Take home messages

• A Riverine Plains survey indicates an increase in prevalence of slug damage over the past three years (2021–2023), which is driven by farming practices and weather.
• Monitoring found that slugs were active across several regions, despite dry autumn and winter conditions in 2024, highlighting the need for proactive slug management.
• A slug’s weight does not indicate its age or sexual maturity – hence when it will lay eggs, which makes management challenging.

Background

The widespread adoption of conservation agriculture improves soil health and conserves soil moisture yet creates favourable conditions for slug populations. These management practices, in combination with the recent wet seasons, have created ideal conditions for slug activity and breeding, leading to increased seedling damage and yield losses from these pests, and increased operational costs to control them. Slugs are establishment pests that can reduce yields primarily by reducing seedling numbers. Canola is particularly susceptible to yield losses, with losses up to 60–80% in previous trial work (Nash et al. 2007).

The ability to predict seasonal slug risk in advance will enable growers to prepare to manage slugs earlier and more intensively. This paper describes three main activities:

• the Riverine Plains survey to better understand the current problem and management practices for slugs
• development of population models to predict slug risk
• population monitoring for three years across southern Australia.

The first year of results on modelling and monitoring are from the current GRDC investment, ‘Optimising slug management: enhancing capacity and capability through population modelling and innovative management strategies (UOA2308-004RTX)’.

Methods

A survey was run by Riverine Plains in 2024 (June–September) in collaboration with the Irrigation Farmers Network, Vic No Till, and Birchip Cropping Group. A total of 17 questions were asked using SurveyMonkey® to gain insight into growers’ slug problems and management.

The GRDC project (UOA2308-004RTX) consists of a modelling, monitoring and bait trial component. A stepwise approach is being developed for grey field slug and black keeled slug based on known life history information and data obtained from cultures.

Monitoring by collaborators involves up to six monitoring paddocks with known slug populations, set up in 2024 for monthly monitoring over 3 years. Twenty slug mats (500mm by 500mm) were set up in two transects consisting of 10 mats each spaced 10m apart. Locations within the paddock for monitoring were selected based on yield maps from previous years where substantial yield losses attributed to slug damage were observed. Monitoring is being conducted monthly to collect data on the number and species of slug under each mat, and a subsample of slugs were sent to SARDI for dissection to identify whether the slugs are adult or juvenile. Environmental measurement includes temperatures (ambient, bare soil, under mat), soil moisture, weather conditions, wind and time of day. All paddocks are being managed using standard agronomic and slug management practices.

Results

Survey

Of the 54 respondees, 72% were growers. A large proportion of respondees (87%) had slug issues over the last 5 years, however the survey also captured responses from those that did not. Cropping rotations are based on canola (94%), wheat (96%), and barley (63%), with faba beans (71%) the dominant pulse grown. Soil ameliorants are widely applied (gypsum 83%, lime 81%), as is ‘high’ rates of urea (74%), with some urea applied pre-sowing (26%). Forty-five respondees indicated slug problems had increased on average by 70% [0–100%] over the last 5 years, based on their worst year. The increase in slug problems is supported by the area to which molluscicides baits were applied (4% no bait applied) and the expenditure on bait: 56% spent $50–100/ha in 2023. Constantly baiting isn’t sustainable.

When looking at farming practices that may contribute to slug threats, a large proportion of respondees practiced some form of conservation agriculture: no-till (39%), minimum tillage (67%), stubble retained (77%). Removal of stubble was suggested by three comments as one cultural method to limit slug numbers. ‘Burning stubble is extremely effective’, yet 69% of respondees are burning stubble with limited control: ‘have had them and baited in fields that have been cultivated, burnt, retained’. Despite rolling being an effective tool to prevent slug damage, only 10% of respondents used this method. Some cultivation is also used, especially to incorporate gypsum and lime: 35% speed tiller and 37% kelly chain.

The increase in slug problems in the last 1–2 years suggests either that these pests may be expanding in range, and/or numbers have increased in areas where they have existed for some time. The damage caused by slugs in 2023 saw large areas across southern NSW, northern Victoria and southeastern SA needing to be resown due to growers being unaware of the extent of slug populations. The yield penalty for late canola crops was estimated at 1t/ha in northeast Victoria, which should be considered an opportunity cost in addition to the direct costs of molluscicides. Previously, canola budgets allowed for $60–80/ha for baiting where slugs were a threat to southwest Victorian crops, yet this was rejected by southern NSW growers. The current survey supports that costing in a high-pressure year (such as 2023). Anecdotally, because growers were proactive in applying bait at sowing (in 2024), the cost of baiting was less and there were no reports of resowing due to slugs.

Modelling

Population models for slugs are being developed with the aim of predicting slug population size based on weather conditions at key times (primarily temperature and rainfall). Laboratory experiments are generating new data on how these variables influence rates of slug growth (Table 1), which are being incorporated into models.

Experimental data show high variability between individuals in the time to sexual maturity for the black keeled slug. The lab results for black keeled slugs (Table 1) are concordant with previous grey field slug studies (Shirley et al. 2020). That is, individual slugs reach sexual maturity at different times independent of temperature, with only a proportion of the population at one time actively breeding.

Table 1: Blacked keeled slug weight data for known ages of individuals reared from the same cohort.

Monitoring

Monitoring found that slugs were active across several regions despite dry autumn and winter conditions in 2024, and slug communities varied considerably in individual paddocks. In Southwest VIC, Wimmera VIC and WA, slugs peaked in June and July, while in Riverina Albury NSW, Eyre Peninsula SA and Southeast and Mid North SA, slug numbers peaked in late July or August. We look forward to seeing how the slug population changes throughout the life of the project.

Results are presented only for the two dominant pest species: black keeled slugs and grey field slugs. Brown field slugs captured were identified by dissection as Deroceras invadens. Striped field slugs captured were identified by dissection as Ambigolimax valentianus.

A total of 2060 black keeled slugs were recorded across all sites up to December 2024. Numbers increased across all sites in late winter early spring, with the highest populations observed in SA Eyre Peninsula and VIC Southwest (Figure 1). Subsequent decreases in spring were recorded in VIC SW, Riverina Albury NSW and WA after those peaks.

A total of 2247 grey field slugs were recorded, however, none were found at Albury, SA Fleurieu, VIC Wimmera or in WA (Figure 2). Grey field slugs are active for longer compared to black keeled slugs. Grey field slugs were found laying eggs in December at one irrigated site in Southeast SA.

Beneficial organisms regulate slug numbers, although understanding of the role of beneficials on slugs is limited. Interestingly, a high proportion of individuals found in several paddocks in Southwestern Victoria had ciliate infections. Further, a reasonable number of predatory carabid beetle were observed under mats. This raises the question of whether natural enemies play an important role in limiting slug populations. This question is partially addressed in the discussion with some simple modelling.

Figure 1. Average black keeled slug population under mats across all sites by region. The y-axis for mean count is different per region, depending on slug number found within the region. Solid lines represent adult slugs and juveniles are represented by dotted lines.

Figure 2. Average grey field slug population under mats across all sites by region Australia. The y-axis for mean count is different per region, depending on slug number found within the region. Solid lines represent adult slugs and juveniles are represented by dotted lines.

Discussion

The grower survey has identified some key factors that increase risk, in particular with canola following faba beans: ‘slug numbers a lot higher after wet spring and faba beans’. Growers in the northeast of VIC commonly grow wheat after faba beans. Cereals are more tolerant of slug feeding than canola. One of the biggest issues facing Australian broadacre farming systems is lack of diversity due to tight crop rotations containing canola, which is thought to increase slug numbers. Models being developed will include crop rotation.

Some modelling supports the role of carabids as a natural enemy and their interaction with mollusc pests (Symondson 1994, 2004). Australia’s complement of carabid beetles includes several species that are opportunistic predators of slugs. The role of these species in slug biocontrol has been found in broadacre cropping situations, where minimum tillage seems to be favouring carabid numbers (Nash et al. 2008a). Australian native carabids, endemic species to Southwest Victoria, demonstrated an ability to reduce populations of the introduced Deroceras reticulatum in an enclosed field cage and field studies (Nash et al. 2008b, Horne and Edward 1998, Hill et al. 2017). Despite these studies examining a limited number of predator species, focused within a single year on the adult stage of their two-year lifecycle, they demonstrated promise for the reduction of slug activity and plant damage.

Both seedling and grower’s low tolerance to damage, and hence, low economic action thresholds, do not favour the conservation of natural enemies in farming systems because there is a lack of prey. Limited prey resources are thought to lead to unstable population dynamics, often leading to pest flares. Understanding the ecology of both carabid beetles and slugs is needed to harness biological control in farming systems.

Predator-prey dynamics refer to the interactions between two species, where one (the predator) hunts and consumes the other (the prey), influencing population sizes and ecosystem stability. These interactions often follow cyclical patterns, with predator populations increasing as prey numbers rise, and subsequently decreasing when prey becomes scarce. Understanding these dynamics is crucial for ecological studies and the potential for biological control of slugs, as it highlights the balance within food webs.

Mathematical models, such as the Lotka-Volterra equations, are often used to predict these dynamics. A basic form of the Lotka-Volterra equation was used to model carabid–grey field slug dynamics using data from previous broadacre studies, to test the validity of releasing carabid beetles to augment their control of slug populations. Models were run in R using the package deSolve. The assumption was a slug population intrinsic growth rate of 0.03, as observed with populations fed canola. Several factors influence predator-prey dynamics, including the availability of resources, environmental conditions, and the adaptability of both species. However, results from a very simplistic model (Figures 3 and 4) suggest when predator (carabid) populations decline due to low prey (slug) numbers, for example, dry seasons or other controls, prey numbers can increase rapidly causing pest flares.

The model (Figure 3b) of unstable community dynamics have been observed in the paddocks before in 1999, 2013–2014 and 2022–2023. The ongoing monitoring of carabids and slugs is slowly un-teasing these food webs, with stepwise models considered an important step forward in understanding slug threats.

Figure 3. Lotka-Volterra models of predatory carabid (red dash line) and slug prey (black line) populations. a) An example of a stable dynamic based on 2004 field data initialised with 5 slugs and 0.8 carabids/refuge – projected that the slug population decreases, but above action thresholds for canola. b) An example of an unstable dynamic – when slug numbers are reduced to almost nil, so are carabid populations, resulting in slug flares approximately two years later.

Conclusion

These initial results have highlighted that, even in years with a decile one rainfall, relative humidity at the soil’s surface can still be high enough for slug activity and reproduction. Overall, slug mats do present a good, inexpensive option for growers if wanting to monitor slug populations if placed onto moist soil. The mats also provided a refuge for other pest and beneficial species such as slaters, millipedes, carabids and spiders. Ongoing research is leading to the conclusion that slugs are highly adaptable, hence the need for new ways of thinking to predict threats and manage this group of pests in a sustainable way.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support. The Riverine Plains slug survey was funded through the Victoria Drought Resilience Adoption and Innovation Hub, which is funded through the Australian Government’s Future Drought Fund. The authors thank retail agronomists and commercial clients that supported the research presented in this paper.

Reference

Hill MP, Macfadyen S, Nash MA (2017) Broad spectrum pesticide application alters natural enemy communities and may facilitate secondary pest outbreaks. PeerJ, e4179.

Horne PA, Edward CL (1998) Effects of tillage on pest and beneficial beetles in the Wimmera region of Victoria, Australia. Australian Journal of Entomology 37, 60-63.

Nash MA, Thomson LJ, Hoffmann AA (2007) Slug control in Australian canola: monitoring, molluscicidal baits and economic thresholds. Pest Management Science 63, 851 - 859.

Nash MA, Thomson LJ, Hoffmann AA (2008a) Effect of remnant vegetation, pesticides and farm managment on abundance of beneficial predator Notonomus gravis (Chaudoir) (Coleoptera: Carabidae. Biological Control 46, 83-93.

Nash MA, Thomson LJ, Horne PA, Hoffmann AA. (2008b) Notonomus gravis (Chaudoir) (Coleoptera: Carabidae) predation of Deroceras reticulatum Müller (Gastropoda: Agriolimacidae), an example of fortuitous biological control. Biologcial Control 47, 328-334.

Shirley M, Howlett S, Port G. (2020) Not all slugs are the same: variation in growth and development of the slug Deroceras reticulatum. Insects 11, 742.

Symondson WOC (1994) The potential of Abax parallelepipedus (Col: Carabidae) for mass breeding as a biological control agent against slugs. Entomophaga 39, 323-333.

Symondson WOC (2004) Coleoptera (Carabidae, Staphylinidae, Lampyridae, Drilldae and Silphidae) as predators of terrestrial gastropods. In 'Natural enemies of terrestrial molluscs'. (ED G.M. Barker) pp. 37-84 (CABI).

Contact details

Michael A Nash
whatbugsyou@gmail.com