Slugs of the genus Arion (Gastropoda: Arionidae) have been classed as a major agricultural economic pests in Asia, Australia, Europe, and North America (Barker, 2002; Ahmadi, 2004; Douglas & Tooker, 2012; Rowson et al., 2014). They cause damage to vegetables, field crops, fruit trees, lawns, and wild plants (Peters et al., 2000; Douglas & Tooker, 2012). They may also appear as stored product pests (Henderson & Triebskorn, 2002). Slug feeding can severely damage the plants and thus subject them to stress (Rowson et al., 2014). Consequently, such plants are less resistant and more susceptible to diseases and are less likely to survive unfavourable weather conditions (Hammond & Byers, 2002).

The economically important agricultural pest species from the Arionidae family in Europe are Arion distinctus Mabille, A. hortensis (Férussac), and A. vulgaris (Moquin-Tandon) (Rowson et al., 2014; Laznik & Trdan, 2016). For the purpose of effective management of any agricultural pest, knowledge of its biology and ecology is required. The life of the Arion slugs is closely connected to its environment, with temperature and humidity directly affecting biological processes (Barker, 2002; Slotsbo et al., 2011). Furthermore, body pigmentation may depend on age, diet or environment (Barker, 2002; Kozłowski, 2005; Slotsbo et al., 2011). Slugs activity is correlated with different factors, such as air temperature, soil surface temperature, wind speed, humidity and soil moisture content (Young & Port, 1989; Kozłowski, 2007; Slotsbo et al., 2013; Rowson et al., 2014). Slugs favour heavier soils, since they can survive over the summer in cracks in the soil and under clods (Barker, 2002). Terrestrial molluscs are generally susceptible to desiccation, and several studies have found habitat selection to be strongly correlated with the availability of water (Carne-Cavagnaro et al., 2006). The most obvious water regulating behaviour of slugs is their preference for moist habitats (Slotsbo et al., 2011). In contrast to snails, slugs lack the physical protection of a shell and must find other ways to reduce water loss. Thompson et al. (2005) suggested that water conductance of the skin is an important factor. However, the ability to survive drought cannot be explained only by water conductance alone but is also influenced by body size and the magnitude of water reserves. Active slugs can lose up to 60% of their initial body weight (Thompson et al., 2005). Furthermore, dehydrated slugs can rapidly recover from dehydration by absorbing water through their integument; this process is termed contact re-hydration (Prior, 1985; Thompson et al., 2005).

Increased mucus secretion is one of the first reactions of slugs to mechanical or chemical irritation (Barker, 2002; Schüder et al., 2003; Simms et al., 2006). The production of mucus enables the slugs to form a protective barrier preventing direct contact between the toxin and the surface of the epithelial cells. Slugs are mainly controlled with bait pellets, which usually contain either metaldehyde or iron (III) phosphate (Garthwaite & Thomas, 1996; Speiser & Kistler, 2002). The major concern related to slug control is that the chemical compounds are also toxic to non-target organisms and pets (Castle et al., 2017). Due to the physico-chemical properties of metaldehyde, it is highly mobile in soil, and hence once applied, it can run off under wet conditions into field drains, gullies and surface waters (Castle et al., 2017). Furthermore, farmers and growers often experience difficulty controlling slugs with bait pellets containing molluscicides. Hata et al. (1997) reported that in wet conditions, the efficacy of these baits can be very low. Slugs can be difficult to kill by contact molluscicides because they are covered by a layer of slime that prevents chemicals from coming in contact with skin, and at the same time, slime may serve to dilute the toxin (Barker, 2002; Speiser & Kistler, 2002; Gonzáles-Cruz & San Martín, 2013). In addition to environmental problems, human health problems also arise from agricultural pesticide usage (Nicolopoulou-Stamati et al., 2016). Many farmers are searching for new alternatives to curb pesticide usage to address the many concerns.

The development of alternative slugs control methods compatible with integrated pest management (IPM) strategies used to control other pests would help satisfy increasing market demands and environmental safety issues. In the early 1990s, a biocontrol method for slugs based on the parasitic nematode Phasmarhabditis hermaphrodita was developed (Wilson et al., 1993). The biocontrol method with parasitic nematodes proved to be a promising alternative to chemical molluscicides (Iglesias et al., 2003). Many countries have strict regulations for using biopesticides that allow only indigenous species to be used (Laznik & Trdan, 2016). Such regulations have slowed the uptake of use of P. hermaphrodita in several countries. Physical barriers, such as continuous lines of sawdust or ash, provide a dry surface which slugs avoid (Barker, 2002; Laznik & Trdan, 2016); however, the effectiveness of these barriers is reduced once they become wet. Copper barriers present an effective mechanical and physiochemical barrier for slugs (Schüder et al., 2003; Laznik et al., 2011). As shown in previous studies, mollusks take up copper through ingestion (Berger & Dallinger, 1989), and directly through the foot, at least in the form of copper compounds (Ryder & Bowen, 1977), causing internal damage and irritation (Schüder et al., 2003).

Due to the negative environmental impacts of pesticides, their non-target effect and increasingly stringent environmental policies, researchers are searching for new, more environmentally acceptable methods of plant protection against pests. One of these measures is the study of plant extracts in the control of economically important harmful organisms (Pavela et al., 2008). To develop an alternative slug control method, we explored the use of plant material from seven invasive plant species: knotweeds (Fallopia japonica [Houtt.] Ronse Decr., F. × bohemica [Chrtek & Chrtková] Bailey), goldenrods (Solidago canadensis L., S. gigantea Aiton), staghorn sumac (Rhus typhina L.), tree of heaven (Ailanthus altissima [Mill.] Swingle), and false indigo (Amorpha fruticosa L.). It was a selection of plants that are widespread in urban areas and whose control is practically impossible in practice. The main idea was to use these plants in order to see if they have any potential in plant protection programmes (Laznik et al., 2018). The aims of our laboratory and semi-field studies were to test (1) the contact control efficacy of an individual use of the selected substances, (2) the barrier effect of the tested substances, and (3) the effect on the slug eating ability.

General analysis of the events at different experiments is presented in Tables 2 and 3.

Due to the price and other benefits, molluscicides represent the most common strategy in terrestrial gastropod control programmes. Molluscicides are considered emerging pollutants and are frequently detected in surface water bodies above the EU statutory drinking water limit (Castle et al., 2017). There are also reports of the negative impact of molluscicides on non-target organisms (Bailey, 2002). Furthermore, growers and farmers often experience difficulty controlling terrestrial gastropods with conventional bait pellets containing molluscicides such as methiocarb (this molluscicide is already being withdrawn from use following the recent ban by the European Commission) and metaldehyde. Due to these problems, researchers are looking for new, more environmentally acceptable ways of protecting plants against harmful organisms, including slugs (Laznik & Trdan, 2016). Recently, the use of environmentally acceptable substances for slug control in agriculture and horticulture has gained unprecedented impetus all over the world (El-Sherbini et al., 2009; Laznik et al., 2011; González-Cruz & San Martín, 2013; Laznik & Trdan, 2016). Different natural substances are promoted due to their wide range of ideal properties, such as high target toxicity, low mammalian toxicity, low cost, and easy bio-degradability (Laznik & Trdan, 2016). One of these measures is also the use of plant extracts against slugs and snails (Barone & Frank, 1999; El-Sherbini et al., 2009; González-Cruz & San Martín, 2013). In our research (laboratory and semi-field), we studied both the contact efficacy, anti-feedant and barrier efficacy with the use of plant material of seven invasive plant species, including knotweeds, goldenrods, staghorn sumac, tree of heaven, and false indigo. A novelty in our research was the idea of how invasive plants could be used to control arionid slugs.

When we rolled the slugs in tested plant powders, the highest mortality of the individuals tested was recorded in R. typhina (30%). These results also led to very low feeding inhibitor effects in experiment A. Klingauf et al. (1988) reported that plant extracts from R. typhina have insecticidal efficacy against aphids (up to 70%). Furthermore, laboratory studies have shown that R. typhina plant extracts also have bactericidal and fungicidal efficacy (Mosch et al., 1989; Rayne & Mazza, 2007). Until now, there had been no reports on the molluscicidal efficacy of R. typhina.

In the second experiment (B), the plant powder of selected plants was used as a barrier for the slugs. After 48 hrs, none of tested slugs fed on lettuce when the barrier was made from the plant powder of R. typhina and S. gigantea. None of the slugs crossed the barrier made of R. typhina; however, 10% of the slugs that crossed the barrier made of S. gigantea did not feed on the lettuce. In all the treatments, the slugs observed produced a large amount of mucus. The effect of the plant powders is similar to that of wood ash and hydrated lime, as all substances cause dehydration of the cuticle and blockage of the airways (Laznik & Trdan, 2016). Prior (1985) reported that slugs are susceptible to dehydration due to evaporative water loss across their body and lung surface and through the deposition of their slime trail. Mucus production is an energy-wasting process that could also impact the feeding behaviour of slugs. More detailed experiments are merited to confirm this thesis in the future.

In experiment C, our goal was to determine whether the foliar application of the tested plant material in a liquid formulation has any effect on reducing the feeding ability of the individuals tested. In our investigation, the concentration of plant water extracts (2.5% or 10% w/v had no influence on slug feeding behaviour. At both concentrations tested, the best anti-feedant effect was confirmed with the F. japonica plant water extract, with slug feeding reduced by up to 40%. The use of knotweed plant extracts had only been cited to control plant diseases, such as powdery mildew (Herger et al., 1988; Neuhaus & Pallut, 1992; Konstantinidou-Doltsinis et al., 2006), or mites (Tetranychus urticae) (Tomczyk, 2006).

In experiment D, our goal was to investigate different techniques of using plant material (as a barrier or foliar liquid formulation) under semi-field conditions. For this matter, only S. gigantea plant material was used. The best results were obtained when we used the combination of both techniques (only 7% of plants were attacked by slugs). In the control treatment, over 70% of plants were attacked by slugs. The most important biologically active compounds in goldenrod plants are flavonoids, saponins and terpenes (Starks et al., 2010). The literature concerning the use of plant extracts with a high content of saponins for the control of terrestrial gastropods is limited. The published reports are mainly related to egg control, feed deterrence, seed treatment and the formulation of baits (Winder & Friedrich, 1996; Barone & Frank, 1999; Iglesias et al., 2002; González-Cruz & San Martín, 2013). González-Cruz & San Martín (2013) concluded in their investigation that the application of plant extracts with a high content of saponins could be a good means for controlling slugs because saponin residues have been demonstrated to be safe in foods and agronomic products in the USA, EU and Japan. However, the results of our investigation confirmed a high level of phytotoxicity when the lettuce was treated with a combination of powder and liquid formulation. More than 10% of lettuce leaves showed phytotoxic symptoms. Phyto-toxicity could be related to the use of a high concentration (10%) of plant extract in our investigation. To support our assumption, further investigation is needed.

The underlying principle of integrated slug control is to reduce the risk of slug damage by means of cultural practices, if possible, and apply molluscicides if necessary. Biological control (P. hermaphrodita) has a role to play, together with other techniques that are especially relevant to organic growers (Wilson et al., 1993; Schüder et al., 2003; Laznik et al., 2011; Laznik & Trdan, 2016). It is important not to attempt to eradicate slugs completely but simply aim to limit their damage to economically acceptable levels. Slug control in organic systems presents particular problems because the use of chemicals is substantially restricted. Various methods of slug control are recommended for organic growers, but most are untested and unproven. Our study showed that plant material (dry or in liquid formulation) could represent an alternative solution for private gardens in plant protection against slugs. However, there are still many challenges to overcome (phyto-toxicity and economics) in further studies. In addition, the potential challenges are the risk of using products derived from invasive pants, as many of the plant species tested in our paper are controlled by legislation throughout the EU.