## 4 Discussion  
Traditionally, slugs are controlled by chemical bait pellets based upon carbamates, metaldehyde, or iron (ferrous) phosphate [9]. However, molluscicides are toxic and give inconsistent results. They are poisonous to vertebrates, entomophagous invertebrates, and earthworms [9,10]. In environments with wet conditions, the efficiency of chemical bait pellets is very low [20]. Decreased efficiency of bait pellets also occurs when slugs cover the pellets with slime thus protecting slugs from coming in contact with skin and diluting the toxin [21,22]. Potential human health implications of exposure to agricultural pesticides in chemical bait pellets forced farmers and industry to search for new alternatives [23]. Recently, new molluscicidal substances that are environmentally acceptable have been proposed [22,24,25]. New natural compounds are characterized as selective and highly toxic to target pests, safe for non-target organisms, low cost, and biodegradable [1,25]. The use of wood ash has been noted among these compounds [2,25]. Therefore, we conducted laboratory and semi-field experiments to assess the contact efficacy, antifeedant, and barrier efficacy of wood ash derived from seven plant species (sessile oak, European beech, European hornbeam, silver fir, European spruce, common alder, and Spanish chestnut). The novelty of the present study lies in the results of interspecific variability of wood ash in controlling arionid slugs. The highest mortality of slugs rolled in the wood ash was recorded under the oak and spruce wood ash treatments (Figure 1). Geochemical analysis of wood ash showed that oak and spruce wood ash comprised relatively high levels of copper (Table 1). Copper has been previously reported as an efficient mechanical and physiochemical barrier for slugs [18,24]. Mollunks take up copper, in the form of copper compounds [26], through ingestion [27] and directly through their foot, causing internal damage and irritation [18]. Furthermore, several wood ashes (beech, oak, fir, and spruce) showed satisfactory results as a physical barrier against slugs. The most effective ashes (beech, oak, fir, and spruce) compared to the ineffective ashes (hornbeam, chestnut, black ader) contain the highest concentrations of the most of identified chemical compounds (Si0\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), MgO, and MnO) and elements (Ba, Co, Cs, Ga, Cu, Pb, and Zn). Silicon dioxide consists of many abrasive crystals, that can cause small abrasions on the body of any pest that comes in contact with the powder. The pest gradually loses its body fluids, dehydrated and dies [18,25,28]. Arionid slugs are reported fairly tolerant to heavy metals which they tend to accumulate in high concentrations in their midgut gland. However, excessive concentrations of zinc, lead, and copper are toxic and can negatively impact the slug's metabolism of other essential metal ions [29]. It has been reported that heavy metals could be  
Figure 4: Effect of wood ash on the damage potential of slugs in semi-field trials. The means (mass of lettuce (g) \(\pm\) S.E.) with different letters significantly differ at \(p=0.05\) (Student’s multiple range test) between the treatments in periods of 24, 48, and 72 h.  
used in slug control programs as an antifeedant and so-called bioactive repellents [30], but there is still inconsistency in efficiency since the toxicity of Zn, Pb or other heavy metals is dosage-dependent [31]. In our study, we found calcium oxide of minor importance for arionid slug since ineffective hornbeam ash had similar concentrations (more than 50%) compared to fir ash, which was among the most effective ashes. This is surprising, since calcium-rich eggshell powder is often recommended as a slug control tool for small gardeners. Furthermore, we detected the lowest concentrations (below 0.0003%) of molybdenum in the four most effective wood ashes, when compared to other three ineffective wood ashes.