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Slugs

Grower, Michael Nash, has reported the presence of slugs in a canola crop (2-4 leaf stage) near Mortlake, in the Western district of Victoria. Slug numbers were monitored by placing terracotta-paving tiles throughout the paddock (approximately 120 tiles in total), then counting the numbers of slugs underneath each week. Michael says numbers increased from early May onwards, and baiting has now been carried out. An iron-chelate based product (Multiguard) was used rather than a traditional chemical, in order to help maintain a healthy population of beneficial species, such as carabid beetles, which are voracious predators of slugs. Slugs have also been reported in other canola crops in the Western district in recent weeks.

Slugs can be problematic to emerging crops and grasses at this time of year, particularly in areas that have experienced good autumn rainfall. They are more likely to be found in paddocks where they have previously been a problem. The most commonly encountered species are the grey field slug (Deroceras reticulatum) and the black keeled slug (Milax gagates), which both grow to about 40-50 mm long. Michael reports finding both of these species, as well as low numbers of the brown field slug (Deroceras panormitanum).

Click here for images of the grey field slug and click here for images of the black keeled slug.

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. Because slugs are usually more active at night, it can be difficult to estimate numbers accurately without monitoring. Using terracotta tiles, or another type of ‘trap’ such as carpet squares or flowerpot bases is the best way to monitor slug numbers. 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 small amount of bait placed under each trap will help to attract slugs if they are present. 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.

The most common control method for slugs is to apply baits, which 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.

There are also a number of other effective control measures for slugs, including cultural and biological methods. Abundant ground cover and stubble can provide ideal moisture levels and shelter for slugs. Good crop hygiene, weed control and removal of refuges can help to reduce the problem over time.

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