PestFacts WA

Diamondback moth activity is increasing in the Geraldton port zone

Two diamondback moth larvae captured by sweep netting canola. Photo courtesy of: Amber Balfour-Cunningham (DPIRD).

Growers are encouraged to get out and monitor canola crops for diamondback moth (DBM) caterpillars after a notable increase was seen recently in some areas of the Geraldton port zone.

A diamondback moth delta trap used as part of DPIRD’s crop surveillance network.  Photo courtesy of: Amber Balfour-Cunningham (DPIRD).

As part of a GRDC project, DPIRD are leading a focus crop surveillance network in all five port zones focused on DBM moth and larvae surveillance by staff from DPIRD, Mingenew Irwin Group, West Midlands Group, Liebe Group, and South East Agronomy Research (SEAR). The focus crops have been monitored since mid-June with low numbers of moths and larvae detected during July, except for some increase in moth numbers near Mingenew and some southern areas at Amelup and Gairdner (see maps below).

Total diamondback moths caught in traps during July 2021. Map courtesy of: Christiaan Valentine (DPIRD).

Total diamondback moths caught in traps during August 2021. Map courtesy of: Christiaan Valentine (DPIRD).

Map of average diamondback larvae sweep net results during early August 2021. Map courtesy of: Christiaan Valentine (DPIRD).

Map of average diamondback larvae sweep net results during the last two weeks of August 2021. Map courtesy of: Christiaan Valentine (DPIRD).

In one of her focus crops near Mingenew, Rachel Mason (Mingenew Irwin Group) has found that DBM larvae have increased over the past fortnight from an average of five per 10 sweeps to 41 per 10 sweeps. This ranged from 22 to 53 per 10 sweeps. Similarly, Rachel’s surveillance showed that two crops at Arrino both increased from an average of 3 per 10 sweeps to 28 per 10 sweeps over the past fortnight, indicating that DBM larvae are on the rise. All crops are at late flowering stage.

In the focus crops across all five port zones, DBM larvae numbers remained low during early August, however the crops mentioned above at Mingenew and Arrino showed a spike in moth numbers in pheromone traps during the previous 2 and 4 weeks. The surveillance data is indicating that this marked increase in moths could have resulted in the increase in caterpillar numbers. If this is the case, then growers at Dalwallinu and in southern areas near Amelup and Gairdner could expect an increase in larvae numbers in coming weeks with a spike in moth numbers detected two weeks ago.

DBM activity is expected to ramp up as spring approaches. However DBM do not always build up to threshold levels in every region and in every year. Last year was a good example where almost all canola focus crops in every port zone did not require insecticide application for DBM because they mostly stayed below 50 per 10 sweeps through to crop dessication/harvest. This is not the case this year with the current rise in numbers detect late August.

Sometimes DBM populations crash naturally as a result of fungal infection (especially with a moist canopy) and natural enemies, so it is important to monitor crops to optimise investments in spring sprays for DBM. A single spray does not kill all DBM present, which comprises eggs, larvae and pupae within the canopy. It is therefore important to do monitoring post spray to assess potential population increase in the weeks following and leading up to crop senescence when DBM are no longer a problem and they die out or fly away.

If you are sweep netting canola crops for DBM caterpillars please use the PestFax Reporter app to report and share your findings. It will help the DBM surveillance project team to determine how well the pheromone traps have worked in predicting outbreak risk and trigger crop protection programs before economic damage occurs.

Managing DBM later in the season

A diamondback moth caterpillar. Photo courtesy of: Svetlana Micic (DPIRD).

DBM thresholds for control are:

• Early to mid-flowering (no stress) - 50 caterpillars or more per 10 sweeps
• Mid to late flowering (no stress) - 100 or more caterpillars per 10 sweeps
• Pod maturation - 200 or more caterpillars per 10 sweeps.

DBM have less impact on yield once canola crops stop flowering.  Visual surface grazing and scarring of pod walls and stems will occur from DBM caterpillar feeding late season and this may result in a minor reduction of grain filling capacity depending on numbers of larvae, soil moisture levels and length of time to harvest.

Growers are urged to abide by label withholding periods for swathing/harvesting canola if applying a registered insecticide to control DBM (see the table below).

The pyrethroids (alphacypermethrin, esfenvalerate, gamma and lambda cyhalothrin) and carbamate (methomyl) registered for DBM are known to provide poor kill on DBM populations because of insecticide resistance.

Pesticide options for the control of native budworm and DBM can be found in DPIRD’s 2021 Winter Spring Insecticide Guide.

For more DBM information refer to:

• DPIRD’s 2021 PestFax Issue 9 Diamondback moth article
• DPIRD’s 2021 PestFax Issue 1 Tips for identifying caterpillars and remember beneficials when spraying article
• DPIRD’s Diagnosing diamondback moth page
• GRDC's Diamondback moth fact sheet
• GRDC’s Managing diamondback moth video.

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: Cindy Webster (DPIRD Narrogin), Dusty Severtson (DPIRD Northam), Alan Lord (DPIRD South Perth) and Svetlana Micic (DPIRD Albany).

Article input: Amber Balfour-Cunningham (DPIRD Northam) and Christiaan Valentine (DPIRD Northam).