Slugs are damaging seedling crops
- Beverley
- West Dale
- Manypeaks
Slugs have been reported damaging a variety of crops in some areas after recent rain.
David Stead (Anasazi Agronomy) has reported slugs, likely to be the invasive field slug (Deroceras invadens), causing damage to a canola crop at east Beverley.
Garren Knell (ConsultAg) has reported extensive slug damage to canola and barley crops at West Dale. These paddocks have had repeated baiting and dead slugs could be seen along the furrows.
James Bee (Elders) has reported ryegrass and clover-based pasture being eaten by black keeled slugs at Manypeaks.
Identification of pest slugs
There are two types of slug species that are pests of broadacre crops, the black keeled slug (Milax gagates) and the Deroceras species, with the more common one being the reticulated slug (Deroceras reticulatum). Other slug species can be present in the paddock, but they are not crop pests.
The black keeled slug is usually black with a prominent ridge down the back, whereas the reticulated slug is often light grey fawn with mottled markings. Black keeled slugs can burrow 20 cm or more below the surface and are readily able to survive paddocks which have been burnt. The reticulated slug does not burrow and is less likely to survive in paddocks that have been burnt.
Like the reticulated slug, the invasive field slug (Deroceras invadens), also known as the Chestnut slug, does not burrow. This is an introduced species, likely to have been present in WA for the last 100 years and was previously misidentified as the brown field slug (Deroceras panormitanum). The spread of this invasive field slug in WA has been constrained by cold winter temperatures.
The invasive field slug can be differentiated from the reticulated slug by its brown colouration and its tail which usually slants vertically upwards. This species tends to be more common in paddocks when pastures are a common part of the rotation.
For more information on slug identification visit DPIRD’s Identification and control of pest slugs and snails for broadacre crops in WA page and GRDC’s Slugs in crop: The back pocket guide.
How to check crops and manage slugs
Crops can be checked for slug activity by observing plant damage and searching for signs of slugs at night. Irregular pieces chewed from leaves and shredded leaf edges are typical of slug presence. Damage to canola and legume crops can be difficult to detect if seedlings are chewed down to the ground during emergence.
Slug numbers as low as 1 per square metre can be damaging to a germinating canola crop. For more information see DPIRD’s suggested snail and slug threshold numbers in broadacre crops.
In emerged crops, baiting will have reduced effectiveness as there is a lot of green material that provides an alternative food source for the slugs. Baiting at the highest registered rate and ensuring even bait coverage will lead to a better chance of slugs encountering the baits and feeding on them. If feeding damage is still occurring and you can’t see any baits remaining on the ground, then consider reapplying baits, especially if there is a future rain event of 10mm or more predicted.
Baiting will generally only kill 50% of a slug population at any one time, and then mainly the larger ones. Younger slugs may emerge in successive waves. Monitoring slug numbers will determine if there is a need for multiple bait applications, and baiting can be confined to areas of high slug density.
Few chemicals are registered in Australia for controlling slugs, and these are baits with the active ingredient metaldehyde and Iron EDTA.
Metaldehyde baiting must be stopped at least 2 months prior to harvest to ensure baits are broken down and do not become a contaminant of grain.
Further information
For more information on slug monitoring and baiting visit:
- DPIRD’s Identification and control of pest slugs and snails for broadacre crops in WA page
- GRDC’s Slug control fact sheet: Successful crop protection from slugs
- GRDC’s Slugs in crop: The back pocket guide.
For more information contact Research Scientist Svetlana Micic, Albany on +61 (0)8 9892 8591.
Article author: Bec Severtson (DPIRD Northam).