Diamondback moth

* DBM numbers are typically below or at threshold in many regions. * Extra considerations are suggested before spraying as crops enter the ripening phase.

 

Where have they been reported?

Currently the diamondback moth (DBM - Plutella xylostella) appears to be the pest causing most concern in the Victorian Wimmera and Mallee, and NSW Riverina and Central West Slopes & Plains districts. The extent of spraying for DBM varies across these regions. In other regions, such as the Victoria’s Northern Country and Northeast, DBM remain low and are not expected to be problematic.

In western Victoria, canola has largely finished flowering and is generally about 2 to 3 weeks from windrowing/harvest. In several reports from around Donald (Victorian Wimmera) the numbers of DBM are greater than have been experienced in some years, but most populations remain below threshold (20 to 80 per 10 sweeps). Damage has varied, with limited pod grazing observed and growers are gnerally not spraying. Further north in the Birchip area, caterpillar numbers are higher (20 to 100 per 10 sweeps) but damage remains marginal (leaf and some minor pod grazing) in many crops. Larvae are mostly mid-development (3-4 mm in length). One incidence of pod boring has been reported. There has been widespread use of Affirm® for DBM control in the Victorian Mallee.

In NSW, many canola crops are within 2 weeks of windrowing while some in the Central West Slopes & Plains district are at mid flowering to mid podding. Again, most DBM populations are below threshold (60 to 80 per 10 sweeps, or 40 to 50 per square metre (using a beat sheet)). In crops around Ariah Park (NSW Riverina), caterpillar numbers (3-4 mm) are still increasing, but have been variable (30 to 200 per 10 sweeps). However, they are not causing much concern because the crops are well advanced. Further north, in the area between Tottenham and Gilgandra, populations are also low to moderate with a small amount of surface grazing to leaves and pods.

Anecdotal reports from South Australia suggest that paddocks that were sprayed with broad-spectrum insecticides earlier in the year, particularly synthetic pyrethroids, have higher populations of DBM, presumably because of the loss of beneficial insects.

DBM habits and behaviour

Larvae feed on all above-ground plant structures, but can be particularly damaging when leaves senesce late in the season and they feed on the green wall of canola pods, slowing or preventing ripening, grain fill and thus reducing yield. This can continue for a period after windrowing/swathing if the pods remain green. Most (but not all) DBM populations have resistance to synthetic pyrethroids and organophosphates.

Our advice

The decision to spray DBM late in crop development is more complex than at other stages. Considerations are:

• the proximity to harvest, and the withholding period of any potential insecticide (3 weeks typically).

• for indeterminate varieties, the likelihood of additional rain to stimulate new flowering and pod set.

• caterpillar densities. It is probable that advanced crops can tolerate over 150 caterpillars per 10 sweeps (200-300 per m2).

• the yield potential. The more effective products are relatively expensive, and the cost-benefit may be questionable. Desiccants applied at windrowing may be a less expensive option.

DBM caterpillars will surface graze or scarify pods, and occasionally burrow into the pod to directly consume the seed. Although pod boring is unusual, there have been a few reports of this behaviour occurring in the last few weeks. However, low numbers native budworm are present in most canola crops at present; in many cases the apparent seed damage may be attributable to this pest. Careful inspections of crops is needed.

See PestFacts Issue No. 10 and the GRDC Diamondback moth Fact Sheet for more information on thresholds and management.

 

* Sources of field reports and thresholds for DBM

Bruce Adriaans – Agronomist, Landmark (Victorian Wimmera)

Greg Baker – Entomologist, SARDI (Adelaide)

Brad Bennett – Agronomist, AGRIvision Consultants (Victorian Mallee)

Chris Dunn – Agronomist, Landmark (Victorian Northern Country)

Terry Edis - Agronomist, Landmark (NSW Central West Slopes & Plains)

De-Anne Ferrier – Research Officer, Birchip Cropping Group (Victorian Mallee)

Mike Laidlaw – Senior Agronomist, Harberger Farm Supplies (Victorian Wimmera)

Peter Mangano – Entomologist, DAFWA (Perth)

Andrew McMahen – Agronomist, Landmark (Victorian Mallee)

David Strahorn – Agronomist, Delta Ag (NSW Riverina)

Damien Tanner – Agronomist, JSA Agronomy (Victorian Wimmera)

Matt Wittney – Agronomist, Dodgshun Medlin consultants (Victorian Mallee)