Diamondback moth activity is increasing in some crops
- Northampton
- Moonyoonooka
- Nabawa
- Alma
- Nangetty
- Carnamah
- Tenindewa
- Bolgart
- Wongan Hills
- Northam
- York
- North Cunderdin
- Kellerberrin
- Merredin
- South Stirlings
- Gnowellen
- North-west Cascade
- Grass Patch.
Growers are encouraged to sweep their canola crops to check for diamondback moth (DBM) caterpillars after high numbers were reported from some crops this week at north-west Cascade, Grass Patch and Merredin.
Lucy Burrows (Clarke and Stokes) found 130 DBM caterpillars per 10 sweeps in moisture-stressed late flowering canola at north-west Cascade. Lucy noted that all DBM growth stages were present in the crop. Lucy also reported sweep netting 6 DBM caterpillars per 10 sweeps in a late flowering crop at Grass Patch. In addition, at Grass Patch, Sam Stubna (South East Rural Traders) has found 60 DBM caterpillars per 10 sweeps in early flowering canola.
In addition, Joshua Cope (Nutrien) found more than 100 DBM caterpillars per 10 sweeps in early flowering canola at Merredin.
Generally, low levels of DBM activity have been reported recently by growers and consultants and Department of Primary Industries and Regional Development (DPIRD) staff monitoring canola focus crops. These include crops from Northampton, Moonyoonooka, Nabawa, Alma, Nangetty, Carnamah, Tenindewa, Bolgart, Wongan Hills, Northam, North Cunderdin, Kellerberrin, York, Merredin, South Stirlings, Gnowellen, Cascades and Grass Patch.
The canola crops that germinated earliest and/or are under stress seem to be experiencing the highest DBM pressure at the moment.
For information on DBM biology and earlier DBM activity this season, refer to the 2024 PestFacts WA Issue 4 article Diamondback moths in moisture stressed crops and Issue 9 article Diamondback moth caterpillars are being found early.
DBM caterpillar activity typically slows down in cold, wet weather conditions and then ramps up in spring.
Management
Previous DPIRD research found that early monitoring is essential in deciding whether chemical control is required. At least four estimates of larval density over 12 days will determine whether the number of caterpillars is increasing or decreasing. On each occasion, five 10-sweep samples should be taken throughout the crop. Sweep netting provides the most precise estimate of caterpillar densities in the shortest time compared to plant sampling or area counts.
If DBM numbers are found to be increasing over the 12-day monitoring period, then insecticide application may be beneficial. However, more than one spray may be required. DPIRD’s research revealed that a single spray in mid-August had little impact on the damage caused by DBM. In contrast, an early program of 2 spray applications, 3 days apart, resulted in significantly better control of caterpillars and reduced yield loss. This two-spray strategy ensures that DBM eggs that survive and hatch after the first application are also controlled. Net sampling of the crop about 3 days after the first spray should be conducted to assess the effectiveness of the spray and determine the number of surviving caterpillars before the second spray is applied.
Infestations of DBM in canola crops can be sporadic and unpredictable, and pre-emptive spraying for this pest is neither effective nor cost-effective.
While DBM activity is expected to ramp up as spring approaches, DBM caterpillars do not always build up to threshold levels in every region and in every year. Sometimes DBM populations crash naturally as a result of fungal infection (especially with a moist canopy) or from other natural enemies such as parasitoid wasps.
Thresholds for control are:
- pre-flowering (stressed crop): 30 or more caterpillars per 10 sweeps
- pre-flowering (no stress): 50 per 10 sweeps
- early-mid flowering: 50 or more per 10 sweeps
- mid-late flowering: 100 or more per 10 sweeps.
DBM are difficult to control because they are resistant to many insecticides, including synthetic pyrethroids and organophosphates. If spraying is required, keep in mind that insecticide sprays have limited coverage in dense canola canopies, and any insecticides that are effective on DBM can only be applied twice per season.
If numbers warrant spraying then growers and consultants can refer to DPIRD’s 2024 winter spring insecticide guide.
Growers should consider insecticide options that are soft on predator insects if spraying.
For more information on beneficials refer to DPIRD’s Know what beneficials look like in your crop page.
For a list of insecticides with their toxicity to beneficial insects, refer to Cesar Australia’s Beneficials Chemical Toxicity Table.
Further information
For more DBM information refer to:
- DPIRD’s Diagnosing Diamondback moth page
- Grains Research and Development (GRDC) Factsheet Diamondback moth is a sporadic but serious canola challenge
- GRDC’s Managing diamondback moth video
- DPIRD’s Protecting WA Crops newsletter March 2024 article Lessons learned from 5 years of monitoring diamondback moths and larvae across the five port zones of WA.
If you are sweep netting canola crops for DBM caterpillars, please use the PestFacts WA Reporter app to report and share your findings.
For more climate information, refer to DPIRD's latest Seasonal Climate Outlook.
For more DBM information contact Research Scientist Dustin Severtson, Northam on +61 (0)8 9690 2160, Technical Officer Alan Lord, South Perth on +61 (0)8 9368 3758 or Research Scientist Svetlana Micic, Albany on +61 (0)8 9892 8591.
Article authors: Bec Severtson (DPIRD Northam)
Article input: Dusty Severtson (DPIRD Northam) and Svetlana Micic (DPIRD Albany).