Arion vulgaris
(Spanish slug)

Arion vulgaris is a slug native to southern France and Catalonia in Spain, that has since spread to much of central, northern and eastern Europe. It is considered to be invasive across western and central Europe, from the Pyrenees to eastern Poland and from southern France to north Italy, Austria and Slovakia and within an isolated range in eastern Bulgaria.

The invasiveness of A. vulgaris is related to several factors. One is its propensity for hybridizing with native large Arion species to produce vigorous adaptable forms. Its ability to colonize environments disturbed by human activities is also of major importance – for example in one study 99% of Swedish records were from synanthropic habitats and only 1% from natural woodlands – as proximity to humans comes with the possibility of passive dispersal through trade, illegal dumping (fly-tipping) of garden waste and particularly transport on living plants. The garden centre trade and horticulture are particularly implicated. In Poland, there is evidence from studies of molecular diversity that A. vulgaris has originated from repeated, separate introductions from other parts of Europe. The ability of A. vulgaris to utilize a great variety of food sources and types has been well-documented and must aid dispersal and colonization. In gardens, plant diversity has been shown to be positively associated with abundance of A. vulgaris.

Moquin-Tandon’s description of A. vulgaris relates to a species inhabiting southern France (Moquin-Tandon, 1855). A. lusitanicus sensu Mabille, on the contrary, is restricted to Portugal (Quinteiro et al., 2005). In addition, there is no evidence to suggest that A. vulgaris occurs on the Iberian Peninsula outside the southern Pyrenees of Catalunya (Chevallier, 1981; Castillejo, 1998; Quinteiro et al., 2005). Theories for its area of origin range from southern Europe (Schmid, 1970) to the Alpine Region. Chevallier (1981) gives a map showing its French range, which is mainly Pyrenean rather than alpine.

Arion vulgaris has been spread by international trade from central and south France and can now be found throughout much of central, northern and eastern Europe. In many regions, it is well-established and has partly replaced the native large arionids: A. rufus and A. ater, especially in anthropogenic habitats, to become a serious pest (Fechter and Falkner, 1990; Turner et al., 1998). There has been considerable interbreeding with rufus and ater throughout its range (Zemanova et al., 2017) and the resulting vigorous hybrids may contribute to notable local increases in numbers. A. vulgaris appears uncommon at higher altitudes, at least in the Alps (ca. 1600 m above sea level; Turner et al., 1998) although this may not apply to the Pyrenees (Chevallier, 1981). Often, there is a clear gradation from pure A. vulgaris in valleys, through hybrids on the slopes, to A. rufus at higher altitudes in European mountains, paralleling ecological gradients (Zemanova et al., 2017).

Extensions in its range have been recorded in a number of countries since 2000. It is now confirmed from Ireland (Anderson, 2010), where it was earlier confused with other taxa. Similarly, it has been identified in the UK mainland, occurring sporadically in disturbed habitats of SE England (Davies, 1987). More recently (2015), it has been cited as especially numerous from localities around Norwich (JIC, 2021). It has recently been found in Iceland for the first time (Ingimarsdóttir and Ólafsson, 2005). Outside Europe, there is a suspected occurrence in the Falkland Islands (Proschwitz, 1988). This report has not been confirmed because the specimen involved was badly damaged. There is also a single unconfirmed report from Cornell University in the USA from October 1998.

It is likely that A. vulgaris will continue to spread northwards and especially eastwards in Europe, to the Baltic States and Russia (Proschwitz, 1997a). It is likely this range expansion will be facilitated by its frequent hybridization with local species of large Arion, allowing vulgaris to assimilate locally adaptive traits (Knop and Reusser, 2012; Zemanova et al., 2017). It may be expected to turn up eventually in the Antipodes and in North America.

A survey of introduced slugs in California, USA, categorizing taxa only by their ITS sequencing using identified UK material as standards, found only A. rufus (R McDonnell, UCLA, personal communication, 2008). But use of only limited numbers of molecular markers provides little power to determine if these animals are rufus or vulgaris-rufus hybrids.

Many authors refer to its propensity for transport with plant materials through the horticultural trade and the gardening trade, a characteristic facilitated by frequently high population densities.

Arion vulgaris is most frequent in agricultural and horticultural habitats with permanent, dense vegetation, such as grassland, fallows and gardens and is often abundant in compost heaps (Dörler et al., 2018). In low numbers, it is present in most agricultural and horticultural land and also increasingly in natural habitats (Fechter and Falkner, 1990; Turner et al., 1998).

Arion vulgaris is a serious pest of diverse vegetable crops (especially Brassicaceae, lettuce, cucurbits), vegetable seedlings, arable crops (Triticeae), forage (helianthus), ornamental plants, low-growing fruits (strawberries) and herbs within gardens, regularly causing severe losses (Dörler et al., 2018). In the early stages of arable crop development (after seedling emergence or after planting), the plants are seriously defoliated or completely destroyed. The leaves, flowers or fruit may be damaged with feeding holes and the potential harvest devalued. In Austria, serious damage to arable agriculture has been reported (Reischütz, 1984). In Poland, A. vulgaris was found to feed on a wide range of plants, including arable crops and commonly occurring weeds. Significantly, vulgaris was found to destroy far more seeds in its gut than native gastropod species (Blattmann et al., 2013). Slug damage was found on 103 plant species (including wild species), preferred crops including Brassica napus (Kozlowski and Kaluski, 2004; Kozlowski, 2005).

Genetics

Arion vulgaris shows comparatively little genetic variation through its central European range (Quinteiro et al., 2005). Material from Switzerland (Zemanova et al., 2017), Britain, France, Italy and Norway (Hatteland et al., 2015) shows genetic evidence of hybridization and introgression with A. ater and A. rufus. These observations are concordant with data on metabolic rates of individuals of these species from Sweden (Hagnell et al., 2003) which appear to suggest hybridization between A. vulgaris and A. rufus.

Interpretation of these data is complicated by the suggestion (Chevallier, 1981) that the A. ater complex in Europe comprises species additional to those presently recognized, i.e. A. ater, A. rufus and A. vulgaris.

Reproductive Biology

Arion vulgaris has an annual life cycle (Davies, 1987). In the British Isles, mating takes place between late July and early September. Courtship starts after sunset and copulation may take between 2 and 3 h (Davies, 1987). The eggs are laid between September and November and the adults then die off. The juveniles appear in spring (March/April) and grow rapidly to maturity in June/July. 

The life cycle is flexible, adapting to local conditions with two generations thought to occur in especially favourable (warm, moist) environments, with a biennial life cycle adopted in less favourable high altitude sites (Knop and Reusser, 2012).  

Although A. vulgaris can self-fertilise, populations of A. vulgaris reproduce more often by outcrossing 

Physiology and Phenology

Due to recent expansions in range, the difficulty in distinguishing A. vulgaris from related species and the possibility of widespread hybridization and introgression, few reliable observations on regional phenotypic variability are reported. 

Chevallier (1977) noted a tendency for dark-coloured individuals to occur at higher altitudes in the species native range in France and for brighter, reddish-coloured individuals to predominate in warmer regions and at lower altitudes. Observations consistent with hybridization of local species such as A. ater and A. rufus in these locations (Zemanova et al., 2017), producing highly plastic and adaptable forms locally (Knop and Reusser, 2012).

No natural enemies have yet been shown to substantially reduce the populations of A. vulgaris. 

General

• Laboratory use
• Research model

The occurrence of A. vulgaris in transported plant materials may involve the adults, juveniles or eggs. 

The adults and juveniles are active after dark and may be detected in the evening or early morning, or by inspecting plant materials stored under cover. Like Deroceras reticulatum, all stages hide under debris, stones and wood and occasionally in the soil around root systems. Smaller stages may hide in leaf whorls.

The eggs are deposited on the soil, under dead leaves or other surface debris and are not buried in the soil. 

Traps containing molluscicides (metaldehyde, carbamate, iron pyrophosphate hydrate) may be used to collect material, but hand collecting is often as efficient and avoids the risk of contaminating produce.

Arion vulgaris belongs to a clade of large round-back slugs inhabiting central, western and southern Europe. These include A. rufus, which has been dispersed with trade to western North America. A. vulgaris and A. rufus are closely related and able to hybridize (Hagnell et al., 2003, Hatteland et al., 2015; Zemanova et al., 2017).

Unfortunately, it is very difficult to distinguish between these species based on external morphology. Reliable methods of discrimination include: dissection of the distal genitalia in mature individuals (Noble 1992; Hatteland et al., 2015); the use of ITS1 sequencing data based on known reference material and microsatellite markers (Zemanova et al., 2017). The spermatophores exchanged during sexual intercourse are highly diagnostic, but they are unlikely to be available routinely. A summary of the internal and external characteristics of A. vulgaris are given by Davies (1987). Quick (1960) provides a useful summary for A. rufus.

Colour phases are often difficult to attribute to this species due to its frequent hybridization and introgression with endemic species: a dark phase and a light phase, are recognized but are most likely a consequence of introgression. 

The dark phase (red-brown, black body colours) of A. vulgaris has a sub-dermal black pigment masked by an overlying reddish pigment. That the red pigment responds to environmental conditions and varies from bright red-brown (warm, dry conditions) to dark-brown or black (cold conditions) (Chevallier, 1977) is likely a consequence of introgression with local species, corresponding with the distribution of either A. rufus (warmer drier conditions) and A. ater (cooler wetter conditions). Colour can alter within a generation if specimens are transplanted from warm to cold areas, e.g. an increase in altitude or movement further north (R Anderson, [address available from CABI], personal communication, 2008). The foot fringe colour matches the overall body colour in the dark phase. In the dark phase of A. rufus, there is no indication of contrasting pigment layers in the dermis. In addition, the foot fringe is brighter, yellow or red to orange, usually contrasting markedly with (duller) body colours that may include grey or pure black. 

The pale phase of A. vulgaris (yellowish to pale-brown body colours) lacks the contrasting dark subdermal pigment and the skin layers are uniform in colour. The pale phase is difficult to distinguish from A. rufus although the latter still tends to have brighter red to orange, contrasting foot fringes, whereas in A. vulgaris the foot fringe and body colours match better.   

Populations of A. vulgaris reproduce more often by outcrossing than by selfing, although this is still possible in the absence of mates. Together with other large Arions such as A. rufus and A. ater, which are facultatively self-fertile, often producing a proportion of each egg clutch by self-fertilization, A. vulgaris lies at the end of a continuum which runs from preferential to occasional facultative self-fertilization.

Resting individuals of A. rufusand A. ater, when stimulated by stroking the back firmly, may react by rolling the body from side to side while in the contracted position. This reaction is not characteristic of A. vulgaris, although individuals which are the product of introgression may show a similar response (see Davies, 1987).

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.

Early Warning Systems

No warning system is available for A. vulgaris. However, in habitats that have been undisturbed for at least 2 years, A. vulgaris often occurs in high numbers and is likely to cause damage.

Cultural Control

Ploughing and other methods of soil cultivation significantly reduce slug populations, and the removal of vegetation deprives A. vulgaris of shelters. In recent years the establishment of fallows has been encouraged in many European countries as part of government-subsidized set-aside programmes. Along with grassland, fallows and other undisturbed habitats are excellent habitats for A. vulgaris. In these habitats, A. vulgaris builds up large populations rapidly and migrates into adjacent crops, often completely destroying the nearest few metres of the field. Thus, crops sensitive to slug feeding should not be sown or planted next to undisturbed habitats and undisturbed habitats should not be created next to a slug sensitive crop.

Biological Control

The nematode Phasmarhabditis hermaphrodita (Rhabditidae) is a moderately effective biocontrol agent for many slug species (Wilson et al., 1993; Glen et al., 1996). Juvenile A. vulgaris are susceptible to P. hermaphrodita, but larger specimens are not (B Speiser, FiBL, Frick, Switzerland, unpublished data).

Chinese ducks are successfully used to control slugs in horticultural crops (B Grimm, Institute of Zoology, Graz, Austria, personal communication; B Speiser, FiBL, Frick, Switzerland) but their use is limited by the labour requirement of herd management. If ducks swallow A. vulgaris, they need access to water to clean their beaks (Sulzberger, 1996). The successful use of ducks for control of snails is described by Sakovich (1996).

Chemical Control

For chemical control of A. vulgaris, bait pellets are normally used. These contain either metaldehyde or a carbamate as active ingredient. The pellets are broadcast on the soil. If A. vulgaris migrates into the crop from an adjacent habitat, it may be sufficient to treat a narrow strip (0.5 m) with twice the dosage recommended for broadcast application (Friedli and Frank, 1998).

Host Resistance

There is little scope to reduce damage by A. vulgaris through the use of resistant varieties.

Quantification of economic impacts is missing from almost all evaluations and should be followed up. An inherent difficulty with such quantification will be recognizing the impact of hybrids and introgressed endemic species, the activity of which is attributable to A. vulgaris.

The status of A. vulgaris in the USA is unclear. Apart from a uncorroborated report on the internet for Cornell University, USA in 1998 no evidence for its occurrence there has been adduced. Despite this, almost the whole of the continental USA falls within the latitudinal and climatic parameters suitable for this species. The relevant authorities are aware of the potential problem, but more studies addressing the identity of introduced large arionids in the USA are required. A molecular study in California, USA (R McDonnell, UCLA, personal communication, 2008) suggests that the invasive, large arionids in that area at least, are attributable to A. rufus; although using a single molecular marker cannot determine the level of hybridization and introgression in these introduced populations.

Europe: DAISIE - Delivering Alien Invasive Species Inventories for Europe, Web-based service, http://www.europe-aliens.org

UK: Malacological Society of London, Canterbury Christ Church University , Kent, CT1 1QU, http://www.malacsoc.org.uk/

07/04/2008 Updated by:

Roy Anderson, Consultant, UK

04/12/2019 Updated by:

Leslie R Noble, Nord University, Norway