Slug solutions for central western slopes mixed farming enterprises

In September 2023, Dr Michael Nash delivered a series of workshops with the aim of raising awareness of the damage slugs can cause in pastures, the associated cost to mixed farming systems and how to manage slug threats, especially when establishing pastures and canola.

To help manage slugs, information was provided to growers and agronomists. Interaction with attendees to understand the local conditions facilitated context-specific management solutions.

Slug biology:

Slugs are hermaphrodites: both mating individuals can produce eggs that are laid in batches into moist soil.

All slugs can delay breeding when conditions are dry, hence should be considered adaptive strategists.

Activity on the soil surface occurs when relative humidity is above 95 - 96% and soil moisture is about halfway between wilting point and field capacity: that is, > 20 - 25% m³ / m³ depending on soil type.

Although all slugs require moist environments to feed and breed, each have specific biology that determines when they are active, what time they breed, and when is the best time to apply baits to protect establishing pastures and crops.

With a body moisture content of around 80%, a slug’s biggest threat is moisture loss, they survive as adults by slowing their metabolism and seeking moist refuges in the ground.

Unlike insects, snails and slugs do not have a set lifecycle, they breed when conditions are favourable.

Brown field slugs and meadow slugs seem to be more common in pastures along the western slopes of NSW and into Qld. Both species breed quite quickly, with up to two to three generations per year.

These small slender slugs require surface plant residues, thistles, or cracks in the ground for refuge to survive when conditions are dry. These species lay their eggs on the soil surface in crevices where the eggs will remain moist.

Black keeled slugs seem to be more common in cropping areas further west. This species breeds slower than other species and actively burrows deeply into the ground to survive. Black keeled slugs lay their eggs into the ground.

Understanding slug risk:

By linking climate tools such as spring rainfall days, NVDI and soil moisture with spring paddock observations of slug activity, growers can start to understand potential slug threats to the next sowing.

For example, a wheat crop with large green canopy in November with plenty of moisture and small slugs feeding in the heads will have a large reservoir of slugs that will survive in the ground over summer.

Despite a dry summer / autumn, resident populations will become active and feed on seedlings late autumn, once conditions are moist enough.  Cultural practices that dry out the soil surface limit slug activity.

Black keeled slugs are likely to migrate from the soil for up to 20 months, making monitoring difficult.

Hence, in the scenario provided, management will need to be focused integrating baiting with cultural practices to ensure successful establishment of canola. In high-pressure years, budgets for susceptible crops need to include the cost of multiple applications of short window baits (e.g. Snailex or Pestmaster) or at least one application of a long window bait (e.g. Axcela®, Metarex Inov® or Sluggoff®) at maximum label rates directly after sowing.

Reactive management, compounded by logistical constraints, often leads to poor crop establishment. Monitoring when slugs are active is the foundation of understanding when individual species are active, mating and breeding, and underpins successful management of snails or slugs.

Knowledge of slug population dynamics in your paddock will assist you to take a proactive approach, including integration of cultural practices, conservation of natural enemies and timely application of baits directly after sowing to protect seed and seedlings at establishment.

Monitoring tips in marginal conditions:

To ascertain the extent of slugs in a paddock, create temporary refuges: take a 20L plastic drum half full of water, tip out 5L, leaving it a quarter full.

This approach allows condensation to attract surface active slugs underneath the drum; check under the drums in the mornings to see how many slugs have been attracted to the moist refuge.

Place four (4) drums spread roughly ten metres apart in areas with a known history of slugs to estimate slug activity. Instead of water-filled drum refuges, you could use other refuges or cameras.

This GRDC video on slug bait timing is a useful resource.

Attractiveness:

To improve slug control, some key factors are needed for baits to be effective.

First, individuals need to encounter a bait, then they must consume a lethal dose.

For example, were the slugs found active after summer rain in Orange attracted to beer, hence likely to feed on bait?

The fermenting process is attractive to slugs, hence baits that contain flour seem to work better once damp for a couple of days.

The result of the “rugby test” between states is that slugs collected in Orange February 2024 were not attracted to any beer. The slugs were more interested in seeking refuge under the rugby field.

Fifteen slugs were released, one went to Qld (XXXX), but turned away at the last minute, and one displayed some interest in SA (Coopers) but did not go there.

These results suggest that baits can be applied too early, when individual slugs are more interested in surviving than feeding.

No wonder control can be hard, because slugs start drinking, then mate before they produce and lay eggs, which coincides with feeding.

Key messages:

To ensure even distribution of baits, calibrate your slug bait spreader.

Best bait applied at the wrong time will never be as effective as any bait applied at the right time: that is, immediately after sowing to protect the seed and seedling.

Pasture solutions central west NSW

• Early spring fallow to limit slug breeding before you resow the next autumn.
• Roll after sowing:
  • increases slug exposure to wind so less feeding activity
  • improves soil seed contact to quicken establishment.
• Heavy spring grazing to reduce slug habitat.
• Incorporate lime prior to sowing:
  • improves establishment and plant health
  • cultivation decreases slug habitat at the soil surface.

Canola solutions central west NSW

• Integrated approach includes cultural and biological controls.
• Prickle harrow and/or roll after sowing:
  • increases slug exposure to wind so less feeding activity
  • improves soil seed contact to quicken establishment.
• Incorporate lime prior to sowing:
  • improves establishment and plant health
  • cultivation decreases slug habitat at the soil surface.

Watch this video on managing slugs in canola in the central tablelands of NSW.