- Bremer Bay
Research scientist Christiaan Valentine (DPIRD) recently found 50 to 100 diamondback moth (DBM) caterpillars per 10 sweeps in a canola crop near Clackline. Yet a canola crop 7 km away was relatively clean.
An extensive survey was carried out by DPIRD in summer to identify any populations of DBM surviving on a green bridge. Four sites were identified and DPIRD positioned a series of traps to measure moth build up in these areas (16 traps total). So far there has not been a large number of moths flying into the Gingin / Regans Ford area, but horticulturists in the region believe that numbers will climb rapidly from now in their brassica crops. Dale has had a steady stream of moths fly in over the last 6 weeks. Amelup has had the largest flights with an average of 208 moths caught per trap since July. DPIRD staff have averaged 33 DBM per 10 sweeps at trap sites at Amelup, while Gingin, Northam and Dale have all averaged less than 10 grubs per 10 sweeps. This survey was funded by GRDC.
James Bee (Elders) has reported finding high numbers 200 caterpillars per 10 sweeps of DBM in late flowering Bonito canola in the Bremer Bay region. James commented that the DBM are likely to have built up in nearby wild radish present in pasture paddocks. The canola was at 80% flowering and moisture stressed. Affirm® has been applied and James will be monitoring surrounding crops which currently have below threshold DBM numbers.
DBM caterpillars are generally spread throughout the plant from top to bottom of the canopy, but time of day and leaves senescing and dropping can cause them to move to more upper parts of the plant where sweep netting picks them up.
Sudden increases in DBM numbers in the sweep net can also be from new caterpillars hatching from eggs. Unlike native budworm, DBM have a short life cycle, especially during periods of warm weather and this gives them the ability to increase populations rapidly.
Growers are reminded that small budworm caterpillars (10-15mm long) can be confused with DBM larvae when sweep netting. The easiest method to tell the two apart is that DBM wriggle rapidly when disturbed and have more of a tapered shape at the ends as opposed to budworm. Hoverfly larvae, which are predators of aphids, also look very similar to DBM, but have a pale to white stripe on the back.
Managing DBM later in the season
DBM thresholds for control are:
• Early to mid-flowering (no stress) - 50 grubs or more per 10 sweeps
• Mid to late flowering (no stress) - 100 or more grubs per 10 sweeps
• Pod maturation - 200 or more grubs per 10 sweeps.
Moisture stressed crops are more susceptible to insect damage, so a lower threshold may be used if extended dry periods are experienced.
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 grubs, 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).
|Active ingredient||Witholding period (days)|
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.
For insecticide options refer to the department’s Winter/Spring Insecticide Spray Chart 2019.
For more DBM information refer to:
- DPIRD’s Diagnosing diamondback moth page
- GRDC's Diamondback moth fact sheet
- GRDC’s Managing diamondback moth video.
For more insect information contact Alan Lord, Technical Officer, South Perth on +61 (0)8 9368 3758 or Svetlana Micic, Research Officer, Albany on +61 (0)8 9892 8591 or Dustin Severtson, Development Officer, Northam on +61 (0)8 9690 2160.
Article authors: Cindy Webster (Narrogin), Alan Lord (DPIRD South Perth), Svetlana Micic (DPIRD Albany), Christiaan Valentine (DPIRD Northam).