Arion vulgaris
(Spanish slug)

The invasiveness of A. vulgaris is related to several factors. Its ability and readiness to colonize humanly-disturbed environments is of major importance. Proschwitz (1997) observed that 99% of Swedish records were from synanthropic habitats and only 1% from natural woodlands. With a proximity to humans, comes the possibility of passive dispersal through trade, particularly in living plants. The garden centre trade and horticulture are particularly implicated (Weidema, 2006). In Poland, there is evidence from studies of molecular diversity that A. vulgaris has originated from repeated, separate introductions from other parts of Europe (Soroka et al., 2007). 

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. 

Other than its country of origin (France), 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.

Moquin-Tandon’s description of A. vulgaris relates to a species inhabiting southern France (Moquin-Tandon, 1855). Arion lusitanicus sensu Mabille, on the contrary, is restricted to Portugal (Quinteiro et al., 2005). In addition, there is no evidence to suggest that 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 (SM Davies, [address available from CABI], personal communication, undated). Chevallier (1981) gives a map showing its French range, which is mainly Pyrenean rather than alpine.

A. 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: Arion rufus and Arion ater, especially in anthropogenic habitats, to become a serious pest (Fechter and Falkner, 1990; Turner et al., 1998). It appears to be 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).

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. It has recently been found in Iceland for the first time. There is a single unconfirmed report from Cornell University in the USA (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, 1997). It may be expected to turn up eventually in the Antipodes and in North America.  

A survey of introduced slugs in California, USA, in which ITS sequencing based on UK material of the species was used, found only Arion rufus (R McDonnell, UCLA, personal communication, 2008).  

Many authors refer to its propensity for transport with plant materials through the horticultural trade and the gardening trade.

A. vulgaris is most frequent in agricultural and horticultural habitats with permanent, dense vegetation, such as grassland, fallows and gardens, and is abundant in compost heaps. 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).
 

A. vulgaris is a serious pest of diverse vegetable crops (especially Brassicaceae, lettuce, cucurbits), vegetable seedlings, arable crops (Triticaceae), ornamental plants, low-growing fruits (strawberries) and herbs within gardens in Central Europe, regularly causing severe losses. 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. Slug damage was found on 103 plant species (including wild species) and preferred crops including Brassica napus (Kozlowski and Kaluski, 2004; Kozlowski, 2005).

Genetics

A. vulgaris shows comparatively little genetic variation through its central European range (Quinteiro et al., 2005). Material from Britain, France and Italy gives no genetic indication of hybridisation with Arion ater or A. rufus. However, data on metabolic rates of individuals of these species from Sweden (Hagnell et al., 2003) appear to suggest hybridisation between A. vulgaris and A. rufus. 

Interpretation of these data is complicated by the suggestion (Chevallier, 1981; SM Davies, [address available from CABI], personal communication, 2008) 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

A. 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 hours (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. 

It has been suggested that two generations can occur in especially favourable (warm, moist) environments.  

Physiology and Phenology

Due to the recent expansions in the range and the difficulty in distinguishing A. vulgaris from related species, few 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.  

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 in leaf whorls, under debris, stones and wood and occasionally in the soil around root systems. 

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 just as efficient and avoids the risk of contaminating produce.

A. vulgaris belongs to a clade of large round-back slugs inhabiting central, western and southern Europe. These include Arion 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). 

Unfortunately, it is very difficult to distinguish between these two species based on external morphology. Reliable methods of discrimination include: dissection of the proximal genitalia in mature individuals; the use of ITS1 sequencing data based on known reference material. 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.  

There are two colour phases: a dark phase and a light phase, which can occur mixed in populations. 

The dark phase (red-brown, black body colours) of A. vulgaris has a sub-dermal black pigment masked by an overlying reddish pigment. 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). 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 by sexual means than by selfing. This is in contrast to other large Arions such as A. rufus, which commonly self. Discovery of copulating individuals, particularly during daylight hours, would tend to indicate A. vulgaris rather than A. rufus or Arion ater. 

Resting individuals of A. rufus, when stimulated by stroking the back firmly, will often react by rolling the body from side to side while in the contracted position. This reaction is extremely rare or non-existent in A. vulgaris (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 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 an effective biocontrol agent for many slug species (Wilson et al., 1993; Glen et al., 1996). Juvenile A. vulgaris are highly susceptible to P. hermaphrodita, but larger specimens are not susceptible (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. 

The status of A. vulgaris in the USA is unclear. Apart from an uncorroborated report on the internet (October 1998) for Cornell University, USA, no evidence for its occurrence there has been adduced. Despite this, almost the whole of 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. It has been established by a molecular study in California, USA (R McDonnell, UCLA, personal communication, 2008) that the invasive, large arionids in that area at least, are attributable to Arion rufus. 

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