Deroceras invadens
(tramp slug)

Pictures

Picture	Title	Caption	Copyright
Title Adult Caption Deroceras invadens (tramp slug); adult, extended. In garigue. Menorca, Spain. Copyright ©Roy Anderson	Adult	Deroceras invadens (tramp slug); adult, extended. In garigue. Menorca, Spain.	©Roy Anderson
Title Adult Caption Deroceras invadens (tramp slug); adult, retracted. In a garden. Belfast, Ireland. Copyright ©Roy Anderson	Adult	Deroceras invadens (tramp slug); adult, retracted. In a garden. Belfast, Ireland.	©Roy Anderson
Title Adult Caption Deroceras invadens (tramp slug); adult, partially extended. In heathland. Co. Londonderry, Ireland. Copyright ©Roy Anderson	Adult	Deroceras invadens (tramp slug); adult, partially extended. In heathland. Co. Londonderry, Ireland.	©Roy Anderson

Identity

Preferred Scientific Name

• Deroceras invadens

Preferred Common Name

• tramp slug

Other Scientific Names

• Agriolimax caruanae Pollonera
• Deroceras caruanae
• Deroceras meridionale Reygrobellet
• Deroceras panormitanum Lessona and Pollonera
• Deroceras pollonerae Pollonera

International Common Names

• English: brown field slug; longneck field slug; Sicilian slug
• French: petite loche maritime

Local Common Names

• Germany: Mittelmeerackerschnecke; Mittelmeerackerschnecke

Summary of Invasiveness

D. invadens is a small, agile slug that is native to the Mediterranean and has been recorded from at least 46 countries worldwide. Until 2011, this species was known as D. panormitanum but molecular work revealed that it comprised two distinct species. This species is similar in appearance to D. laeve and as a result, the exact distribution and impact of this species is unknown. This is a particular problem in countries such as the USA and Australia and probably also in South America. D. invadens is regarded as a significant pest of agricultural crops in New Zealand (Barker, 1999) but is highly likely to be damaging in many other countries as well. References to slug damage in agricultural crops by D. laeve are very likely to refer to D. invadens. In addition to this, D. invadens is an aggressive slug which may compete with native slugs, decreasing biodiversity.

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Metazoa
•         Phylum: Mollusca
•             Class: Gastropoda
•                 Subclass: Pulmonata
•                     Order: Stylommatophora
•                         Suborder: Sigmurethra
•                             Unknown: Limacoidea
•                                 Family: Limacidae
•                                     Genus: Deroceras
•                                         Species: Deroceras invadens

Notes on Taxonomy and Nomenclature

D. invadens, formerly referred to in the scientific literature as D. panormitanum Lessona & Pollonera, 1882, and before that as D. caruanae Pollonera, 1891 or Agriolimax caruanae Pollonera, 1891, has had a long and confusing history of misidentification and misapplication of names.

Not until the age of DNA bar-coding (in tandem with extensive behavioural studies) has it been possible to untangle a skein of confused taxa with an extensive list of names which have been applied or mis-applied to this taxon (Reise et al., 2011). The origins of D. invadens are currently unknown but are likely to be somewhere in or around Italy. The name formerly applied to what will be called here the tramp slug, i.e. D. panormitanum, relates to a different species native to Sicily and Malta (Reise et al., 2011). It appears that two other names in circulation, D. pollonerae Simroth, 1889 and D. caruanae are synonyms of D. panormitanum.

Reise et al. (2011) performed breeding experiments and DNA analysis on material from Sicily, Malta, Sardinia and England. These confirmed that two closely similar taxa exist, D. invadens the tramp slug with a worldwide distribution and D. panormitanum (=pollonerae = caruanae) native to Sicily and Malta and now found also at single sites in Wales and Ireland (Rowson et al., 2016).

Description

D. invadens is a small, agile, slug species with a reputation for pugnacity towards other slugs. Size range is 25-35 mm. The body is cylindrical, narrowing to a short but strongly truncate keel at the tail. The mantle is moderately large but less so proportionately than in D. laeve, so that the tail part of the body is clearly longer than the mantle. In living specimens the mantle is transversely wrinkled in front as in D. laeve. The body colour is variable. In Mediterranean countries a pinkish flesh-coloured ground colour is common with a translucent cuticle and few if any darker spots. This form can also occur in northern Europe. In north-west Europe two forms predominate, these are slightly or considerably darker colour forms. The most common is mid gray and translucent with lighter mantle, through the cuticle of which the shell and pale internal organs can be seen even in the field. There is a marbling of tiny darker spots, but these are difficult to see with the naked eye. In hilly or exposed areas a darker form occurs, with mid to dark grey ground colour and contrasting pale mantle on which darker spotting is particularly obvious. The respiratory pore is white-rimmed, more clearly marked in darkly pigmented specimens. The sole in most specimens is translucent grey and paler than upper body pigments. Pedal and body mucus is colourless. Internally D. invadens has a rounded, compact penis with two fairly symmetrical, slightly elongate and inturned, ‘side pockets’ comprising the penial caecum and penial lobe (see Reise et al., 2011).

Distribution

D. invadens is native to Italy and has been introduced widely into a number of countries around the world; it is now present in the continents of Africa, North America, South America, Central America, Europe and Oceania. The distribution of this species is believed to be under reported. This is in part due to lack of interest and the similarity with D. laeve (Forsyth, 2014).

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

D. invadens (as Agriolimax caruanae) was first reported outside its native range (which is probably in Italy), in England around 1930. It was then recorded in Australia in 1936. Post-World War II, it appears to have spread rapidly into north-west Europe and Iberia. In North America it was found in California by 1940, Canada by 1966, South America by 1962, and South Africa by 1963. Country specific dates can be found in the Introductions table. Despite rapid colonization, it has not yet been reported from the Russian Federation, Central Asia, or China. In most places from which this species is recorded, it has retained a close association with man and there is little indication of full naturalization in some of these countries (Hutchinson et al., 2014).

Introductions

Risk of Introduction

As world trade continues to grow, it is increasingly likely that D. invadens may appear in major conurbations around the world. Its affinity for environments disturbed by anthropogenic activitys, particularly gardens and commercial horticultural enterprises, its tolerance of a wide range of environmental temperatures and ability to adhere to clothing, animals and vehicles, makes it readily transportable and adaptable to new environments.

Habitat

D. invadens adapts well to disturbance and is a typical ‘garden’ species in many parts of its range. After introduction to new areas it is mostly associated with urban and roadside habitats, gardens and farm buildings (Rowson et al., 2014a). In Britain and Ireland, while expanding its range in the 1970s, it became very common at fly-tipping sites where garden rubbish was deposited.

Once established it can invade semi-natural woodland, rough pasture, wetland margins and agricultural fields in the general countryside especially where ground conditions are moist. In the warmer climate of the west Mediterranean it is restricted mainly in irrigated hotel gardens and in and around garden centres and has not been observed in dry forests or garigue, especially in the summer months.

Habitat List

Hosts/Species Affected

D. invadens is a generalist slug and has been recorded causing agricultural damage to crops. Examples of these species include; Asparagus officinalis, Avena sativa, Brassica napus, B. oleracea, B. rapa, Cucurbita maxima, C. pepo, Daucus carota, Franaria vesca, Hordeum vulgare, Lactuca sativa, Solanum tuberosum, Triticum aestivum and T. durum.

Host Plants and Other Plants Affected

Growth Stages

List of Symptoms/Signs

Biology and Ecology

Reproductive Biology

D. invadens is an annual species which is variable in its seasonality. It can self-fertilise but most reproduction occurs by outcrossing (Foltz et al., 1984). The eggs are small and are laid in clutches of up to 50 (Quick, 1960). The lifecycle of this species is short and up to three generations may exist per year (AnimalBase, 2015).

Physiology and Phenology

D. invadens is an agile slug and more irritable and fast moving than D. laeve. It has been observed to tail-flick and bite other slug species (Rowson et al., 2014a). It has no particular affinity for water, although it requires damp or well watered habitats.

Longevity

The maximum life span of D. invadens is about one year.

Activity Patterns

D. invadens can breed and is active at all times of year, except during drought or under freezing conditions.

Nutrition

Barker (1999) has observed D. invadens as a pest in gardens in New Zealand, but there are remarkably few observations on feeding habits or preferences elsewhere. In Ireland young specimens have frequently been observed doing damage between lettuce leaves (Lactuca sativa) offered for sale, so it is probably a serious pest in horticulture. No verifiable reports of damage to cereal or root crops has been reported although it is likely to cause damage to winter cereals.

Environmental Requirements

D. invadens has a reasonably wide temperature tolerance, but this is more biased towards environmental heat than cold. For example this species has been recorded widely in the tropics. The watery mucus it secretes requires readily available water, so it is intolerant of or inactive in, truly arid conditions. 

Climate

Natural enemies

Notes on Natural Enemies

There is little information on the natural enemies of D. invadens however, they likely include the European hedgehog (Erinaceus europaeus) slow worms, ground beetles, tachinid flies, toads and birds such as the runner duck, Anas platyrhynchos. In Europe and North America beetles of the genus Silpha (S. atrata in Europe) and Cychrus (C. caraboides in Europe) have evolved mouthparts and behaviour to specifically target pulmonate molluscs, including slugs. Some Diptera (Phoridae) are specific parasites of agriolimacid slugs (Robinson and Foote, 1968). 

Means of Movement and Dispersal

Natural Dispersal

D. invadens is an active and fast moving species which will naturally move locally into new areas.

Accidental Introduction

It is possible that D. invadens can adhere to clothing, tyres, vehicles etc. and be transported into new locations this way. In addition this species is likely to be accidentally introduced through the transportation of fresh produce, plants for the garden trade or in rubbish from gardens.

Pathway Causes

Pathway Vectors

Impact Summary

Economic Impact

References to slug damage of agricultural crops by D. laeve are very likely to refer widely to D. invadens. The impact on crops produced in temperate climates can be high, especially where grass is part of the cycle, or where horticulture is practiced. Winter cereals in temperate climates may be particularly at risk.

Environmental Impact

Impact on Biodiversity

D. invadens can be aggressive, wounding other slugs when in high-density populations; they may defend themselves by tail-wagging and slapping and by quickly fleeing (Rollo and Wellington, 1979; Rowson et al., 2014a). This, therefore, decreases the biodiversity of native slugs.

Risk and Impact Factors

• Proved invasive outside its native range
• Highly adaptable to different environments
• Is a habitat generalist
• Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
• Pioneering in disturbed areas
• Tolerant of shade
• Capable of securing and ingesting a wide range of food
• Benefits from human association (i.e. it is a human commensal)
• Fast growing
• Has high reproductive potential
• Reproduces asexually

• Negatively impacts agriculture
• Negatively impacts livelihoods
• Reduced native biodiversity
• Threat to/ loss of native species

• Competition - monopolizing resources
• Interaction with other invasive species

• Highly likely to be transported internationally accidentally
• Difficult to identify/detect in the field
• Difficult/costly to control

Uses

D. invadens may be used in laboratories as a research model.

Uses List

General

• Laboratory use
• Research model

Similarities to Other Species/Conditions

D. invadens and the Sicilian species D. panormitanum closely resemble each other in external characters. Careful comparison of populations of D. panormitanum from Cardiff, UK and Kilmacanogue south of Dublin, Ireland with D. invadens has allowed the following observations.

In size, general habitus and colouration, there is overlap between the two species. Medium to dark pigmented D. panormitanum, however, differ from similarly dark D.invadens in pigmentation of the mantle which seems always to be more contrastingly paler in D.invadens with white internal organs showing through, than it is in D.panormitanum. The dark colouration on the upper surface of D. panormitanum also frequently has a significant violaceous sheen which D.invadens does not possess. Paler forms of both species are less easily separated. Internally D. panormitanum has a similar layout of the penis as D. invadens, but the ‘side pockets’ are asymmetric and while the penial lobe is a more or less rounded lobe, the penial caecum is elongate and bent, tapering towards the tip (Reise et al., 2011). The pockets are almost symmetrical in D. invadens i.e. slightly elongate but of even width along their length (Wiktor, 2000).

D. invadens is also similar in appearance to D. laeve. D. laeve differs from both of the above in its smaller size, even, usually chestnut colour, lack of pale rim on the respiratory orifice, longer mantle and sole the same colour as the upper surface. D. laeve often has the penis missing or partly missing (Foltz et al., 1984). Where it is fully formed the shape is elongate and somewhat spirally twisted with no side pockets.

Prevention and Control

Control

Biological Control

Phasmarhabditis hermaphroditais a nematode parasite of slugs which, though most effective in controllingD. reticulatumand may also killD. invadens (Speiser et al., 2001). However, this form of control is uneconomic for field crops at present.

Chemical Control

Prystuppa et al. (1987) found that 2% methiocarb was the most effective treatment with copper failing to provide significant mortalities against Deroceras slugs. 

Gaps in Knowledge/Research Needs

It is important to accurately identify which Deroceras species are associated with crop damage across the globe, to promote development and more effective targeting of control measures. Present perception is that most damage is attributable to D. reticulatum, followed by D. laeve, with D. invadens having a minor role. In fact, if identification issues were resolved, D. invadens is potentially the second most important pest of these three, with D. laeve in third position. These differ in their means of reproduction, seasonality and climatic constraints and so may require different control strategies.

More work is also required on the specific responses of D. invadens in relation to control.

References

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Contributors

26/11/15 Original text by:

Roy Anderson, Consultant, Northern Ireland

Distribution Maps