Take Home Messages
- Slugs were active around the Wimmera region, even in a decile 1 growing season.
- Carpet mats are a good, inexpensive way to monitor slug activity
Background
The widespread adoption of no/low till farming and stubble retention management practices on farm, whilst benefiting soil moisture retention, has also inadvertently favoured slug populations. These changes to management practices, in combination with recent wet seasons in the Wimmera, have created ideal conditions for slug activity and breeding. Ultimately, this has led to increased seedling damage and yield losses from these pests, as well as higher operational costs for controlling them. Slugs are establishment pests and reduce yields primarily by cutting seedling numbers. Canola is particularly susceptible to yield losses, with previous trials finding losses of up to 60–80 per cent (GRDC, 2024).
A major challenge for growers dealing with slugs is managing bait applications to best avoid seedling damage and yield losses. Baiting is not only expensive, typically costing Victorian growers $30/ha–$120/ha (GRDC, 2024) but can also be inefficient when applied reactively. Slugs are tricky targets, and baiting is only effective if conditions are adequate for surface activity and slugs are feeding. Baits do not attract slugs, so they must encounter them to be effective. If baits are applied too early in the season, slugs may not yet be active and the baits may need to be reapplied, wasting resources. If applied too late, once seedling loss has already occurred and establishment is slower, slugs may be breeding and require higher rates of bait over a longer period to protect seedlings. Previous research has established thresholds for Deroceras reticulatum (grey field slug) populations and crop damage, with populations as low as 1 slug per refuge trap causing seedling losses (Nash et al, 2024). Economic thresholds are yet to be developed for other key pest slug species under Australian conditions.
BCG is in the first of a three-year project funded by GRDC, and managed by SARDI and the University of Adelaide. This project seeks to gain a better understanding of slug population dynamics, improve in-field monitoring and evaluate spring baiting as a population control strategy. The goal is to provide growers with well-informed strategies for monitoring and managing slug populations.
Method
This project has two components: monitoring, and a bait trial to be conducted in 2025. Six monitoring paddocks with known slug populations were set up in March 2024 for slug monitoring over the next three years (see Table 1 below). Twenty slug mats (250mm by 250mm) were set up in two transects of 10 mats spaced 10m apart. Locations within the paddock for monitoring were selected based on yield maps from previous years where significant yield losses attributed to slug damage were observed.
Monitoring was conducted monthly. Data on the number and species of slug under each mat were collected, and a subsample of slugs was sent to SARDI for dissection to identify slug development stage (adult or juvenile). At the time of monitoring, several environmental metrics were taken including temperature (ambient, bare soil, and under mat temperature), soil moisture (ranked by feel such as dry, moist, wet), weather conditions, wind, and time of day.
Growers managed paddocks as per usual, including current slug management practices. All sites were baited after sowing with the exception of Site 4.
Results & Interpretation
Seasonal overview
Coming into 2024, slug populations were likely elevated due to the relatively wet seasonal conditions experienced during 2022 and 2023. Slugs can live for up to two years and, being opportunistic breeders, they breed whenever conditions are suitable rather than seasonally. The decile 1 growing season rainfall experienced in the Horsham area in 2024 meant slug activity was relatively lower than previous years (Figure 1).
Over summer, slugs remain dormant in patches of moisture within the soil and typically only emerge to the soil surface when it reaches 96 per cent relative humidity, and soil gravimetric moisture is 20–45 per cent (GRDC, 2024). These conditions often occur after the autumn break. In 2024, the autumn break was not a distinctive rainfall event. However, 29mm of rainfall in the first week of April, in combination with higher-than-average stored soil moisture from a wet summer was thought to have been adequate for slugs to become surface-active. As such, slugs were observed at all monitoring sites after this ‘false break’. Slug activity decreased after April and throughout winter, due to several factors. Firstly, all sites excluding site 4 were baited after sowing. Secondly, the soil surface at this time remained relatively dry, a product of low rainfall and slow growing crops reducing ground coverage and increasing evaporation area. However, as crops reached canopy closure, soil moisture was trapped at the surface and increased relative humidity. This along with accumulated rainfall and higher temperatures going into spring, likely created a more ideal environment for slug activity, which increased in late winter and early spring. Further temperature increases and a reduction in crop canopies as crops reached maturity resulted in less soil surface relative humidity and a decrease in slug activity in late spring.
While the above outlines the general trend observed at all sites, several different species were observed at certain sites. Not all slug species behave in the same way and have different temperature and moisture thresholds for surface activity; this is explored in further detail below in the section, ‘Species Breakdown’.
Damage
The autumn break meant slug activity in 2024 was relatively lower than previous years during crop establishment. Significant damage to the crop was only observed on faba beans at Site 4 (Figure 3). Some feeding damage in the other faba bean paddock (Site 6) and both canola paddocks (sites 2 and 3) was observed early in the season, however damage was minor and often only observed on the crop when it had passed the growth stage when yield would be affected (4L canola) (Figure 3).
Species Breakdown
Black Keeled Slugs
Black keeled slugs were the most common species observed in 2024, distinguishable from other species by the keel (ridge) running along their tail (see Figure 4). During March, black keeled slugs (BKS) were only observed at site 1 but peaked the following month at multiple sites (Figure 5). Activity then decreased in early winter before increasing again in late winter to early spring. The soil moisture threshold for surface activity is generally higher for black keeled slugs relative to other species (50mm to 100mm rainfall), this means they are generally observed later in the season than other species. However, as the area received average summer rainfall and the soil profile held greater moisture, this threshold may have been reduced coming into the 2024 growing season and as such they were first observed at a similar time to other species.
Where other slug species are primarily surface-dwelling and seek refuge in cracks in the soil, BKS burrow down into the soil to seek moisture. The burrowing behaviour often leads to underestimated populations for this species, particularly in dry years. This may be why they were only observed in low numbers throughout winter, with a combination of baiting after sowing and dry conditions increasing their burrowing behaviour. BKS can be more damaging in lower numbers than other species because they can burrow into the seed bed and feed on seeds, reducing plant density. Canola is particularly susceptible (GRDC, 2024).
Brown Field Slugs
Brown field slugs (BFS) are ambiguous in appearance but can be distinguished from other species by their brown/grey colour, clear mucus, and tail flicking behaviour when agitated (see Figure 6). Dissections determined the species was Deroceras invadens. BFS were only observed at one site at the beginning and towards the end of the growing season (see Figure 7). Brown field slugs are typically more active in warm conditions, which is why they became active later in spring than BKS, after temperatures had further increased. During the October count, eggs were also observed under one of the mats, suggesting despite low rainfall, the crop canopy and slug mats allowed for sufficient relative humidity for this species to breed (see Figure 7). This species is typically less damaging to crops than other species and mainly found in pastures. These slugs were only found in an oat paddock, and no damage to the emerging crop was observed.
Striped Field Slugs
The Striped field slug (SFS) is easily distinguishable from other slugs by the dark brown tram tracks (two or three stripes) running down their body (see Figure 8). Limited dissections suggest this species was Ambigolimax valentianus. This species was continually observed at site 4 throughout the growing season but was also observed in April at site 3 in low numbers at the beginning of the season (see Figure 9). Compared to other slug species, SFS seemed to be more active in drier conditions. These ideal environmental conditions and the fact that the paddock was not baited or rolled may explain why this species was observed in greater numbers, with up to 82 slugs counted under a single mat. They are not known to cause economic damage. However, they were observed doing the greatest feeding damage of any species. This may have been a product of a high population density and relatively low food sources. Damage was also mostly to the lower canopy of faba beans after they were established, past a growth stage where yield would be affected.
Commercial Practice and On Farm Profitability
These initial results highlight that, even in years with decile 1 growing season rainfall, relative humidity at the soil surface can still be high enough for slug activity and reproduction.
One of the objectives of this project is to validate the efficacy of carpet slug mats for monitoring slug activity and their practicality for farmer use. Traditionally, ceramic tiles or bait lines have been used, however these methods have limitations that often lead to populations being underestimated. The slug mats used in this study presented some challenges in monitoring slug populations, particularly around maintaining an ideal environment for slugs. The mats are good insulators but due to seasonal conditions were put down wet, onto dry soil. The mats then dried out and protected the soil underneath from rain, meaning the environment underneath stayed dry. Moving the mats onto damp soil remedied this issue, as the area under the mats was then able to stay moister relative to bare soil. Temperature data collected at the time of monitoring also showed soil temperature was often cooler under the mat than at the bare soil, creating a more ideal environment for slug activity. Overall, these mats present a good, inexpensive option for growers who want to monitor slug populations, as long as they are placed onto moist soil. Baits can be placed under the mat for ease of monitoring. The mats also provided a refuge for other pest and beneficial species, such as slaters, millipedes, carabids and spiders. Highlighting they could be useful for monitoring other soil surface dwelling pests and beneficials of interest.
Further years of monitoring by BCG and other groups in the medium rainfall zone will be used to produce a model that assesses management options and takes an integrated approach to slug control, with the aim of providing growers with more efficient and economic options for slug control.
References
GRDC, 2024, Improved Management of Snails and Slugs, <https://grdc.com.au/research/projects/projectid=664>.
GRDC, 2024, Strategies to limit slug threats other than baits, <https://grdc.com.au/resources-andpublications/grdc-update-papers/tab-content/grdc-update-papers/2024/07/strategies-to-limit-slugthreats-other-than-baits>.
Nash M., Thomson L., and Hoffman A., 2007, Pest Management Science. ‘Slug control in Australian canola: monitoring, molluscicidal baits and economic thresholds.’ pp 851–859.
GRDC, 2024, Slug Control Fact Sheet, <https://grdc.com.au/__data/assets/pdf_file/0018/613116/SlugControl-Fact-Sheet-GRDC-241205.pdf>.
Acknowledgements
This research was funded by GRDC as part of the ‘Optimising slug management: Enhancing capacity and capability through population modelling and innovative management strategies’ project (UOA2308-004RTX).
Thank you to Micheal Nash, University of Adelaide/La Trobe University, and Thomas Heddle, University of Adelaide, for providing a technical review of this article.
BCG sincerely thanks Sam Eagle, Nathan Plush, Tim Rethus, Adrian Scott, Justin Ruwoldt and Rob Staehr for hosting the monitoring sites, as well as Nick Zordan and Mitch Fromm for their support throughout the project.
