Effectiveness of Different Beer Types in Slug Trapping: A Two-Year Field Study on Arion vulgaris Moquin-Tandon and Limax maximus L.

Simple Summary

Slugs are a persistent pest in agriculture, and their control is increasingly challenging due to the environmental drawbacks of chemical molluscicides. This study was motivated by the need for safer, non-chemical methods—specifically, to determine whether the type of beer used in the traps influences slug attraction. Over two field seasons, we compared five commercial beers and found that Paulaner Weissbier and Union Lager were significantly more attractive to slugs than other beers or ethanol alone. Chemical analysis revealed that certain fermentation-derived volatile compounds, especially esters and limonene, likely explain these differences. Slug activity was also strongly influenced by temperature but not by rainfall. These results provide practical insights for improving the effectiveness of beer traps and point toward the future development of synthetic or yeast-based attractants for sustainable slug management.

Abstract

Slugs are significant agricultural pests, causing extensive crop damage and economic losses. While chemical molluscicides are commonly used for control, concerns about their environmental impact have driven interest in alternative methods, including beer traps. This study evaluated the effectiveness of different beer types as attractants for slug trapping in field conditions over two consecutive years (2022–2023). Five types of beer—Union Lager, Paulaner Weissbier, BrewDog Punk IPA, Guinness Draught, and Chimay Blue—were tested alongside ethanol (10%) and a control treatment. The results demonstrated that Paulaner Weissbier and Union Lager were the most effective attractants, followed by Guinness Draught and Chimay Blue, while BrewDog Punk IPA had moderate effectiveness. Ethanol (10%) and the control treatment failed to attract slugs, confirming that volatile compounds, rather than alcohol alone, drive slug attraction. Gas chromatography–mass spectrometry (GC-MS) analysis revealed that Paulaner Weissbier contained high levels of isoamyl acetate and limonene, while Union Lager exhibited elevated ethyl esters, which likely contributed to their effectiveness. Environmental factors influenced slug activity, with higher temperatures correlating with increased slug capture rates, while precipitation had no significant effect. These findings highlight the role of fermentation-derived volatile compounds in slug attraction and suggest that optimizing beer traps based on beer composition and environmental conditions could improve their effectiveness as a non-chemical slug control method. Future research should explore the long-term stability of beer attractants, the potential of synthetic formulations, and alternative yeast-based attractants to enhance slug management strategies.

1. Introduction

Slugs are economically significant pests in both agricultural and horticultural systems, causing substantial damage to a variety of crops including leafy vegetables, cereals, and fruit-bearing plants [1,2,3]. Their feeding activity not only reduces yield and quality but increases the risk of secondary infections, as their mucus and feeding wounds facilitate pathogen entry [4]. Their preference for humid microhabitats and nocturnal foraging habits make slugs particularly difficult to monitor and control.

Chemical molluscicides—primarily formulations based on metaldehyde and iron (III) phosphate—remain widely used for slug management [5,6,7]. However, growing environmental and ecological concerns have led to regulatory scrutiny. Metaldehyde is highly water-soluble and has been frequently detected in surface waters, where it poses risks to aquatic organisms, pets, birds, and non-target invertebrates, prompting bans or restrictions in several EU countries [8]. Although iron (III) phosphate is often marketed as a safer alternative, studies have suggested that it may negatively impact beneficial soil fauna, including earthworms and especially free-living soil nematodes, which are key indicators of soil health and ecosystem function [6,7].

In response to these drawbacks, attention has shifted toward sustainable, non-chemical alternatives, including physical barriers, cultural techniques, and biological control agents [9,10,11,12,13]. Among biological methods, the parasitic nematode Phasmarhabditis hermaphrodita (Schneider) has shown promise due to its specificity toward slugs [14]. However, its high market cost and sensitivity to temperature and soil conditions limit its widespread adoption in field settings [15,16]. Physical barriers, such as copper tape, diatomaceous earth, and abrasive materials, offer variable efficacy and are generally impractical for larger-scale operations due to labor demands and cost [9,10].

One simple and widely used non-chemical method is the beer trap. These traps rely on the slugs’ attraction to the volatile fermentation by-products—primarily esters, alcohols, and terpenes—present in beer, which mimic natural olfactory cues used by slugs when foraging [17,18,19,20,21,22]. Despite their popularity among gardeners, beer traps have not been rigorously evaluated under controlled field conditions. Additionally, there is little consensus on which beer components are most effective, and how environmental factors, such as temperature or precipitation, might modulate their success [18,19,20,21].

This study aims to address these knowledge gaps by evaluating the effectiveness of five commercially available beer types for attracting slugs under real-world field conditions over two consecutive years. The selected beers differ in their alcohol content, malt and hop profiles, and fermentation characteristics—variables expected to influence their volatile compound composition and attractiveness.

This study is guided by the following hypotheses:

(i)

Beer traps capture slugs in significantly greater numbers than water or ethanol-only controls;

(ii)

The effectiveness of beer traps varies according to the type of beer used;

(iii)

Specific volatile compounds in beer are associated with increased slug attraction;

(iv)

Environmental conditions—particularly temperature—modulate trap effectiveness.

By identifying the characteristics of beer and the environmental factors that optimize trap success, this research contributes to the refinement of practical, ecologically sound slug control measures compatible with integrated pest management (IPM) systems.

2. Materials and Methods

2.1. Study Site and Experimental Setup

The field experiment was conducted over two consecutive years (2022 and 2023) at the Laboratory Field of the Biotechnical Faculty in Ljubljana, Slovenia (46.0508° N, 14.4695° E). The experiment was designed as a randomized block design consisting of five blocks, each containing six treatments. Each block measured approximately 4 m × 4 m and was located in a small, forested area within the experimental field, previously identified as a slug hotspot based on related studies [12,13]. No crops were planted in the blocks to eliminate crop-specific influences on slug activity.

This study included five different types of beer: ref. [1] Union Lager, produced by Pivovarna Union (Heineken) in Slovenia, contains 4.9% alcohol and represents a widely consumed lager in the region; ref. [2] BrewDog Punk IPA, crafted by BrewDog Brewery in Scotland, has an alcohol content of 5.4% and is characterized by its strong hop profile and fruity aroma; ref. [3] Paulaner Weissbier, a traditional German wheat beer from Paulaner Brauerei, features 5.5% alcohol and is known for its unfiltered, yeasty, and slightly spicy taste; ref. [4] Guinness Draught, brewed by Guinness Brewery (Diageo) in Ireland, is a famous stout with a creamy texture and lower alcohol content of 4.2%; ref. [5] Chimay Blue, produced by Chimay Brewery (Scourmont Abbey) in Belgium, is a strong Trappist ale with an alcohol content of 9.0%, featuring rich malt flavors and complex fermentation aromas. The selection of these beers was based on their varying chemical compositions, particularly in terms of fermentation by-products, ethanol levels, and aromatic compounds, which are hypothesized to influence their attractiveness to slugs. Water was used as a negative control. In 2023, a 10% ethanol solution was included as a comparison treatment to test whether ethanol alone, without beer-derived volatile compounds, could attract slugs.

2.2. Beer Trap Setup and Slug Sampling

Plastic containers (20 cm × 14 cm × 5 cm) were used as slug traps. The containers were distributed 10 m apart across the experimental field to minimize the influence of neighboring traps and to ensure an even distribution of sampling points. Each trap was placed at ground level to facilitate slug entry and was partially buried to provide stability and to prevent accidental displacement due to wind or animal interference. Each evening, 250 mL of freshly opened beer was poured into each trap. To maintain consistency, all beer types were stored under identical conditions and poured directly from sealed containers to prevent evaporation or degradation of the volatile compounds. The beer was replenished daily to ensure maximum freshness and to maintain consistent attractiveness throughout the experiment.

Traps were inspected every morning between 06:00 and 08:00, and all captured slugs were carefully removed using sterile forceps to avoid contamination. Slugs were temporarily held in clean plastic containers lined with damp paper towels to prevent desiccation prior to species identification. Identification was performed using a comprehensive identification key [2]. The key diagnostic features, such as body morphology, coloration, and mantle patterns, were recorded.

The number of slugs captured per trap was recorded daily throughout the experiment. To account for seasonal variation in slug activity, sampling was conducted over a five-month period (June to October) in both 2022 and 2023. Each year, 15 sampling days were randomly distributed within the peak slug activity season (June–September) to capture environmental variability and temporal trends. This design provided robust seasonal representation while maintaining logistical feasibility. Notably, more than 500 L of each beer type were used across the two years, making a higher sampling frequency impractical due to the substantial cost and labor requirements.

In 2022, sampling occurred on 20, 21, and 30 June; 1, 5, 7, 8, 12, and 13 July; 25 August; and 13, 14, 15, 21, and 22 September. In 2023, sampling was conducted on 21 and 27 June; 15, 18, and 27 July; 1, 8, 12, 16, and 25 August; and 2, 7, 17, 25, and 28 September. To evaluate the potential influence of weather conditions on slug activity and trap effectiveness, the average nighttime temperature (°C) and precipitation (mm) were recorded for each sampling date using an on-site meteorological station.

2.3. Evaluation of Beer Aroma Profiles, Alcohol Content, and Extract Composition

To analyze the volatile aroma compounds present in the tested beers, 10 mL of each sample was transferred into a 20 mL headspace vial, to which 1 g of sodium chloride (NaCl) was added. The addition of NaCl increases the ionic strength of the liquid phase and facilitates the release of volatile organic compounds into the headspace through the “salting-out” effect, thereby improving extraction efficiency for headspace gas chromatography–mass spectrometry (GC-MS).

The analysis was conducted using an Agilent 8890 GC System (Agilent Technologies, Santa Clara, CA, USA) coupled with a 5977B mass spectrometer (Agilent Technologies, USA) and an automated headspace sampler (PAL RSI 120, CTC Analytics, Zwingen, Switzerland). Volatile compounds were separated on an HP-5MS capillary column (30 m × 250 µm × 0.25 µm, Agilent Technologies, USA) using helium as the carrier gas. Instrument parameters, including oven temperature programming and injector conditions, followed standard operating protocols optimized for beer volatile compound profiling. Data acquisition and processing were performed using Agilent MSD ChemStation Enhanced Data Analysis software (Rev. F.01.03.2357). Compound identification was based on retention times and mass spectral comparisons with reference libraries. All analyses were carried out at the Laboratory for Agrochemistry and Brewing, located at the Slovenian Institute of Hop Research and Brewing in Žalec.

The alcohol content was measured using near-infrared spectrometry (NIR) within the range of 800–2500 nm, using an Anton Paar Beer Analyzer system (DMA 4500M with Alcolyzer ME and PFD modules) (Graz, Austria). The device automatically controlled the sample temperature at 20 °C and calculated the alcohol content based on pre-calibrated absorbance equations. Prior to analysis, each sealed beer sample was inverted 10 times to ensure homogeneity. The beer was then introduced into the device using the PFD sample injector. After securely closing the safety door, the system extracted and analyzed the sample automatically. All results were digitally logged, and each sample was measured in duplicate to ensure analytical precision. The analyzer was cleaned between each measurement cycle using a specialized cleaning solution followed by demineralized water.

2.4. Statistical Analysis

To assess the influence of the beer type, the experimental block, and the sampling date on the slug capture efficiency, a two-way analysis of variance (ANOVA; α = 0.05) was conducted. This approach allowed for the evaluation of both the main effects of these factors and their interaction effects, providing a comprehensive understanding of their combined influence on slug attraction. Duncan’s multiple range test (p < 0.05) was applied for post hoc pairwise comparisons to determine statistically significant differences between treatments. Data were expressed as a mean ± standard error (SE). Prior to statistical analysis, the Shapiro–Wilk test was used to assess the normality of data distribution, while Levene’s test was employed to check for homogeneity of variance. In cases where normality assumptions were violated, a non-parametric Kruskal–Wallis test was conducted as an alternative. Additionally, Pearson’s correlation analysis was performed to investigate potential relationships between the slug capture rates and environmental variables, such as temperature and precipitation. All statistical analyses were performed using Statgraphics Plus for Windows 4.0, and graphical representations, including bar plots and trend analyses, were generated using MS Office Excel 2010.

3. Results

3.1. Beer Type Significantly Influences Slug Capture (H1, H2)

To evaluate whether the beer type affects the slug capture efficiency, a two-way ANOVA was performed for each study year. In 2022, a highly significant main effect of the beer type was observed (F = 13.2, p < 0.0001), indicating that clear differences in attractiveness among the tested beers (

Table 1. ANOVA results for slug capture in beer traps in 2022.

Factor	Sum of Squares	Df	F	p
Beer type (BT)	3934.0	5	13.2	0.0001
Block (BL)	7294.6	4	30.5	0.0001
Sampling date (SD)	6727.3	14	8.0	0.0001
BT x BL	4267.4	20	3.6	0.0001
BT x SD	6426.4	70	1.5	0.0081
BL x SD	9305.2	56	2.8	0.0001
Residual	16,726.8	280
Total (Corrected)	54,681.7	449

). Block (F = 30.5, p < 0.0001) and the sampling date (F = 8.0, p < 0.0001) also had significant effects, reflecting variability in the slug presence across space and time.

Notably, significant interaction effects were found between the beer type and the block (F = 3.6, p < 0.0001), as well as between the beer type and the sampling date (F = 1.5, p = 0.0081). This suggests that the beer effectiveness was partially influenced by local environmental conditions and temporal patterns. The interaction between the block and the sampling date (F = 2.8, p < 0.0001) further underscores the importance of microclimatic and seasonal variation in slug behavior.

In 2023, statistical patterns were similar. The beer type again had a highly significant effect on slug capture (F = 57.7, p < 0.0001), reaffirming the consistency of beer-based differences across the years (

Table 2. ANOVA results for slug capture in beer traps in 2023.

Factor	Sum of Squares	Df	F	p
Beer type (BT)	21,035.4	6	57.7	0.0001
Block (BL)	8331.0	4	34.3	0.0001
Sampling date (SD)	3357.1	14	4.0	0.0001
BT x BL	5305.9	24	3.6	0.0001
BT x SD	3946.3	84	0.8	0.9216
BL x SD	8772.7	56	2.6	0.0001
Residual	20,416.4	336
Total (Corrected)	71,164.8	524

). The block (F = 34.3, p < 0.0001) and the sampling date (F = 4.0, p < 0.0001) remained influential, as did the beer type × the block interaction (F = 3.6, p < 0.0001). However, the beer type × the sampling date interaction was not significant (F = 0.8, p = 0.9216), indicating more stable beer performance over time compared to 2022.

These findings confirm that both the type of beer used and environmental variation significantly affect the trap effectiveness. This aligns with the hypothesis that beer composition plays a central role in slug attraction, and that slug activity is context-dependent.

Figure 1 presents the mean number of slugs captured per day per trap for each beer type tested, along with a control treatment, during the 2022 experiment. A total of 2420 slugs were captured in beer traps. The Spanish slug (Arion vulgaris Moquin-Tandon) was the dominant species, accounting for 93% of the total catch, while the great gray slug (Limax maximus L.) comprised only 7%. No other slug or snail species were recorded in 2022. Notably, no slugs were captured in the control treatment, confirming that the beer played a crucial role in attracting the slugs. Among the tested beer types, Chimay Blue exhibited the highest average slug capture rate, followed by Paulaner Weissbier and Union Lager (Figure 1). Guinness Draught and BrewDog Punk IPA demonstrated moderate effectiveness, whereas the control treatment remained ineffective in capturing slugs. In all beer treatments, A. vulgaris was the predominant species, while L. maximus was captured in significantly lower numbers.

Figure 2 presents the mean number of slugs captured per day per trap for each beer type tested, along with the ethanol (10%) and control treatments, during the 2023 experiment. A total of 5034 slugs were captured in beer traps. The Spanish slug (A. vulgaris) was the predominant species, accounting for 93% of the total catch, while the great gray slug (L. maximus) comprised only 7%. No other slug or snail species were recorded in 2023. Notably, no slugs were captured in either the control or ethanol (10%) treatments, confirming that the beer played a crucial role in slug attraction. Among the tested beer types, Paulaner Weissbier and Union exhibited the highest average slug capture rates, followed closely by Guinness Draught and Chimay Blue. BrewDog Punk IPA demonstrated moderate effectiveness, capturing fewer slugs than the top-performing beers. The results reveal significant variation in the slug capture efficiency among the different beer types, emphasizing the importance of beer selection for slug monitoring and control strategies.

3.2. Volatile Compound Profiles Explain Differences in Slug Attraction Among Beer Types (H3)

The tested beers exhibited considerable variation in alcohol and extract content, which may influence their chemical composition and attractiveness in biological applications. Chimay Blue had the highest alcohol (9.52%) and extract content (18.94%), indicating a rich and complex composition. Paulaner Weissbier (5.31% alcohol, 12.49% extract) and BrewDog Punk IPA (5.44% alcohol, 11.94% extract) contained slightly higher alcohol levels compared to Union Lager (4.82% alcohol, 10.94% extract) and Guinness Stout (4.27% alcohol, 9.80% extract), which had the lowest values among the tested samples. These differences in alcohol and extract content suggest variations in the fermentation processes, yeast strains, and ingredient compositions, potentially affecting the concentration of volatile compounds and sensory properties for each beer.

Gas chromatography–mass spectrometry (GC-MS) analysis revealed significant differences in volatile compound composition among the tested beers. As expected, Paulaner Weissbier exhibited the highest concentrations (compared to other samples) of isoamyl acetate and ethyl acetate, which are responsible for banana and pear aromas, as well as cinnamyl compounds. Additionally, Paulaner Weissbier contained the highest levels of limonene essential oil among all samples.

BrewDog Punk IPA had the highest myrcene concentration, which aligns with expectations given its IPA beer style. By contrast, myrcene was below the limit of quantification (LOQ) in Union, Paulaner, and Guinness. BrewDog Punk IPA also contained the highest concentrations of ethyl propanoate, isobutyl isobutyrate, isoamyl isobutyrate, linalool, and ethyl octanoate, most of which were below the LOQ in other beer samples. Interestingly, limonene levels in BrewDog Punk IPA were lower than in Paulaner Weissbier, likely due to the specific composition of the hop essential oils used in the brewing process.

Union Lager exhibited the highest concentrations of ethyl hexanoate, ethyl octanoate, and ethyl caprate among all tested beers. Ethyl caprate was not detected in any other sample, suggesting that the presence of high levels of ethyl esters in the Union beer is likely attributed to the specific yeast strain used in the fermentation process.

Chimay Blue had the highest alcohol and extract content among all tested beers. It was rich in volatile compounds, with concentrations similar to those found in Paulaner Weissbier and BrewDog Punk IPA. Notably, pentanol stood out as a dominant volatile compound in Chimay Blue, with significantly higher levels than in other beers. However, this beer was relatively low in heavier volatile compounds and hop-derived essential oil components, indicating a different aromatic profile compared to the other tested beers. These results highlight the substantial variability in volatile compound composition across the different beer types, which may influence their effectiveness in slug attraction.

3.3. Seasonal Weather Patterns and Their Influence (H4)

The weather data collected from the on-site meteorological station during the study period (June–September) in 2022 and 2023 revealed notable variations in the temperature and precipitation patterns between the two years. In 2022, the average daily temperature ranged from 16.0 °C (September) to 24.5 °C (July), with the warmest conditions observed in July (24.5 °C) and the coolest in September (16.0 °C). The precipitation patterns were highly variable, with relatively low rainfall in June (35.8 mm), July (86.5 mm), and August (54.1 mm), followed by a significant increase in September (471.8 mm), indicating a late-season peak in precipitation. In 2023, the temperatures were generally lower, ranging from 19.1 °C (September) to 22.7 °C (July), with July (22.7 °C) and August (21.8 °C) recording the highest temperatures. Rainfall was markedly higher in June (142.9 mm), July (260.0 mm), and August (298.5 mm) compared to the same months in 2022. However, in September 2023, the precipitation decreased significantly to 65.5 mm, contrasting with the high rainfall observed in September 2022.

Overall, 2023 was characterized by lower temperatures and higher precipitation during the summer months (June–August), while 2022 exhibited higher temperatures and a delayed peak in precipitation occurring in September. These climatic differences may have influenced the ecological interactions and activity patterns of the slugs observed in this study.

Table 3. Nighttime Temperature and Precipitation During Sampling Dates in 2022 and 2023.

2022			2023
Sampling Dates	Average Night Temperature (°C)	Precipitation Amount (mm)	Sampling Dates	Average Night Temperature (°C)	Precipitation Amount (mm)
20 June	21.5	0	21 June	21.3	0
21 June	21.3	0	27 June	23.5	0
30 June	21.8	0	15 July	17.8	0
1 July	21.1	0	18 July	19	0
5 July	17.1	9.7	27 July	17.6	1.9
7 July	14.3	3.4	1 August	20.1	0
8 July	15.7	4.8	8 August	18.9	0
12 July	17.1	0	12 August	14.5	0.6
13 July	19.1	0.3	16 August	19.4	0
25 August	18.8	0	25 August	18.8	0
13 September	15.1	0	2 September	12.9	0
14 September	17.9	0	7 September	18.6	0
15 September	13.4	3	17 September	5.4	94.9
21 September	6.1	0.4	25 September	7.7	3.1
22 September	5.6	0	28 September	11.8	6.9

presents the nighttime temperature and precipitation amounts recorded on the sampling dates between June and September in 2022 and 2023. The data reveal notable differences in temperature trends and rainfall distribution between the two years. In 2022, the nighttime temperatures ranged from a high of 21.8 °C (30 June) to a low of 5.6 °C (22 September). Precipitation was generally low, with several sampling dates recording 0 mm of rainfall, except for 5 July (9.7 mm) and 15 September (3 mm). The coldest nights occurred in late September, when the temperatures dropped below 6 °C.

In 2023, the nighttime temperatures followed a similar seasonal trend but were slightly lower overall. The warmest night was recorded on 27 June (23.5 °C), while the coldest was on 17 September (5.4 °C). Rainfall was notably higher on 17 September (94.9 mm), marking a sharp contrast with the dry conditions observed on most other sampling dates.

Overall, both years exhibited warm summer nights in June and July, followed by progressively cooler conditions in September. However, the precipitation patterns differed significantly, with 2023 showing more localized heavy rainfall events, while 2022 experienced generally drier conditions with sporadic minor precipitation events. These climatic variations may have influenced the environmental conditions and biological activity of the slugs during the study period.

3.4. Temperature Positively Correlates with Slug Capture (H4)

A Pearson’s correlation analysis was conducted to assess the relationship between the slug capture rates and the environmental variables (temperature and precipitation) in both 2022 and 2023. Prior to the correlation analysis, the Shapiro–Wilk test was performed to check the normality of the data. In 2022, the slug capture rate (p = 0.118) and temperature (p = 0.072) followed a normal distribution, whereas the precipitation (p < 0.0001) did not. In 2023, the slug capture rate (p = 0.569) followed a normal distribution, while the temperature (p = 0.025) and precipitation (p < 0.0001) deviated from normality. Despite the non-normal distribution of some variables, a Pearson’s correlation analysis was applied for consistency.

The correlation between the slug capture rate and the temperature was found to be moderate but not statistically significant in 2022 (R = 0.351, p = 0.199); whereas, in 2023, it was moderate to strong and statistically significant (R = 0.594, p = 0.020). This indicates that the slug capture rates were more strongly associated with temperature in 2023 than in 2022, suggesting that warmer conditions had a greater influence on slug activity or trap effectiveness.

The slug capture rate showed no significant correlation with precipitation in either year. In 2022, a weak positive but non-significant correlation was observed (R = 0.122, p = 0.664); while, in 2023, the correlation was weakly negative and also non-significant (R = −0.110, p = 0.696). These results suggest that rainfall had little impact on the slug capture rates, indicating that the beer traps remained effective regardless of the precipitation levels. The stronger negative correlation between the temperature and precipitation in 2023 suggests that the environmental conditions varied between the two years, potentially affecting slug behavior and trap efficiency.

4. Discussion

This study investigated the effectiveness of the different beer types in attracting slugs under field conditions, and explored the roles of volatile compounds and environmental variables in influencing trap success. Our findings confirm that the beer composition is a key determinant for slug attraction, validating the use of beer traps as a non-chemical pest control method. The results directly address our hypotheses concerning variability in the beer effectiveness (H2), the chemical drivers of attraction (H3), the environmental influence (H4), and the species response (H1).

As predicted, the beer type had a significant effect on the slug capture rates. Paulaner Weissbier and Union Lager consistently ranked as the most attractive, followed by Guinness Draught and Chimay Blue. BrewDog Punk IPA was the least effective. These patterns were consistent across both years, and support previous findings that slugs are more responsive to specific fermentation by-products and aroma compounds than to ethanol alone [17,18,19,20]. This was further supported by the lack of captures in both the 10% ethanol and water control treatments, confirming that alcohol concentration alone is not the main attractant.

A chemical analysis using gas chromatography–mass spectrometry (GC-MS) supported our hypothesis (H3) by revealing the distinct volatile compound profiles among the beer types. Paulaner Weissbier was rich in isoamyl acetate and limonene—compounds known to elicit behavioral responses in A. vulgaris [20]. Union Lager contained high concentrations of ethyl hexanoate and ethyl octanoate, esters associated with fruity aromas. These likely contributed to their high attractant performance. By contrast, BrewDog Punk IPA contained elevated levels of hop-derived compounds, such as myrcene and linalool, which, although intense in aroma, did not correspond with higher slug attraction. This suggests that certain volatile compounds, particularly esters and limonene, play a more significant role in stimulating the slug foraging behavior than general aromatic intensity.

The traps were designed to attract multiple slug species, and our data confirmed this (H1), although A. vulgaris accounted for over 90% of captures in both years. This likely reflects its ecological dominance in the area and stronger behavioral responsiveness to the beer volatile compounds compared to L. maximus. Still, the presence of multiple species demonstrates that the beer traps have a broader utility for gastropod monitoring.

A key observation was the markedly higher slug capture in 2023 compared to 2022—more than double the total count. While part of this difference can be attributed to the interannual variability in weather conditions (as examined under H4), other ecological factors likely contributed. In 2023, the nighttime temperatures during the sampling were moderately high and more stable, promoting consistent slug activity. The precipitation, although more abundant in some months, did not correlate significantly with slug captures in either year. This suggests that rainfall may not inhibit the effectiveness of the beer traps, which is notable given that chemical molluscicides often perform poorly in wet conditions. However, the climatic explanation alone does not fully account for the interannual difference in capture rates. Additional contributing factors may include fluctuations in natural food availability in the surrounding habitat, which can affect the slug foraging behavior [21]. The slugs may have been more inclined to enter the traps in 2023 if natural food sources were less abundant due to weather-related plant stress. Changes in slug population dynamics, including higher reproductive success in 2023 following a mild winter, could also be a factor. Although the experimental blocks were unplanted, the surrounding vegetation, ground litter, and microhabitat structures, such as moisture-retaining debris or shaded cover, may have influenced the slug density and behavior.

Spatial variability in trap performance, demonstrated by significant beer type × block interactions in both years, underscores the role of the microclimate. Factors such as soil texture, local humidity, and surrounding vegetation structure may affect both the slug presence and the diffusion of the beer volatile compounds in the environment.

Our findings align with those of Piechowicz et al. [18,19,20], who reported esters as key attractants, and identified some beer compounds (e.g., CO₂ or acrylic acid derivatives) as neutral or inhibitory. Although our study did not analyze these specific inhibitory compounds, the consistent trends across the years reinforce the conclusion that beer attractiveness is driven by a specific subset of volatile compounds.

Despite their effectiveness, beer traps have practical limitations. Volatile compounds degrade quickly under field conditions, reducing long-term efficacy. Future research should assess how long attractant compounds remain effective under real-world exposure. Additionally, synthetic baits based on isoamyl acetate, limonene, and ethyl esters may provide more consistent performance across diverse environmental settings. Alternative low-cost attractants—such as fermenting bread dough or yeast-based formulations—have shown promise in previous studies [22]. Their comparative effectiveness under different temperature, humidity, and soil conditions warrants further investigation.

In summary, this study contributes new evidence that beer type, chemical composition, temperature, and site-specific factors jointly influence slug trap performance. Optimizing the attractants based on volatile compound composition could support the development of more sustainable, scalable, and cost-effective strategies for gastropod management.

5. Conclusions

In conclusion, this study confirms that the type of beer used significantly influences the slug trap success, and that key fermentation volatile compounds—especially esters and citrus-scented terpenes—are central to slug attraction. Environmental factors, particularly temperature, modulate slug activity and should be considered when deploying traps in the field. These insights contribute to the development of more sustainable, species-specific, and environmentally friendly slug management strategies within integrated pest management (IPM) frameworks.