Paddock Practices: Be on alert for slugs in the HRZ
Date: 22 Mar 2021
Key points:
- A year-round integrated pest management strategy is essential for slug control.
- Stubble management and weed control help reduce slug populations.
- Use strategies at seeding to protect emergent crops.
- Avoid vulnerable crops such as canola in paddocks with a known slug problem.
Slugs are a significant pest in the high rainfall zone of the southern cropping region.
Milder, wetter summers such as that experienced in many areas in 2020-2021 are favourable for slugs and therefore growers need to be aware of the potential risk to emerging crops.
Wet springs can also allow slugs to extend their breeding season, leading to an increased risk of damage to crops for the following growing season.
No single, once-a-year tactic will successfully control slugs and year-round integrated pest management is essential.
Understanding slug behaviour
Moisture availability is a key regulator of slug populations. Paddock conditions that support soil moisture retention will generally favour slugs.
This moisture requirement means most slug species seek shelter in the soil and travel in cracks or between clumps of dirt. Weeds provide moist refuge and food for slugs through the summer.
Slug eggs cannot survive hot and dry conditions, but adults will breed opportunistically as soon as soil moisture permits.
Species that burrow into the soil, particularly the black-keeled slug, need moisture to penetrate down through the soil profile before they become active.
The timing and level of slug activity will be determined by weather conditions in autumn, as slugs respond to the more sustained moisture events that coincide with seeding in late April or early May.
These behaviours mean crops are most vulnerable to damage from slug feeding during establishment. Understanding slug behaviour and paddock history will provide a valuable guide for monitoring slug activity and risk.
Activities that limit paddock moisture retention and deny slugs of shelter, especially in late summer, can help minimise slug numbers before the onset of autumn rain events.
Increased slug feeding and breeding usually coincides with crop seeding, when baits should be used to protect emergent crops.
Management begins with monitoring
Continual monitoring of slug populations and slug damage is essential for planning control and mitigation strategies.
In recent camera studies of slug movement in South Australia and Western Australia by Department of Primary Industries and Regions research division, South Australian Research and Development Institute (SARDI), and WA Department of Primary Industries and Regional Development (DPIRD) (GRDC Project DAS00160), peaks in autumn and winter activity varied between sites but generally occurred in periods where the soil water volume at a 10 centimetre depth was at least 0.05-0.45 m3/m3 and the relative humidity at ground level was greater than 96 per cent.
Slug activity can be assessed using surface refuges or bait lines. Refuges should be checked early in the morning before slugs seek shelter deeper in the soil as the day gets warmer. An accurate assessment will require more than 10 refuges per 10 hectares. Several rows of refuges should be placed across representative areas, as slug numbers can vary over quite small distances.
Recent work (SARDI, DPIRD) established that silver slug mats are more attractive to slugs than tile refuges and are the recommended material for refuge-based monitoring if they can be sourced.
The work also found that bait lines are a very effective alternative to refuge traps, with the benefit of being easier to deploy for monitoring slug activity just before seeding time.
Bait lines are simply laid along a furrow and checked two days later for dead slugs. The baits will need to be refreshed every two to three weeks.
The presence of dead slugs indicates that slugs are active and feeding, so baiting should begin.
Control strategies
Steps to minimise slug numbers before they can breed should be taken well before the arrival of seasonal rain in autumn. An IPM strategy from mid to late summer could include:
Stubble removal
Grazing or baling to remove stubble during summer can help reduce slug refuges and reduce moisture retention, by compromising the slug habitat with hotter, dryer conditions. Slashing can also help the sun to heat the upper soil. However, cut stubble or harvest trash can aid moisture retention and should be cleared. Stubble removal can also make subsequent bait applications more accessible.
Stubble burning
Where acceptable, burning stubble or windrows is an effective way to remove slug habitat, while heating and drying the soil surface. The impact of burning on soil, moisture, air quality and public perception must all be weighed against the benefits of slug control in the cropping system.
Stubble removal and burning is most effective against surface-active species, such as the grey field slug, but will have limited impact on burrowing species.
Baiting
While snail baiting should begin by late summer, slug baiting is primarily for crop protection and can usually begin with seeding. Research conducted by Southern Farming Systems with GRDC investment found emergent plants are most susceptible up to growth stage 14 in cereals and the four true leaf stage in canola (Midwood, 2014).
In trials in western Victoria (Midwood, 2014), baiting at seeding and again eight to 10 days later was found to be most effective.
Recent grower innovations for distributing mouse bait from the seeder’s air commodity cart may also be adaptable to slug bait. (See ‘Useful Resources’ below.)
Baits may even be sown with the seed to target burrowing species.
Registered slug baits include:
- Metaldehyde baits, which may be less effective when temperatures drop below 10°C, particularly on smaller slugs (Nash et al., 2016).
- Methiocarb-based baits which is also toxic to carabid beetles, one of the only natural slug predators in Australia.
- Iron EDTA complex bait.
Baits must be stored and applied carefully to avoid failure. Metaldehyde baits lose their effectiveness when exposed to heat, including during storage. Bran-based baits break down quickly when exposed to rain and bait reapplication may be necessary.
Rolling
Rolling after sowing consolidates the soil, making it harder for burrowing slugs to find the seed. Heavy rolling will also crush clods and flatten stubble to deny slugs of hiding places, restrict their movement and make bait encounters more likely.
Seed bed compaction offers the added benefit of aiding rapid crop establishment, shortening the period of maximum vulnerability to slug damage. (See below.)
Crop selection
Canola, sunflowers and field peas have been found to be more susceptible to slug damage than cereals, and appeared to encourage slug populations (Nash et al., 2016.). Faba beans and linseed were found to have the least effect on slug populations.
Sowing canola or peas into paddocks with a history of slugs may be risky, especially if soil moisture levels are high. In these conditions, growers should bait at sowing and monitor crops closely for signs of slug damage.
Early establishment
After a wet summer or autumn, early sowing may help vulnerable crops such as canola to establish strongly before slugs emerge. Rolling to consolidate the seed bed can help seed to soil contact and promote strong establishment. Choosing a vigorous variety should also help the crop to out-grow the immediate threat to seedlings.
Manage slugs to minimise damage
It is essential to deny slugs of their habitat before the onset of autumn rain events so the population is minimised before breeding begins.
Once slugs are actively breeding, no current control measure will reduce populations below the established thresholds. For canola, this is as low as one slug per square metre. Slugs can breed twice between sowing and harvest in particularly wet and mild seasons.
The precise timing of slug breeding will vary with local environmental conditions. It is essential to use an IPM strategy to minimise numbers year-round. This strategy should be based on population monitoring to establish risk, denying moisture and shelter over summer, baiting from sowing to protect seedlings, and supporting vigorous plant establishment in high-risk areas.
Useful resources:
Slug Control Fact Sheet – GRDC, 2013
Snail and slug baiting guidelines – PIRSA-SARDI, 2016
GRDC Stubble Initiative, 2019
Mitigating snails, slugs and slaters in Southern Western Australia – GRDC, 2019
Seeder-mounted mouse bait spreaders:
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