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Slugs

Slugs have been favored by the higher than average summer rainfall this season. Agronomist, Craig Drum (Tatyoon Rural), reports finding up to 10-20 slugs per m² in various paddocks between Ararat and Lake Bolac, in the Western district of Victoria. Agronomist, Eoin Flett (Williams & Jackson Pty Ltd) has also noted that slugs are very active in paddocks west of Geelong, in the Central district of Victoria. The majority of slugs being found are grey field slugs (Deroceras reticulatum). The other common species in this area is the black-keeled slug (Milax gagates), however this species usually becomes active later in the season.

In a typical year, slugs are stimulated out of their summer aestivation following autumn rains and will generally be problematic in paddocks where they have previously been a problem. Heavier soils are preferred, typically heavy red loams, gravelly loams and grey clays. The grey field slug is opportunistic and this year they have emerged much earlier, and have now bred up to high numbers in some regions.

Grey field slugs are light grey to fawn in colour with darker brown mottled markings. They secrete characteristic milky-white mucus when disturbed. Black-keeled slugs have a uniform grey-black body and a distinctive ‘keel’ or ridge running along the dorsal midline. Both species grow to approximately 50 mm in length. Slugs will feed on pastures and all crop types, however canola is most susceptible to damage. When slugs chew canola seedlings they can eat off the growing point above ground, often killing the plant.

Because slugs are more active at night, it is difficult to estimate numbers accurately without monitoring. Using terracotta tiles, or another type of refuge ‘trap’ such as carpet squares is the best way to monitor 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 few pieces of slug bait under each trap will help to attract slugs if they are present. Be aware that traps will be less successful in attracting slugs in paddocks when there are alternative hiding places, such as heavy stubble.

The most common control method for slugs is to apply baits, which, under normal conditions should be applied early in the season after good germinating rains to coincide with slug emergence. Economic thresholds for slugs vary depending on the species. For the grey field slug, suggested thresholds are 1-2 slugs per m² in canola and pulses, and 5 per m² in cereals and pastures, whereas for the black-keeled slug, suggested thresholds are 1-2 per m² in canola, cereals and pulses and 5 per m² in pastures. These thresholds are only preliminary and require further validation, however it is likely that slug numbers are already well above threshold levels in many paddocks.

Baiting efficacy can be reduced when there are higher slug populations, an increased availability of refuges, and higher amounts of plant material. When baiting, it is important to consider the number of baits that are spread per m², and how this relates to the numbers of slugs present. Because many paddocks are already wet, consideration should also be given to using baits that are rainfast (e.g. metaldehyde based products). Other management options that should be considered this season are burning stubbles to remove refuges, and removing green material from paddocks prior to sowing. In some situations it is advisable to sow less susceptible crops, as well as faster establishing varieties.

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