Slugs

Be aware of the slug risk, and if necessary apply quality baits at appropriate densities prior to crop emergence.

 

Where have they been reported?

According to SARDI slug expert, Dr Michael Nash, emerging crops this season are at high risk of being damaged by resident slugs. The recent rainfall received in many cropping areas will encourage slugs to emerge more synchronously and to forage freely. Michael is currently working at Jindera and Henty in the NSW Riverina to establish bait assessment trials in canola, and reports that the black keeled slug has started to emerge in these trials. Resident slugs are likely to emerge in regions that received soaking rains over the past 2 months, especially in moderate to high rainfall regions.

Slug habits and behaviour

The main pest species of slugs attacking broad-acre crops are the black keeled slug (Milax gagates) and the grey field slug (Deroceras reticulatum). The brown field slug (Deroceras invadens) can also be a serious pest, with species often co-occurring. Soils that retain moisture are preferred by slugs, typically heavy red loams, gravelly loams and grey clays. In a typical year, adult slugs are stimulated out of their summer aestivation following autumn rains and will generally be problematic in paddocks where they have previously been an issue. With the recent rainfall across many regions, slugs will become more active over the coming weeks.

Our advice

Use terracotta tiles or a similar type of refuge ‘trap’ to assess relative numbers of slugs. Traps should be placed on the soil surface when it is visibly wet, and then checked after a few days for the presence of slugs underneath. A few pieces of slug bait under each trap will help to attract slugs if they are present.

Economic thresholds for slugs vary depending on the species and field conditions, hence are only a rough guide. For the grey field slug, suggested thresholds are 1 slug per m² in canola and pulses, and 5 per m² in cereals and pastures, whereas for the black keeled slug, suggested thresholds are less than 1 per m² in canola, cereals and pulses and 5 per m² in pastures.

Control options

Slugs are best managed using both chemical and cultural methods, while also preserving the role of soil dwelling beneficial insects such as carabid beetles to reduce the seasonal incidence of slugs.

The most common chemical control method for slugs is the use of baits. These must be applied at sowing after good germinating rains to coincide with slug emergence and before seedlings emerge. When baiting, it is important to consider the conditions, number of baits used and how this relates to slug densities. Baiting efficacy can be reduced when there are higher slug populations, an increased availability of refuges, and higher amounts of plant material. Michael says that field trials conducted in 2011-2013 showed that: (i) ingestion of active toxicants is more effective than spray applications; (ii) 3% rather than 1.5% metaldehyde a.i. on baits is economically more justifiable; and (iii) uniform sized baits rather than cylindrical baits spread more evenly and therefore provide greater control of slugs.

Cultural management options include rolling after sowing to consolidate the seedbed and the use of shallow discs to break the soil into a fine tilth. Other options that could have been considered earlier in the season are burning stubbles to remove refuges, and removing summer/autumn weeds from paddocks prior to sowing.

Click here for further information on slug management.

 

* Source of field reports of slugs

Dr Michael Nash - Researcher, SARDI (Adelaide)