Slug baiting for integrated control
With seeding approaching, now is the time to monitor paddocks for slug activity. Slug baiting in infested areas should occur at or immediately after sowing, before seedling emergence, to protect seeds and seedlings from actively feeding slugs.
The seasonal risk from slugs is elevated in parts of SA in early 2021 following generally favourable conditions during spring and summer 2020/2021. Above average rainfall in September† (mainly Kangaroo Island and Lower South East regions) and October† (most of SA’s cropping zone, to a lesser extent the South East region) may have prolonged slug reproduction last spring. A generally mild summer with below average temperatures across SA, especially in December† and February†, potentially aided slug survival.
Slug biology and damage
The black-keeled slug, Milax gagates, and the grey field slug, Deroceras reticulatum, are the two major slug pests of all crop and pasture types in southern Australia. Canola is especially susceptible to damage. Other species that may be present include brown field slug (most likely Deroceras invadens or D. laeve; some taxonomic uncertainty exists) and striped slug, Lehmannia nyctelia.
Slugs are generally more problematic in higher rainfall climates (> 500mm) and seasons, and in paddocks or paddock area with heavy soils that retain moisture or crack. Slugs attack crops at all stages but cause most damage to seedlings. Grey field slugs feed mostly on the soil surface, attacking plants at ground level and consuming cotyledons, leaves and stems, sometimes severing seedlings. Black-keeled slugs feed similarly at the soil surface but also burrow below ground to attack germinating seeds.
Slugs are mostly inactive during summer. The black-keeled slug burrows to depths of 20 cm or more to survive the heat, whereas the grey field slug seeks refuge under rocks, logs or summer weeds and debris, or by moving into cracks in the soil. Soil-penetrating rainfall and cooler temperatures increase slug activity surface on the soil surface in autumn and winter.
Monitoring and baiting slugs
Monitor slug activity using surface refuges or bait lines. Focus monitoring in areas with a history of slug problems and/or moisture-retaining soils.
Silver slug mats are more attractive to slugs than tiles for refuge-based monitoring of slugs (more than 4-fold more slugs captured), shown by recent research by SARDI and DPIRD [1]. Place refuges along several rows (i.e. transects) in representative paddock areas, with pellets underneath each refuge, and check for slugs after a few days. Slug numbers can vary greatly between transects.
Bait lines are a useful alternative to refuges for slug monitoring. Place a line of pellets (e.g. 100m in length) along a seeding furrow and check for dead slugs after a few days. Do this in several furrows. Bait lines are easy to apply and monitor but may need re-application after 2-3 weeks or when pellets break down.
The presence of dead slugs under refuges or in bait lines indicates slugs are actively feeding, and bait should be applied. After application, monitor and re-apply as necessary. For information on bait products, refer to the SARDI snail and slug baiting guidelines.
Integrated control
Baiting slugs at sowing is part of year-round integrated slug management, together with cultural controls. Slug populations can be reduced by fine tillage followed by rolling. Slugs can be managed in no-till systems by well-timed baiting at sowing in combination with agronomic practices to achieve rapid crop establishment. These include early sowing before slugs are active, use of hybrid rather than open-pollinated seed varieties, rolling to compact the seed bed and restrict slug movement, and weed management. Further slug management advice is available in a recent GRDC Paddock Practices article.
References and further reading
† Source: Australian Bureau of Meteorology, climate data maps for South Australia, monthly anomalies for rainfall and mean maximum temperature).
[1] Perry KD, Brodie H, Fechner N, Baker GJ, Nash MA, Micic S, Muirhead K (2020b). Biology and management of snails and slugs in grains crops. Final report for GRDC (DAS00160). South Australian Research and Development Institute, December 2020.
SARDI snail and slug baiting guidelines ().
PestNote – Black-keeled slug ()
GRDC Paddock Practices - Be on alert for slugs in the high rainfall zone
