field in northern California to determine the effectiveness of the indigenous coleopterous predator Scaphinotus striatopunctatus in the biological control of C. aspersum. Release of the predator in the field under light metal sheets, together with colonization by garter snakes (Thamnophis elegans) from an adjacent field, resulted in a significant reduction in snail populations.

In South Africa, the native predacious gastropod Natalina cafra was investigated as a potential biological control agent against C. aspersum, with special attention to the possibility of establishing a viable population of the natural enemy in captivity (Joubert, 1993), but this approach seems not to have been implemented.

Research by the Entomology Division of the Plant Protection Department, Cukurova University, Turkey, on the importation of predators and parasitoids as biological control agents (mainly for citrus pests) included the coccinellid Hippodamia convergens as a potential predator of C. aspersum (Uygun and Sekeroglu, 1987).

Ducks, chickens or guinea fowl can provide long-term control in citrus orchards and vineyards, if an appropriate breed is chosen and properly cared for. Growers take the animals each morning into the orchard for as little as half an hour to scavenge for food. This solution can be very effective but involves extra labour in managing the animals and protecting them from predators (Sakovich, 2002; Davis et al., 2004).

Chemical Control

Application of molluscicides is the most widely implemented approach to controlling snail pests. However, they do not totally control snails and must be used properly and in conjunction with other methods (physical, biological) in integrated management (Flint, 2003). Molluscicides are generally delivered as baits (e.g. de Boodt et al., 1990), and also exist as sprays and dusts but have proved less effective under these formulations. There are three major classes of compounds present in molluscicides: metaldehyde (induces desiccation and loss of locomotor activity), carbamates (causes paralysis and loss of muscle tone), and metal chelates such as iron EDTA (interacts with oxygen uptake and triggers arrest of feeding) (Barker and Watts, 2002).

However, baits are toxic to the decollate snail and to other non-pest gastropods, and certain materials may poison pets (Dreistadt et al., 2004).

To be effective, baits must be sprinkled preferentially when the snails are active (Flint, 2003). The formulation and dose to apply will depend on environmental conditions (for more information, see Barker and Watts, 2002).

Laboratory and field experiments have been conducted in Egypt to investigate the molluscicidal activity of weed extracts from Ambrosia martine, Citrullus colocynthis, Cymbopogon proximus and Glinus sp., and of five fertilizers, against C. aspersum. Ambrosia martine extract was shown to be the most toxic, followed by C. colocynthis and C. proximus, while Glinus sp. was the least effective. Snail mortality increased gradually with time. Some commonly used fertilizers showed considerable initial and residual molluscicidal effects. Ferrous sulfate was the most effective against C. aspersum, followed by ammonium nitrate, ammonium sulfate and superphosphate (Zidan et al., 1997).

A patented copper complex, formulated as a spray and referred to as copper silicate, shows molluscicidal activity (Davis et al., 1996). The product was tested in both spray and granular formulations against C. aspersum in a vineyard and a citrus orchard in Western Australia, where it was compared with copper oxychloride spray. The copper complex spray demonstrated a high level of repellence to C. aspersum for extended periods in the field, and proved superior to the other products tested in reducing snail numbers in vines/trees. The product has potential in the management of pest molluscs, especially as part of IPM programmes, but it seems not to have been further developed.

Control by Utilization

In some places gathering snails free for personal consumption is permitted, e.g. from artichoke, kiwifruit, avocado and citrus growers, although it is recommended to purge them at least 3 days to remove traces of poison baits that might have been ingested.

Much work on C. aspersum has been conducted on aspects of its life history under artificial conditions because of the economic value of heliciculture. Some fundamental points of its biology have also been studied from an evolutionary point of view. However, only a few studies deal with the ecology of natural populations (demography, dispersal, interspecific interactions), and no-one has studied phenotypic evolution in response to global change, i.e. adaptive responses to the variation in landscape structure and environmental pressures. Studies on resistance to molluscicides (present formulations do not seem to be very effective against C. aspersum) and to pesticides more generally, would be valuable because these substances have direct toxic effects that may result in enhanced mortality and/or adaptive responses.

Thus, a research programme based on theoretical and empirical studies that would lead to generalizations on (i) the genetic architecture of the combinations of life-history characteristics involved in invasions, (ii) responses to selection and (iii) the model of spread of populations in recently invaded areas is clearly needed. Such a programme is predicated on invaded areas being identified beforehand.

07/08/15 Updated by:

Robert Cowie, Pacific Biosciences Research Centre, University of Hawaii, Honolulu, Hawaii, USA

24/07/09 Original text (for ISC) by:

Armelle Ansart, Université de Rennes 1, 263 Av. du Gal Leclerc, CS 74205, 35042 Rennes Cedex, France

Luc Madec, Université de Rennes UMR CNRS 6553 ECOBIO, Campus de Beaulieu, Bât. 14A Av du Général Leclerc, F-35042 Rennes Cedex, France

Annie Guiller, Laboratoire de Parasitologie, UMR CNRS 6553, Université de Rennes, Av. du Pr. Léon Bernard, 35043 Rennes, France