Species identification has been underlined as a critical aspect of effective control of slugs in the high rainfall zone of the southern cropping region.
A Grains Research and Development Corporation-funded evaluation of various management strategies to reduce damage to crops by slugs has shown that different species become active and feed on crops at different times, and therefore control tactics should be timed in accordance with the emergence of each individual slug species.
The evaluation of slug management strategies – a “fast track project” instigated by the GRDC’s Regional Cropping Solutions Network in the HRZ – has been conducted by Southern Farming Systems in the Western District of Victoria.
SFS chief executive officer Jon Midwood said the project aimed to demonstrate and evaluate a range of management strategies that could effectively reduce slug damage to crops at emergence and during establishment.
Mr Midwood said slugs were a major pest that regularly damaged emerging and seedling canola, fodder rape, pasture legumes and to a lesser extent cereal crops and pulses. The adoption of stubble retention has favoured slug population growth and activity through increased soil moisture-holding capacity and shelter for the pests.
The major slug species that cause crop damage are the grey field slug and the black keeled slug, while several other species also occur in the HRZ.
Mr Midwood said growers, agronomists and advisers often lacked effective management strategies that consistently controlled slugs below thresholds for growing canola.
“The most common strategies used by growers include burning of stubble, light cultivation, rolling and baiting post-sowing,” he said. “However, growers’ approach to management is quite often reactive when damage has occurred and where baiting is seen as the only available control option.”
Mr Midwood said the project involved three trial and demonstration sites at Inverleigh, Hamilton and Skipton and that treatments included in the trials initially aimed to include a range of cultural practices and chemical options (baits), but with the very dry summer of 2012-13, some of the cultural controls became impractical.
The treatments that were included were based on a draft strategy developed by Dr Paul Horne from IPM Technologies Pty Ltd, using knowledge of life cycles, behavior, habitat and predators of the major slug species.
Key elements of Dr Horne’s draft strategy that differ from current farm practices are monitoring population dynamics in early spring and the use of strategic baiting over summer and autumn as well as several cultural practices.
Consistent with Dr Horne’s draft strategy, the trials conducted by SFS highlighted the importance of species identification in the implementation of an effective slug control program.
“At both the Hamilton and Inverleigh trials, damage levels were higher than at the Skipton trial. One of the reasons for this may well have been the presence of two species of slugs which can live at different depths in the soil,” Mr Midwood said.
“The grey field slug is mainly surface active and can have up to three generations a year. It will generally breed in autumn and spring, however, if conditions are favourable this species will breed any time – a pair can produce up to 1000 eggs a year.
“The second species identified at these two sites was the black keeled slug which can burrow up to 20 centimetres underground to escape the heat and find some moisture. A breeding pair can lay up to 200 eggs a year.”
Mr Midwood said the importance of identification of the species related to the emergence of each species as the autumn break developed. “At the very early emergence stage of the canola, only grey field slugs were causing plant damage, but as the wet front penetrated the soil profile with increased rain, the black keeled slugs became active.
“Just applying the initial bait treatment post-sowing and prior to emergence isn’t going to be sufficient to control the later emerging species. Growers and advisers need to be able to accurately identify the different slug species and know which species are active and where in a paddock to help with choosing appropriate control strategies.”
Other guidelines for growers include:
- Slug damage is much greater after leafy crops. Crop residues (especially in the autumn), as well as weeds and volunteers from previous crops, provide slugs with a source of food and shelter which encourages them to breed
- The best time to monitor for slugs is when the weather is mild and the soil is damp. Concentrate monitoring on areas in the paddock where slugs have been a problem in the past
- There are many cultural techniques that can be used to improve canola establishment and the best approach is to be prepared to incorporate them into a broader integrated pest management strategy. The trials showed a positive result from rolling immediately after sowing compared to not rolling
- Always use the highest possible label rates with baits or adjust the rate to the perceived size of the slug population
- Broadcasting is the best method of bait application and kills slugs more quickly than pellets that are drilled with seeds. Broadcast pellets as soon as possible after drilling.
Jon Midwood, SFS
03 5265 1666 or 0400 666 434
Sharon Watt, Porter Novelli
Caption: The brown field slug is a species that is becoming more prevalent in the high rainfall zone. Photo by Michael Nash.
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