Strategies to limit slug threats other than baits

Strategies to limit slug threats other than baits

Take home messages

  • Enhance ecosystem services by providing food for beneficial invertebrates and limiting disruption to their populations.
  • Incorporate bottom-up approaches to Integrated Pest Management: for example, improve crop tolerance to herbivores by increasing seedling vigour.
  • Integrate management practices to limit slug activity: that is, cultivation prior to and rolling after sowing before applying molluscicide baits.

Background

The adoption of conservation tillage systems to retain soil moisture in marginal Australian farming systems is associated with changing pest threats. Modern farming practices include the increased use of pesticides yet fail to reduce threats to production (Nash et al. 2023). Published research indicates an increased prevalence of slugs in broadacre cropping systems due to the application of insecticides, either as a foliar sprays and/or seed treatments, which affect carabid beetles, a natural predator of slugs (Hill et al. 2017, Douglas et al. 2014).

Slugs are particularly damaging to establishing canola, with yield losses in untreated areas of experiments at 60–80% (GRDC DAS00134 data). One way to estimate the cost of slugs is expenditure on molluscicide baits, which continues to increase in Australia, with over $49 million spent last season (2022–23 APVMA data). Locally costs are greater, for example growers in southwest Victoria spend $30–$120/ha on bait to protect canola from slugs. Additionally, 95% of canola is sown into burnt and/or cultivated ground in these areas. In areas where slugs are a high risk, some growers have shifted away from growing canola, especially where they cannot implement strategic burning and cultivation. That lost opportunity cost is estimated upwards of $270 million annually to the canola industry. A 5% production loss by slug and snail activity represents a loss of more than $130 million to the Australian canola industry.

Management changes that are improving canola establishment, hence tolerance to slugs, include:

  • increased usage of baits applied as crop protectants
  • earlier sowing
  • increased seedling vigour
  • improved plant nutrition; and
  • integration of cultural practices that improve germination and growth.

To establish crops where slugs are a threat, molluscicides are used as a crop protectant, integrated with cultural controls to achieve successful establishment of canola. One key factor in successful establishment has been the shift to earlier sowing (Figure 1) when soil temperatures are still warm and the crop emerges and grows more quickly, if moisture is available. However, slugs have been, and continue to be, a major threat despite a dry autumn and late break in 2024.

This paper presents strategies adopted by Australian growers which complement molluscicides and improve canola establishment in areas threatened by slugs.

Discussion

Limiting disruption to natural enemies

Enhancing natural enemies is a cornerstone of IPM, including reducing slugs by maintaining predatory beetles (Hill et al. 2017). By limiting the use of disruptive pesticides, ecosystem services; such as pest control, pollination (for example, bees) and soil engineering (for example, ants); will be maintained. To help growers and advisers make informed choices around pesticide use in Australian grain crops, a rating score can be used to calculate cumulative disruption over two years, with values above this score expected to disrupt natural enemies (Nash et al. 2008). Impacts may vary in the field, especially if multiple applications of a chemical occur. Evidence of non-target impacts due to seed treatments is scant, but does exist for Carabidae (Douglas et al. 2014, Douglas and Tooker 2016), a ground beetle that feeds on slugs, earthworms and caterpillars. Some data are available for fungicide impacts from the International Organisation for Biological and Integrated Control (IOBC) database, however these do not account for additive impacts. For example, pyrethroid and triazole/imidazole fungicide combinations increase the toxicity to beneficial invertebrates, by reducing repellence of pyrethroids, hence increasing exposure. Table 1 provides information for growers about the pesticides they commonly use that are limiting slug control by predatory ground beetles, and potentially reducing some soil functions. Despite knowing the disruption pesticides can cause to ecosystem services provided by beneficial invertebrates, in particular insecticides (CESAR Australia), the grains industry continues to use them prophylactically as a ‘cheap’ form of insurance against sporadic production threats. For a change to management to occur, perceptions need to shift.

Bottom-up IPM

A new approach is needed that is underpinned by host plant resistance, new cultural practices, ecological indicators, reliable predictors and infrequent emergency intervention strategies that move away from heavy reliance on monitoring and economic thresholds traditionally supported by rigid chemical-based management strategies (Nash and Hoffman 2012). This bottom-up approach to IPM (Han et al. 2022) needs to be based on developing stable crop environments that can limit fundamental niches available for exploitation by sporadic pest populations and increases crop resilience to resident herbivores.

Top-down forces have been conceptualised for practices in agriculture (for example, release of predatory wasps), yet bottom-up forces have received little attention in the framework of IPM. Bottom-up effects are major ecological forces in crop-invertebrate pest-natural enemy multitrophic interactions and need to be considered to optimise IPM. Irrigation, fertiliser use, crop resistance, habitat manipulation, organic management practices and landscape characteristics have all been shown to trigger marked bottom-up effects and thus impact pest management (Han et al. 2022). An experiment in the US (Le Gall et al. 2022) demonstrated a reduction in damage caused by slugs where maize was sown directly into cover crops after no-till soybeans, compared to no cover crop or terminated cover crop. This result points to other processes, rather than top-down control of slugs by predatory beetles, as natural enemy activity-density was greatest in bare plots. Green plots had the lowest activity-density. This leads to the hypothesis that the plants growing in the green plots were less favourable for slug populations to increase, and/or something had changed in the unterminated, green-on-green biome.

Australian research on cover crop interactions with pests leads to two questions:

Are the cash crop plants following cover crops ‘healthier’?

Can Australian growers grow ‘healthier crops’ more tolerant to establishment pests?

Table 1: The impacts of pesticides commonly applied in Australian broadacre cropping on beneficials, based on IOBC ratings that relate to reduction in the tested species’ ability to provide pest control (from Nash 2023). Ratings range from 1 to 4, where 1 = harmless <25% (green), 2 = 25%–50% (yellow), 3 = 50%–75% (orange), 4 = >75% (red). Some rates are based on specific references as listed in the following notes.

LD50 values <2μg/bee are considered toxic, 2–11μg/bee moderately toxic, 11–100μg/bee slightly toxic, and >100μg/bee not toxic. Only fungicides that are disruptive to predatory ground beetles are included. ND = No Data available; ST = applied as seed treatment; B = applied as bait.

Product example

Active compound (group)

Target

Ants

Ground Beetles

LC50

μg/bee

Overall Rating

Ironmax Pro®

iron

slugs

1

1

 

1

Vivus Max

NPV (31)

heliothis

ND

1

 

1

Dipel® SC

Bt (11)

caterpillars

ND

1

 

1

Vantacor®

chlorantraniliprole (28)

caterpillars

ND

1

>100

1

Metarex Inov®

metaldehyde

slugs

1

1

 

1

Apron® XL

metalaxyl-M  (4)

damping off

ND

1

97.3

2

Spin Flo®2

carbendazim (1)

fungal diseases

ND

1

>100

2

Impact®

flutriafol  (3)

black leg

ND

1

>100

2

Sumisclex®

procymidone (2)

fungal diseases

ND

2

>100

2

Steward®

indoxacarb (22A)

caterpillars

4

1

0.0266

2

Success® Neo

spinetoram (5)

caterpillars

ND

1

3.0

3

Methomyl 225

methomyl (1A)

broad

ND

4

9.5

4

Gaucho®

imidacloprid  (4A)

aphids / mites

4

3

0.007

4

Cruiser®

thiamethoxam  (4A)

aphids / mites

4

3

0.005

4

Talstar®

synthetic pyrethroids (3A)

broad

4

4

0.015

4

Lorsban®

organophosphates (1B)

broad

4

4

0.059

4

Cosmos®

fipronil (2B)

broad

4

4

0.0125

4

Poncho® Plus1

clothianidin (4A)

broad

4

4

0.004

4

MethioSHIELD™

methiocarb

slugs / broad

4

4

 

4

Veritas®3

tebuconazole (3) + azoxystrobin (11)

fungal diseases

ND

4

>200

4

Notes: 1Poncho® Plus also contains imidacloprid so ratings are based on both; 2carbendazim disrupts earthworms, hence the rating; 3The individual actives of Veritas are not toxic to predatory beetles, yet in combination they are toxic.

Early sowing

One strategy to avoid establishment pests, such as slugs, is to create a mismatch in crop/pest phenology. By sowing susceptible crops before slugs emerge from the soil, the plants get a chance to establish before slugs become active on the soil surface associated with relative humidity above 96%. However, in irrigated situations and seasons where a full soil moisture profile exists, combined with full stubble retention, slugs are often active early: for example, southeastern Australia in 2023 when slugs were observed causing seedling losses to lentils, wheat and faba beans.

The main advantage of early sowing is quick establishment of susceptible crops. In effect, the management aim is to outgrow the herbivorous slugs. The thermal time for canola emergence is reported to be between 90°C.d and 115°C.d. In southern Australian environments, this typically translates to 4–5 days under average late March to early April soil temperatures of 25°C, 7–8 days at 15°C in late April to early May, and over 12 days in May when temperatures drop below 10°C (McDonald and Desbiolles 2023). With canola now generally being sown a month earlier, in April, across southern Australia (Figure 1), this has seen quicker establishment. However, a late break hinders this strategy. Current GRDC investments are researching ways to improve establishment, such as deeper sowing with improvements to seed quality, including novel traits as used in Europe, to overcome pest issues and seeding equipment requirements.

Sowing dates of canola extracted from two NVT sites in southwest Victoria. These indicate sowing of canola is now one month earlier than when canola was traditionally sown in May. The y-axis presents julian days as a count of days since Jan 01 being one (1).

Figure 1. Sowing dates of canola extracted from two NVT sites in southwest Victoria. These indicate sowing of canola is now one month earlier than when canola was traditionally sown in May. The y-axis presents julian days as a count of days since Jan 01 being one (1).

Cultivation before and rolling after seeding before applying baits

Research (GRDC project SFS00023) demonstrated ‘a positive result from rolling immediately after sowing compared to not rolling. This was especially noticeable at Inverleigh and at Hamilton where there were higher slug numbers and damage. This was nicely demonstrated at Hamilton where the control treatment was rolled and resulted in less crop damage compared to applying bait but not rolling. This is a cheap, non-chemical, cultural control technique which restricts slug movement in the seed bed and also helps to consolidate soil around the newly sown seed, and therefore, improve establishment.’

Research (GRDC project DAS00136) also demonstrated light cultivation with speed discs prior to sowing reduced slug activity, equivalent to a single application of a 50g/kg metaldehyde bait, but with some loss in canola plants (Figure 2). Cultivation as a single factor was not able to significantly reduce seedling loss to slugs (Χ2 2.5, P = 0.113), however there was a significant interaction with baits (Χ2 11.6, P = 0.021), indicating cultivation combined with slug baits improved canola seedling survival where slugs were active. This result was concordant with a second site tested in 2014 and previous results which demonstrated that speed tillers reduce slug activity (Nash et al. 2008). Unfortunately, cultivation has a deleterious impact on carabid beetles that feed on slugs (Nash et al. 2008).

Old data highlight the advantages of cultural practices to limit slug damage, especially when dry sowing. These advantages are:

  • protecting the seed from slugs that are active
  • reducing soil surface relative humidity below 96%, which is considered optimal for slug activity, and
  • improving moisture conditions around the seed to aid germination.

Grey field slug (Deroceras reticulatum [D_ret) relative surface abundance (columns with error bars being s.e. mean) prior to and after application of cultivation, sowing of ATR Wahoo canola and bait treatments May 2014. Squares with error bars (s.e. mean) indicate seedling numbers with two true leaves, June 2014.

Figure 2. Grey field slug (Deroceras reticulatum [D_ret) relative surface abundance (columns with error bars being s.e. mean) prior to and after application of cultivation, sowing of ATR Wahoo canola and bait treatments May 2014. Squares with error bars (s.e. mean) indicate seedling numbers with two true leaves, June 2014.

The “one percenters” that make a difference in canola establishment

A few, but not all suggestions, are provided below:

  • new cultivars

- longer season allows for earlier sowing

- seedling vigour

- herbicide tolerance that allows for knockdown in-crop

  • grade for larger seed – that is, >1.8mm – increases biomass
  • reduce stubble to increase light interception and reduce damping off
  • time of sowing – mid April into warm soils, that is, >14°C
  • improved seeding equipment, thus better seed placement – that is, do not sow >5cm
  • seeding speed <8km/h
  • nutrient placement – that is, higher rates of N and P (>25kg/ha MAP) placed below the seed especially with disc seeders, and
  • avoid herbicides and seed treatments that reduce seedling vigour.

Burning

Removal of slug habitat by burning is used to improve crop establishment but does burning reduce slug populations? Overseas research indicates retention of straw does increase grey field slug numbers (Glen et al. 1984, Symondson et al. 1996). An Australian survey (in press 2024) of native roadside vegetation across southwest Victoria used generalised linear models to estimate ‘slug’ density. Black keeled slugs were positively associated with fire frequency (F1= 9.34, P = 0.004): that is, the more often a roadside was burnt, the more slugs were found under tiles.

How does burning improve canola establishment? Limited data have been collected to answer this question, but the available evidence supports three hypotheses:

  • Burning increases light interception. It has been recorded that 8% less light intercepts two leaf canola seedlings when comparing stubble retained (20cm) to burnt ground at Streatham VIC in May 2016.
  • Burning decreases damping off. Where Apron XL was applied to canola seed, all seedling loss was accounted for in models testing significance of slugs causing seedling loss.
  • Camera analysis of slug activity indicated slugs were less active on windy nights. Burning increases soil surface wind speed, hence reduces slug activity.

Burning may be thought of as a simple management option, however the underlying mechanisms reducing slug impacts on canola establishment in response to burning are complex. Further research is needed to fit the many pieces of the ecological puzzle together to improve canola establishment, hence reduce the cost of establishment pests, such as slugs, to industry.

Conclusion

Management of slugs under Australian conditions can be difficult due to seasonal climate differences. Invertebrate communities are changing in response to conservation agriculture (Nash et al. 2019) and intensification: that is, overuse of pesticides. Yet industry often fails to attribute the true cost of intensification to growers’ bottom line (Hill et al. 2017). To manage crop threats like slugs in a cost-effective way, the biology of those pests and the context in which controls are applied must be understood. Ecological knowledge is necessary to improve canola establishment, empowering growers to shift to bottom-up IPM.

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 authors also thank the retail agronomists and commercial clients who supported the research presented in this paper.

References

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Hill MP, Macfadyen S, Nash MA (2017) Broad spectrum pesticide application alters natural enemy communities and may facilitate secondary pest outbreaks. PeerJ5, e4179.

Douglas MR, Rohr JR, Tooker JF (2014) Neonicotinoid insecticide travels through a soil food chain, disrupting biological control of non-target pests and decreasing soya bean yield. Journal of Applied Ecology 52(1), 250-260.

Nash MA, Thomson LJ, Hoffmann AA (2008) Effect of remnant vegetation, pesticides and farm management on abundance of the beneficial predator Notonomus gravis (Chaudoir) (Coleoptera: Carabidae). Biological Control 46(2), 83-93.

Douglas MR, Tooker JF (2016) Meta-analysis reveals that seed-applied neonicotinoids and pyrethroids have similar negative effects on abundance of arthropod natural enemies, PeerJ 4, e2776.

Nash MA, Hoffmann AA (2012) Effective invertebrate pest management in dryland cropping in southern Australia: the challenge of marginality. Crop Protection42, 289-304.

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Le Gall M, Boucher M, Tooker JF (2022) Planted-green cover crops in maize/soybean rotations confer stronger bottom-up than top-down control of slugs. Agriculture, Ecosystems and Environment 334, 107980.

McDonald G, Desbiolles J (2023) 'Crop establishment and precision planting: Southern and Western regions.' (GRDC: Kingston ACT).

Nash MA, Thomson LJ, Horne PA, Hoffmann AA (2008) Notonomus gravis (Chaudoir) (Coleoptera: Carabidae) predation of Deroceras reticulatum Müller (Gastropoda: Agriolimacidae), an example of fortuitous biological control. Biological Control 47(3), 328-334.

Glen DM, Wiltshire CW, Milsom NF (1984) Slugs and straw disposal in winter wheat. Proceedings of the 1984 British Crop Protection Conference: Pests and Diseases, pp 139–144. (British Crop Protection Council: Croydon UK).

Symondson WOC, Glen DM, Wiltshire CW, Langdon CJ, Liddell JE (1996) Effects of cultivation techniques and methods of straw disposal on predation by Pterostichus melanarius (Coleoptera: Carabidae) upon slugs (Gastropoda: Pulmonata) in an arable field. Journal of Applied Ecology 33(4), 741-753.

Nash M, Severtson D, Macfadyen S (2019) New approaches to manage invertebrate pests in conservation agriculture systems – uncoupling intensification. In 'Australian Agriculture in 2020: from conservation to automation.' (Eds: J. Pratley, J. Kirkegaard) pp. 189–202. (Agronomy Australia and Charles Sturt University: Wagga Wagga NSW).

Nash M (2023) Slugs and no till. From the Ground Up 70,18-25.

Contact details

Michael A Nash
whatbugsyou@gmail.com

GRDC Project Code: MAN2204-001SAX, UOA2308-004RTX,