Slugs (and snails) are becoming active as crops emerge

With increasing rainfall around south-eastern Australia, slugs are a risk to emerging crops, and even snails are on the move. Baiting at sowing is a key control tactic.

Where have they been reported?

According to SARDI entomologist, Dr. Michael Nash, black keeled slugs have begun damaging seeds and seedlings of emerging lucerne pastures east of Holbrook in the NSW Riverina. The damage was exacerbated by the bait being applied a couple of weeks after sowing in the thought that dry conditions would prevent slug damage. Michael has also reported that small pointed snails are actively feeding on emerging canola seedlings in some areas of Western Victoria. On a positive note, Michael reports that the bait applied at sowing is doing a great job of controlling them. He says that slugs are slow to emerge in the drier areas west of Streatham (South West district of Victoria) but elsewhere they are starting to become active.

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) (see photos below). The brown field slug (Deroceras invadens) can also be a serious pest, with multiple species often co-occurring. Soils that retain moisture are preferred by slugs, particularly 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.

Grey field slug. Photo by Andrew Weeks, Cesar Australia

Black keeled slug. Photo by Andrew Weeks, Cesar Australia

Our advice

Monitor all paddocks in high rainfall areas (>500 mm) especially in crops being sown to canola. The most important time to monitor is the previous spring to understand where problems might occur, then after rain prior to seeding. Traditional methods used to assess relative numbers of slugs, like refuges traps (e.g. terracotta tiles), rely on slugs seeking refuge under them. While these work for grey field slugs, black keeled slugs often do not use these surface refuges, and estimating black keeled slug numbers using refuge traps can be misleading. Refuge traps for grey field slugs 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.

Once the crop has been sown and germination has commenced, crops should be visually examined for damage and to ensure adequate rates of baits applied at sowing remain active to protect seedlings. Damp and mild conditions are optimal for slug activity; hence mornings after these conditions are the best time for monitoring.

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.

A carabid beetle attacks a striped field slug. Photo by Andrew Weeks, Cesar Australia

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. These approaches have been shown to reduce grey field slug populations. 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.

The most common chemical control method for slugs is the use of baits. Baits are a crop protectant and must be applied immediately after sowing and rolling, prior to crop emergence. Drilling of baits will protect seed but not seedlings, hence a follow up surface application is recommended. Thirty pellets per m² are needed to ensure adequate chance of encounter.

There are several molluscicidal baits registered against slugs in broad-acre crops with guidelines for effective usage being updated regularly by SARDI. Be aware that some baits are far more stable than others under adverse weather conditions including cold temperatures and significant rainfall. Iron III and metaldehyde formulated baits are effective in providing seedling protection against slugs and have limited effects on natural enemies. Remember to check affected areas after baiting and be aware that re-baiting may be required under high slug infestations or split emergence of populations due to different species co-occurring in the same field.

Description

Black keeled slugs grow to 60 mm in length and are usually dark brown-black in colour, with a prominent ridge (keel) running along the back. The body size, the colour and the keel are the distinguishing features of this slug species.

The grey field slug can grow up to 50 mm long and varies in colour from fawn to grey with netted dark brown markings, thus it is also known as the reticulated slug. A distinguishing feature is that they produce a milky coloured mucous when irritated.