Deroceras invadens
(tramp slug)

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.

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).

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.

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.

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.

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. 

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). 

General

• Laboratory use
• Research model

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.

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.

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. 

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.

26/11/15 Original text by:

Roy Anderson, Consultant, Northern Ireland