Slugs

Black-keeled slugs (Milax gagates) have been found in a canola crop south of Dubbo, in the Central West Slopes and Plains district of New South Wales. Agronomist, Glenn Shepherd (IMAG Consulting), says numbers were relatively low, however damage to seedlings was evident so the affected area (part of the paddock only) has been baited to prevent further damage. The paddock contained a large amount of wheat stubble, is under a no-till regime, and has also been quite wet throughout summer and autumn. These are all favourable conditions for slug survival.

Grower, David Cook, reports finding both black-keeled slugs and grey field slugs (Deroceras reticulatum) in two paddocks on his property east of Shepparton, in the Northern Country district of Victoria. Highest numbers were found in a wheat crop sown into faba-bean stubble, where 4-5 slugs were found in a 0.5 m² area. There were also slugs in a young canola crop that was sown into a long-term pasture paddock. David says some damage was evident such as chewed leaves and stems being nipped off, and the affected areas will need to be baited. Agronomist, Craig Drum (Tatyoon Rural), has also reported significant issues with black-keeled slugs and grey field slugs around Tatyoon and Lake Bolac, in the Western district of Victoria. Craig says canola, cereal and pasture paddocks have been affected, and that some paddocks will require re-sowing due to the extent of damage.

Black-keeled slugs have a uniform grey-black body and a distinctive ‘keel’ or ridge running along the dorsal midline. Grey field slugs are light-grey to fawn in colour with darker brown mottling and they secrete a milky white mucus when disturbed. These slugs feed on all crop types, however canola is very susceptible to damage. At this time of year, baiting is the most effective management approach. Foliar applications of insecticides are not effective against slugs. For more information about monitoring and control options for black-keeled slugs and grey field slugs, refer to PestFacts Issue No. 1.

Slugs have also been reported from properties near Apollo Bay, in the Western District of Victoria. Carol Wilmink reports finding high numbers of large, dark coloured slugs on her property and says they have caused problems by leaving a slimy secretion on pastures used for sheep grazing. They have also caused feeding damage to fungi. Researcher, Michael Nash (University of Melbourne), has identified the slugs as Arion ater, which are commonly known as the black slug or the black arion.

The black slug is an introduced species and has previously only been found in a limited number of locations in Australia, including near Melbourne, Adelaide and Sydney. Carol says that within her region the slugs have been found on several other properties, and there is concern about them spreading further. Black slugs are common garden pests in Europe, however their distribution within Australia is likely to be limited to only very moist environments. Michael says the drier climatic conditions in Australia makes it unlikely they would become a significant pest in broad-acre agricultural areas.

Black slugs grow to a large size, 100-150 mm long, and are generally jet black in colour. There is very little known about their life-cycle in Australia. Carol says she has found highest numbers and greater activity in autumn and spring, and has found they burrow into the ground (sometimes quite deep) during winter. When disturbed, black slugs contract their body and begin to rock from side to side. This is thought to be a defense mechanism that confuses potential predators.