Slug warning

Slugs can be problematic to emerging crops and grasses at this time of year, particularly in areas that have experienced good autumn rainfall. Agronomist, Tim Paramore (Tim Paramore Agronomic Services), reports the presence of slugs in an emerging canola crop near Corowa, in the South West Slopes of NSW. Tim says the affected paddock is irrigated and contains some stubble on the soil surface; conditions that have likely favoured the build-up of slug numbers. Irregular areas of canola seedlings were eaten, with large patches of the paddock containing no or few seedlings. It was only discovered that this damage was caused by slugs after traps were placed on the soil surface and checked after a few days.

The most common slug species are the Reticulated Slug (Deroceras reticulatum) and the Black Keeled Slug (Milax gagates), which both grow to about 40-50 mm long. Slugs damage plant seeds and seedlings, mainly feeding at night. Damage to seedlings is often in patches and can result in the death of plants. Although all seedling crops and pastures may be attacked, canola is particularly susceptible. Slugs were problematic in parts of the Western district, in Victoria, at this time last year and it is best to keep an eye out again this season. Slugs are commonly found in paddocks where they have previously been a problem.

Because slugs are usually more active at night, it can be difficult to estimate numbers accurately without monitoring. Slug numbers can be monitored by placing traps (carpet squares, tiles or flowerpot bases) on the soil surface and counting the number under them after a few days. It is important to set traps when the soil surface is visibly wet. Once the crop has been seeded and germination has commenced, crops should be examined at night for slug activity. It is also a good idea to continue these checks to determine the effectiveness of control methods.

There are a number of effective control measures for slugs, including chemical, cultural and biological methods. The most common control method is to apply baits, but these should be applied early in the season after good germinating rains. This is when slugs are emerging from their hiding places and actively looking for food. This means there is little alternative food to compete with the bait pellets and adults are likely to be killed before they have laid eggs. Baiting will generally only kill 50% of the slug population at any one time, mainly the larger ones. Baiting (using a suitable application method) in early autumn around crop edges may minimise the impact of slugs invading from surrounding pasture or fence lines.

Paul Horne (IPM technologies) says iron-chelate based compounds rather than traditional chemicals (e.g. Mesurol) can help maintain a healthy population of beneficial species, such as carabid beetles which are veracious predators of slugs. Abundant ground cover and stubble can provide ideal moisture levels and shelter for slugs. Good crop hygiene, weed control and removal of refuges can reduce the problem over time. For more information on the biological control of slugs click here.