A branch of biology - ecology - is plumbing new depths of pest species knowledge to enable better prediction of the seasonal risk diamondback moth poses to canola this spring.
Unmasking this insect nemesis of canola, described as the oilseed crop's principal spring pest, was a collaborative study led by South Australian Research and Development Institute (SARDI) entomologist Kym Perry.
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"Predicting insect pressure, which remains a major goal in pest management, requires deep ecological understanding at a species level," Dr Perry says.
Chasing this objective, researchers and agronomists collected moth data - snaring insects in pheromone traps and sampling plants for larvae - across South Australia and western Victoria from 2014 to 2016.
Sampling concentrated on the moth's plant habitat, both cultivated and wild, since the science of relationships between organisms and the environment, so-called ecology, is derived from the Greek 'oikos' meaning household, home or place to live.
"A major goal was to understand whether the insect survived in local areas, or had migrated from further afield," Dr Perry says.
To this end, insect data was collected from host plants, such as wild brassicas, during autumn, before the growing season commenced.
Agronomists monitored a network of 30 sentinel canola paddocks, once a week, to detect the in-crop arrival of diamondback moth populations.
Predicting insect pressure, which remains a major goal in pest management, requires deep ecological understanding at a species level
Modelling, incorporating climate data for each sampling site, was used to predict first oviposition or egg-laying, signalling when diamondback moth crop colonisation was likely to occur.
The modelling also indicated whether in-crop climatic conditions were suitable for early-season build-up of moth numbers, helping to identify whether populations had local or long-distance origins.
Such targeted data collection has seen researchers develop a more accurate ecological profile of diamondback moth - outlining its population origins, colonisation patterns and seasonal pressure risk in South Australia.
Dr Perry says that overall, the new profile, drawing on sampling and modelling, as well as light-trapping data, suggests early season outbreaks of diamondback moth stem from wild brassica plants, growing locally over summer.
The data further showed wild brassicas support widespread populations of insecticide-resistant moths over summer.
And the insecticide-resistance observed in moths collected from brassica weeds and canola was nearly identical, suggesting the pest moved frequently between wild and cultivated brassica plants.
Wild brassica species known to host the pest on SA's Eyre Peninsula are: Lincoln weed (Diplotaxis tenuifolia), dog weed (Diplotaxis muralis) and the coastal sand dune species, sea rocket (Cakile maritima). Brassica forage crops also host diamondback moth.
Moths tended to colonise most canola paddocks soon after crop emergence, from May to June. This colonisation pattern suggests spring outbreaks, from September to November, may be caused by population build-up over the season, rather than from migration, known to drive colonisation of Helicoverpa moth species.
The research also showed pre-growing season rainfall was linked to moth pressure in autumn. This is because rainfall spurred germination of brassica weeds, providing habitat for the moths and a green bridge, leading to crop colonisation early in the season.
"In 2014 and 2016, rain between February and April promoted weed growth and diamondback moth populations. Canola crops were then colonised soon after germination," Dr Perry says.
"In contrast, dry conditions in 2015 severely limited pre-season host plant availability, leading to low diamondback moth numbers, and canola crops were colonised, on average, later in the season."
New 'cryptic' moth findings
Another four-year SARDI study found a new Australian species of diamondback moth, discovered by Canadian scientists in 2013, poses a low risk of damage to canola country in southern and western growing regions.
The study from 2014 to 2017 looked at the biology, distribution and pest status of this new species, Plutella australiana. It is described as 'cryptic' because it is identical-looking, but genetically distinct from the dominant Australian species, Plutella xylostella.
Of 1400 moths collected from brassica plants in 2014 and 2015, molecular assay screening showed only 10 per cent were P. australiana, and 90 per cent were P. xylostella.
Insecticide bioassays showed the new Australian species had not evolved insecticide-resistance, unlike its more prevalent counterpart, P. xylostella.
And P. australiana was 19 to 306-fold more susceptible than P. xylostella to insecticides commonly used for diamondback moth control. These insecticide chemistries are alpha-cypermethrin (Group 3A, synthetic pyrethroid); emamectin benzoate (Group 6); and spinetoram (Group 5).
The GRDC-invested research found major differences between the genetics and preferred host plants of the two moth species, which are both known to attack canola and wild brassicas.
Insecticide resistance management
Dr Perry urges canola growers to implement five main management tactics as part of an industry-wide effort targeting insecticide resistance in the predominant species of diamondback moth, P. xylostella.
These best practice insecticide resistance management (IRM) tactics are:
- Only spray when necessary, using a threshold-based approach
- Avoid using synthetic pyrethroid insecticides (Group 3A) for control in canola crops
- Consider using Bacillus thuringiensis (Group 11A) products in insecticide rotation strategies
- Rotate use of insecticides - avoiding repeated use of the same chemistry in one or consecutive seasons
- Adjust rotations when controlling diamondback and Helicoverpa moth species together.
Speaking at the GRDC Research Updates in Adelaide, SA, Dr Perry outlined that while five chemical sub-groups are currently registered for controlling the pest, only a few remain 'reliably effective'.
These are the synthetic insecticides spinetoram (Group 5) and emamectin benzoate (Group 6), as well as Bacillus thuringiensis var. kurstaki strain products (Group 11A).
Whereas synthetic pyrethroid (Group 3A) and organophosphate (Group 1B) insecticides are 'partially or totally ineffective' for diamondback moth control.
Dr Perry says there is no geographic pattern to widespread insecticide resistance, which is fairly consistent across growing regions, and within canola crops and weed populations, suggesting high gene flow between different moth populations.
GRDC Research Codes: UA00146, DAS00155, DAS00094
More information: Kym Perry, email@example.com, 08 8429 0738.
Useful resource: GRDC 'Resistance management strategy for diamondback moth in Australian canola': https://grdc.com.au/fs-resistancestrategydiamondbackmoth