Slugs, earwigs, millipedes, and slaters – current management options

Slugs, earwigs, millipedes, and slaters – current management options

Take home messages

  • Soil pests remain challenging in retained stubble systems, and the loss of chlorpyrifos reduces control options.
  • Control millipedes and slaters before applying slug bait.
  • Baiting slugs at sowing is critical to protect crop seedlings from feeding damage.
  • A new GRDC slug project is underway to improve slug prediction and management options.

Background

Slugs, earwigs, slaters and millipedes are favoured by modern no-till farming systems that retain stubble and plant residues, which increase soil moisture and organic matter. Consequently, these pests have become increasingly prevalent over recent years (Micic et al. 2008, PIRSA 2018, Umina 2019). In recent seasons, some wet spring conditions have contributed to higher than usual slug populations in some regions. This paper briefly summarises the current situation and control options regarding these pests.

Earwigs, millipedes and slaters: who’s the culprit

These pests are all favoured by similar conditions, such as high stubble loads, and may occur together. It is critical, but can be challenging, to identify which pest is responsible for direct plant feeding damage before deciding on control options (otherwise they may be costly and ineffective).

A recent study, funded by SAGIT (S-UA1420), investigated the feeding activity of earwigs, millipedes and slatersin six cropping fields of SouthAustralia(van Helden and Brodie 2022). Infrared cameras were used to observe invertebrate and vertebrate activity on seedlings of seven different crop types – barley, wheat/oat, canola, chickpea, faba bean, lentil, and vetch. Video images were analysed to determine which pests were feeding (n = 16,645 individual pest observations across all crops).

In the six paddocks, most direct feeding on crops was caused by slugs, earwigs and mites (Figure 1). The slater species, Australiodillo bifrons, was abundant but did not cause direct feeding damage. Similarly, Portuguese millipede was commonly seen but caused minimal damage.

Millipedes feed largely on organic matter and often do not damage crops, even when in high numbers. Occasionally, millipedes cause feeding in emerging canola, lupin and lucerne (Douglas et al. 2019). Higher millipede abundance is usually associated with organic soil with higher stubble loads. Often, millipedes can be blamed for damage that may be caused by other less visible pests, such as earwigs and certain slater species.

Regarding slaters, species identification is critical, as only some species are damaging. The introduced pill bug, Armadillidium vulgare, is highly damaging in emerging lentils and canola and can reach densities of several thousand individuals per m2 in stubble/trash prior to crop sowing (Perry, unpublished data). This species rolls into a ball when disturbed.

European earwigs are highly damaging in emerging canola and lupins, and to a lesser extent cereals. They become active at night and are often not visible during the day.

Per cent of invertebrate and vertebrate pests observed feeding (yellow) on crops or not feeding on crops (blue). SAGIT project (S-UA1420).

Figure 1. Per cent of invertebrate and vertebrate pests observed feeding (yellow) on crops or not feeding on crops (blue). SAGIT project (S-UA1420).

Management of earwigs, millipedes and slaters

For earwigs, millipedes and slaters, there are currently limited management options available, and no pesticides registered for in-crop use in broadacre crops. Fipronil-based and thiamethoxam-based seed treatments are registered for earwigs in some crops, but provides limited protection of seedlings from chewing damage or loss. Broad-spectrum insecticides that are registered in-crop for use on other pests in SA may have some efficacy against these invertebrate pests, but control is usually limited by lack of coverage in heavy stubble and growers must only use products according to the registered label. Always read and follow product labels.

How best to manage these pests requires more research. The best available strategies at present are likely to be to reduce refuges and food sources in paddocks, such as by removing stubble and trash residues. If high numbers of earwigs or slaters are detected prior to crop sowing, avoid planting canola or other susceptible crops. In canola, rapid plant establishment (early sowing, high vigour varieties) is one of the best defences against pest attack.

Although rarely damaging to crops, millipedes will consume snail and slug bait and should be controlled before applying molluscicide baits.

Slugs

The grey field slug (Deroceras reticulatum)and the black keeled slug (Milax gagates) are damaging species that have become more prevalent in some locations in the past few years. Slugs require moist habitats to survive and are typically found in wetter regions (for example, >400mm annual rainfall) or in wetter seasons. Current integrated management strategies for slugs include cultivation, stubble burning, rolling to compact the seed bed, molluscicide baits, and biological control (for example, carabid beetles, flatworms).

Slug monitoring project (UOA2308-004RTX)

Optimizing slug management: Enhancing capacity and capability through population modelling and innovative management strategies (Dr. Kate Muirhead, Dr Kym Perry, Dr. David Logan, Dr. Thomas Heddle)

A new study funded by GRDC, ‘Optimising slug management: enhancing capacity and capability through population modelling and innovative management strategies’ (UOA2308-004RTX), is investigating predictive models of slug risk to broadacre agriculture in southern and western growing regions of Australia. Thirty sites across SA, VIC, NSW and WA are being monitored monthly to determine the abundance of slug species. The project aims to provide growers with decision support tools to identify and manage slug risk.

From seven sites, 1039 slugs have been collected to date from which four species have been confirmed: the black keeled slug (Milax gagates), striped field slug (Ambigolimax sp.), the grey field slug(Deroceras reticulatum) and the brown field slug(Deroceras sp.).

This project, which will continue through to 2026, aims to identify the triggers for emergence and feeding damage of these species, and will investigate new management tactics, such as spring baiting.

Slug populations across South Australia including Eyre Peninsula, Fleurieu Peninsula, Mid-North and Southeast.

Figure 2. Slug populations across South Australia including Eyre Peninsula, Fleurieu Peninsula, Mid-North and Southeast.

Tips for baiting slugs

In areas at risk, baiting is an essential part of effective integrated slug management. Below are key management tips for effective baiting.

Monitoring

Monitor high-risk paddocks using refuges for slugs, such as mats, tiles or similar (for example, wood, bricks, drums with water). Paddocks at risk include those with previous slug damage. Canola is particularly susceptible to seedling damage or loss, especially in paddocks with clay soil. Slugs begin emerging with cool, moist conditions. Most slugs prefer temperatures ranging between 14–18°C. Monitor risk areas before and at sowing.

Bait lines are an effective alternative to refuges – simply apply a line of baits along a seeding row and check for dead slugs after 1–3 days. Bait lines need to be re-applied every 7–14 days depending on rate of bait breakdown.

Baiting for best results

To protect seed and emerging seedlings from slugs, baiting needs to be immediately after sowing, following rolling if this culture method is integrated into management. If populations are high, or other pests are feeding on baits, baits may need to be reapplied as per label rates. Monitoring after bait application is necessary to ensure bait remains to protect seedlings: that is, if there is no bait left, reapply baits that day.

Control slaters and millipedes

Slaters and millipedes will consume slug and snail pellets. If present in high numbers before crop sowing, these pests must be controlled before applying baits. In some scenarios, if baiting fails to control slugs and/or snails and there are high populations of slaters and millipedes, these species may be consuming the baits with no consequences to their survival before slugs/snails get to the baits.

Know your slugs: ‘Slugs in crops: the back pocket guide’

Different regions may have more than one species of slug attacking crops. Use the ‘Slugs in crops: the back pocket guide’ (see link below) to identify your slug species. If you don’t know which species you have, you can send photos to PestFacts or the Slug Team at SARDI/University of Adelaide.

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 SAGIT for funding support. We wish to acknowledge all the growers and consultants who have helped collect the data for these projects.

References

Douglas J, Hoffmann A, Umina P, Macfadyen S (2019) Factors influencing damage by the Portuguese millipede, Ommatoiulus moreleti (Julida: Julidae), to crop seedlings. Journal of Economic Entomology 112(6), 2695-2702.

Micic S, Hoffmann AA, Strickland G, Weeks AR, Bellati J, Henry K, Nash MA, Umina PA (2008) Pests of germinating grain crops in southern Australia: an overview of their biology and management options. Australian Journal of Experimental Agriculture 48(12), 1560-1573.

PIRSA (2018) PestFacts SA (http://www.pir.sa.gov.au/research/services/reports_and_ newsletters/pestfacts_newsletter)

Umina PA (2019) Potential role of seed treatments in the management of emerging arthropod pests of canola. Crop and Pasture Science 70(10), 890-898.

van Helden M, Brodie H (2022) Caught red-handed: Revealing invertebrate species and conditions causing seedling damage in field settings.

PestFacts south-eastern

Slugs in crops: the back pocket guide

Contact details

Thomas Heddle
School of Agriculture, Food and Wine
University of Adelaide
Main Waite Building, Waite Road, Urrbrae SA 5064
0401 575 159
thomas.heddle@adelaide.edu.au
@ThomasHeddle1

GRDC Project Code: UOA2308-004RTX,