Proactive pest management in retained stubble in the Victorian and Tasmanian high rainfall zones
- Retained stubble provides suitable habitat for crop seedling pests to survive and multiply
- Burning and baiting increased crop establishment where there were high populations of millipedes in 2016
- Mesurol bait significantly increased millipede death in SARDI trials in 2016
- Many pests common in retained crop stubble have no registered insecticide controls
- Canola and lupin seedlings are very susceptible to damage, cereals less so
Retained stubble increases food supply for detritivores (European earwigs, Portuguese millipedes, slaters) and provides protection from predation and heat damage (slugs and snails) for many of the common crop seedling pest species. This allows pests to survive and multiply rapidly at the same time that winter crops emerge. Many growers and advisers recommend burning stubble to minimise any potential for damage to emerging crops.
Canola is especially vulnerable to pest damage. In the GRDC 2012 report, ‘Current potential cost of Invertebrate pests’, yield loss to slugs was 11 per cent, earwigs 13.3 per cent and millipedes was 6.7 per cent for canola grown in the Southern region. These losses were valued at $5.71M annually. The value of control measures used were calculated at $8.47M/annum or $14.07/ha.
The cool moist conditions experienced in the HRZ combined with retained stubble are expected to significantly increase the potential for seedling damage and associated control costs attributed to these pests.
Challenges of retained stubble in the high rainfall zone
The major pest issue in retained stubble paddocks is seedling damage. Stubble decomposing provides a food source for detritivores such as millipedes, slaters and scavengers such as European earwigs. It also provides a physical refuge against predation and the hot dry conditions experienced over summer for the grey field slug.
Growers and advisers adopt a wide range of strategies to minimise pest damage in emerging crops. Common chemical and cultural control measures for each pest species are listed below.
Grey field slug: (Deroceras reticulatum) is a surface dwelling mollusc that can damage emerging crop seedlings.
- Dry conditions in the previous spring and summer will reduce numbers
- Control summer weeds and crop volunteers
- Burn stubbles, hotter is more effective
- Heavy rolling after sowing to crush clods used as refuge by slugs and consolidate the seed bed making it harder for slugs to find the seed
Molluscicide baits – spread immediately after sowing at 20-30 baits/m2. Paddocks may require follow up baiting/s after rain or if damage is still occurring. Monitor slug numbers with tiles in slug prone areas, 1-2/m2 will require action.
Black keeled slug: (Milax gagates) is identified by a raised angular edge on the top of the tail end of the slugs body. They survive over summer by burrowing into the soil to a depth of 20cm.
- Dry conditions in previous spring and summer reduce numbers; they require moist conditions to become active and feed
- Sow early with high vigour seed to avoid high populations
- Deep cultivation or reforming raised beds can reduce numbers
- Heavy rolling will kill emerged slugs and reduce refuge sites
- Parasitic nematodes, ciliates and carabid beetles are natural predators
Molluscicide baits – spread immediately after sowing at 20-30 baits/m2. Paddocks may require follow up baiting/s after rain or if damage is still occurring. Monitor slug numbers with tiles in slug prone areas. 1-2/m2 will require action.
European earwigs: (Forficula auricularia) have been identified as damaging winter crop seedlings when numbers increase to greater than eight per metre square. They are scavengers that eat decomposing plant matter and live and dead insects. In vineyards and orchards they are considered beneficial because they can attack pest species. Stubble retention systems favour increases in European earwig populations by providing a large food source. They are nocturnal, and will shelter under stubble during the day and feed on plants at night. Feeding damage to leaves can appear similar to that caused by slugs.
To confirm that earwigs are causing damage inspect crops at night. Turn over stubble, rocks and logs on the ground around the edge of the field during the day to see where they are during daylight hours. If necessary collect earwigs to confirm identification (there are many native earwig species).
- Reduce stubble residues by burning, incorporation or grazing.
- Cultivation will disturb egg burrows.
- Encourage natural predators such as carabid beetles, birds and small lizards.
- Control summer weeds and crop volunteers
No insecticides are registered for control of earwigs in broad acre crops.
Fiprinil (500g/l) is registered as a seed treatment on sorghum for control of black earwigs. There are also some insecticides registered for earwigs in horticulture such as carbaryl and chlorpiryfos.
Portuguese millipedes: (Ommatoiulus moreleti) can damage seedlings but damage is considered rare. Large populations will not always result in seedling damage, and crops may be susceptible to damage for only a very limited time. The Portuguese millipede is a detritivore and well suited to stubble retention systems. It is active in autumn and spring.
- Reduce stubble residues by burning, incorporation or grazing.
- Sow early with high sowing rates and high vigour seed to avoid high pest populations.
- Control summer weeds and crop volunteers.
No insecticides are registered for control of millipedes in broad acre crops.
Slaters (Porcellio scaber, Armadillidium vulgare). There are a large number of slater species in Australia, and they are generally detritivores. Slaters have been observed causing damage to canola seedlings, wheat, lentils, lucerne and chickpea crops. However, they can be present in high numbers and not cause damage to crops.
- Reduce stubble residues by burning, incorporation or grazing
- Use rotations that include crop species less susceptible to damage
No insecticides are registered for control of slaters in broadacre crops.
If you observe high slater numbers in your crop you should confirm that they are causing damage to the plants. If necessary collect a sample to confirm identification.
SFS have conducted field trials investigating the impact of pests in stubble retained cropping systems.
Trials comparing 15cm and 30cm high retained stubble in 2015 and 2016 experienced no slug damage. Large populations of Portuguese millipedes were observed in trials in 2015 and 2016 (26-29/m2), however canola seedling damage was only evident in 2016. Millipede numbers were the same in trials sown with disc and tyne seeders, however more plants suffered millipede damage in the disc sown plots. This may be attributed to the later germination of the disc sown plots.
Immediately after sowing, 10m wide strips of bait were applied in four locations at 90 degrees to the sowing direction. The bait was a mix of Metarex® at 8kg/ha and Mesurol® at 2kg/ha. There was a noticeable reduction in establishment in the areas where no bait was applied (Figure 1). There was also a noticeable decrease in the percentage of plants damaged in the baited strips (Table 1).
SARDI researcher Dr Michael Nash tested the efficacy of a range of slug baits on Portuguese millipedes (Figure 2). Chlorpyrifos at 300ml/ha (IBS) reduced damage to canola seedlings in the paddock surrounding the SFS trial area.
Table 1. Canola seedling damage comparing stubble versus burn, plus and minus bait. SFS 2016 DvT trial
|Treatment||% damaged||Increase in damage||Treatment||% damaged||Increase in damage|
|Stubble + bait||42||Disc + bait||17|
|Stubble no bait||61||32% more||Disc no bait||27||37% more|
|Burn + bait||39||Tine + bait||67|
|Burn no bait||51||24% more||Tine no bait||95||30% more|
The 2016 research has indicated that baiting with Mesurol® has a reasonable level of control for Portuguese millipedes and can have a noticeable impact on reducing canola seedling damage. More research is required in retained stubble to test the efficacy of other chemical and cultural control measures before sound recommendations can be made.
Stubble seriously increases pest densities and the development of reliable monitoring techniques is critical to management decision making. Rotations that avoiding species susceptible to pest damage (such as canola) may help to reduce losses and retain profitability.
SARDI / Caesar PestNote Southern 2013, European Earwig http://cesaraustralia.com/sustainable-agriculture/pestnotes/insect/European-earwig
GRDC Factsheet, Oct 2013. Black Portuguese millipedes and Slaters in no-till systems https://grdc.com.au/__data/assets/pdf_file/0028/159319/final-hr-grdc-millipedes-slaters-fact-sheet-pdf.pdf.pdf
GRDC Factsheet, Nov 2013, Earwigs in the medium and high rainfall zones. https://grdc.com.au/__data/assets/pdf_file/0019/134245/final-lr-grdc-earwigs-fact-sheet-pdf.pdf.pdf
GRDC report, 2013. The current & potential costs of invertebrate pests in grain crops, https://grdc.com.au/__data/assets/pdf_file/0026/159281/grdcreportcurrentpotentialcostsinvertebratepests-feb2013pdf.pdf.pdf
This research is being conducted by SFS as part of the GRDC Maintaining profitable farming systems with retained stubble initiative (project BWD00024 ‘Maintaining profitable farming systems with retained stubble in Victoria and Tasmania’).
Disclaimer: Any recommendations, suggestions or opinions contained in this publication do not necessarily represent the policy or views of the Southern Farming Systems (SFS) or the Grains Research and Development Corporation (GRDC). No person should act on the basis of the contents of this publication without first obtaining specific, independent professional advice. SFS and GRDC and contributors to these guidelines may identify products by proprietary or trade names to help readers identify particular types of products. We do not endorse or recommend the products of any manufacturer referred to. Other products may perform as well as or better than those specifically referred to. SFS and GRDC will not be liable for any loss, damage, cost or expense incurred or arising by reason of any person using or relying on the information in this publication.
Stubble project overview: This guideline has been developed for SFS Farming Systems Group as part of the Maintaining Profitable Farming Systems with Retained Stubble initiative, funded by the Grains Research and Development Corporation (GRDC). The initiative involves farming systems groups in Victoria, South Australia, southern and central New South Wales and Tasmania collaborating with research organisation’s and agribusiness to explore and address issues for growers that impact the profitability of cropping systems with stubble, including pests, diseases, weeds, nutrition and the physical aspects of sowing and establishing crops in heavy residues.