WithTheGrain: Stubble pest watch-outs for 2018
Author: Alistair Lawson | Date: 11 Apr 2018
An array of invertebrates has been causing headaches for growers in the southern region and are the subject of several GRDC investments looking at ways of controlling the pests.
Earwigs, slaters and millipedes, as well as established foes such as slugs and snails, have been impacting crops at establishment, in some cases forcing growers to re-sow – resulting in a large yield penalty and making for patchy, uncompetitive crops which allow weed escapes.
While stubble retention has been a key driver in the emergence of these pests, cesar director Dr Paul Umina says other factors such as the uptake of minimum/zero till farming, more continuous cropping, earlier sowing and increasing pesticide use, have also been important.
There are several species of earwigs present in grain crops, however Dr Umina says the European earwig (Forficula auricularia) is the most important.
It is mostly found in paddocks with heavier soils and high stubble loads. Earwigs cause irregular chewing damage to leaves, cotyledons, stems and flowers, mostly in canola.
“Earwigs are social insects, so a nest of them is likely to mean a hotspot within a crop,” Dr Umina says.
“Management of earwigs is difficult with very limited chemical control options. Insecticide seed treatments and baits do offer some control of the pest, but we still aren’t clear on what the most appropriate chemical control options are.”
Growers with big earwig problems are forced to rely on burning or cultivating to reduce numbers. However, paddock risk factors remain uncharacterised which does limit cultural control options.
“Spiders and carabid beetles are known natural enemies of earwigs and preserving these species through integrated pest management strategies is important,” Dr Umina says.
The GRDC is currently investing in research to explore the lifecycle and distribution of earwigs within cropping regions. Work is also underway to examine the impact of earwigs on different crop types and growth stages.
Three main species of slaters are present in southern region grain crops, with the most widespread being the pill bug (Armadillidium vulgare) and the common slater (Porcellio scaber).
Slaters feed on many crops, including canola, lupins, chickpeas, lentils, cereals and Lucerne. They are highly dependent on moisture and can impact crop establishment with uneven, rasping-type chewing damage and ring-barking of stems and young branches.
The complicating factor in managing slaters is that they are detritivores, meaning they are often happy feeding on dead organic material such as decaying stubble and therefore play a beneficial role as nutrient recyclers.
“While insecticide seed treatments and baits can offer some control of the pest, little is currently known about the general biology of slaters, if there are feeding behaviour ‘switches’ and, if so, what the triggers might be,” Dr Umina says.
“We also don’t know whether or not stubble burning can assist in control or whether it exacerbates damage by slaters.”
As part of his PhD studies, University of Melbourne student Joshua Douglas is assessing the feeding habits of slaters and millipedes.
The black Portuguese millipede (Ommatoiulus moreletii) is the main millipede species of concern for southern region growers.
Like slaters, millipedes are predominantly detritivores, but can also attack lupins, Lucerne and canola, and early-sown crops have been shown to exacerbate issues.
However, the feeding behaviour of millipedes is very unpredictable, Dr Umina says.
“In some cases, there can be high numbers of millipedes but no damage to crops,” he says.
“When they do attack crops, they can cause economic damage but just how much is currently poorly understood. For many crops, it is likely to be very little.”
“We do understand the lifecycle and distribution of the black Portuguese millipede fairly well, but we do not know the conditions that favour plant feeding behaviours or the most appropriate control options growers should use.”
According to South Australian Research and Development Institute (SARDI*) entomologist Greg Baker, snails are benefitting from conservation farming practices such as minimum-till or zero-till farming and stubble retention with less burning and less grazing practiced than previous eras.
“Less soil disturbance, a better ground habitat with more moisture and shelter as well as more off-ground summer refuges for snails are providing conditions for their greater survival, growth and reproduction and less chance that they encounter bait,” Mr Baker says.
The challenge in controlling snails is that the four main species – the common white snail or vineyard snail (Cernuella virgata), the pointed snail (Cochlicella acuta), the white Italian snail (Theba pisana) and the small pointed or conical snail (Prietocella barbara) – all have different impacts and require different management approaches.
Research being conducted by SARDI researcher Helen Brodie as part of a GRDC investment is looking to provide improved predictive recommendations on bait timing.
This has involved monitoring snails with cameras – the original idea coming from Michael Richards of Natural Resources Management Northern and Yorke – to help generate data on the behaviour and activity patterns of snails related to local weather data and recording snail physiological measures.
Preliminary findings from this research indicate that the snail’s life stage influences the effectiveness of bait control.
“We have been collecting snails from the field, bringing them back to the lab and, under controlled conditions putting some of the snails into pots with metaldehyde bait to assess the mortality achieved. We also put some of the snails with bait that does not contain the metaldehyde so we can measure how much the snails were eating,” Ms Brodie says.
“Throughout the year the snails eat a varying amount of bait, and the mortality effect differs markedly. The mortality generally drops through the winter months and is higher through the autumn period. However, the relationship between bait consumption and mortality does vary, and we’re working towards understanding the underlying cause(s) of this variability.”
Previous research and grower experience has shown that baiting programs are most effective where bait is applied when snails are actively feeding and before egg-laying commences when season-breaking rains begin.
Like snails, Mr Baker believes slugs are also benefitting from the rise in conservation farming methods.
Both the black keeled slug (Milax gagates) and grey field slug (Deroceras reticulatum) are common pests in the high-rainfall zone of south-eastern Australia and impact canola crops particularly.
Previous research conducted as part of GRDC investments has found that baiting at sowing is the most critical factor in chemical control of slugs.
Some other important rules of thumb come down to cultural practices such as:
- Recognise that slug risk factors vary from year to year, understand what is going on in your paddock and monitor each year.
- Use cultivars or agronomic practices that ensure rapid crop establishment (for example, with canola, sow early and use either hybrid seed or grade open pollinated varieties).
- Roll stubbles prior to application of bait at sowing.
- Follow-up monitoring of emerging seedlings at least every two days; watch for further slug activity, especially if wet.
Sometimes, problem areas may need re-baiting which can depend on the weather. When applying baits, it is important to adhere to label instructions and note the withholding periods for each product.
*SARDI is a division of Primary Industries and Regions SA
GRDC research codes: CSE00059, UM00054, DAS000160, DAS00134, DAS00095, YPA00002
Dr Paul Umina,
03 9349 4723,
08 8303 9537,
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