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Agronomist, Paul Parker (NSW DPI), reports that diamondback moths (Plutella xylostella) have been observed in some canola crops between Lockhart and Albury, in the southern part of the South West Slopes of NSW. Diamondback moths (also referred to as cabbage moth) have also recently been reported on canola in South Australia and in many parts of Queensland. These observations serve as a timely reminder to check crops for the presence of larvae, which chew on leaves resulting in holes or windows of damage. Radish, turnip and cruciferous weeds are frequent hosts for diamondback moth and should also be monitored.
Diamondback moth larvae are pale yellowish-green, slightly tapered at each end and approximately 10mm long when mature. The moths are about 10mm long and are grey-brown in colour. They have a characteristic whitish strip of uneven width down the back, which resembles diamond patterns.
The warm and dry conditions recently experienced favour diamondback moth outbreaks. Higher temperatures increase development rates of diamondback moth, whereas rainfall (greater than 5-8mm within a 24 hour period) is a major factor that contributes to larval mortality due to dislodgment, drowning or fungal disease (e.g. Zoophthora radicans).
Diamondback moth populations are notorious for their rapid resistance to synthetic pyrethroids, which is common in Australia. Furthermore, insecticides are not effective in killing adults, eggs and the first larval stage which occurs (mines) inside the leaf. Normally, diamondback moth damage is not severe enough to warrant control, and chemicals should only be used when completely necessary. Given the present moth numbers and crop conditions, Paul says the economic benefit of applying chemicals in many areas is questionable.
Diamondback moth populations are often kept under control by beneficial insects such as wasps, spiders, lacewings and hoverflies. Crops should be checked for these natural enemies. Bacillus thuringiensis (Bt) is the softest insecticide that has the least impact on beneficial insects.
What is PestFacts?
PestFacts south-eastern keeps growers and advisers informed about invertebrate pests and beneficials in broadacre crops and pastures during the winter-cropping season in Victoria and southern New South Wales.
PestFacts south-eastern is run through IPMforGrains: Best Practice Insect Pest Management, a project delivered by the National Pest Information Network (cesar, DPIRD, QDAF, NSW DPI, and SARDI). This project aims to provide grain growers and advisors with information on invertebrate grain pest occurrence and equip industry with the knowledge needed to implement integrated pest management practices. This initiative is a GRDC investment and includes in-kind contributions from all project partner organisations.
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PestFacts is produced on an ʻas-needsʼ basis and relies upon pest observations and field reports from our subscribers. If you have recently observed invertebrate pests (or beneficial species) in broadacre crops, please contact us to report your observations.
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