Diamondback moth

* Numbers of DBM caterpillars in canola have grown dramatically in recent weeks in western Victoria. * The economic threshold is 100 per 10 sweeps in post flowering crops. * The use of synthetic pyrethroids and organophosphates is not recommended.

 

Where have they been reported?

In the Victorian Mallee, numbers of diamondback moth (DBM) caterpillars (Plutella xylostella) have built up dramatically since the last week of August. They are now widespread across many canola crops, particularly in the northern Mallee. Counts of 50 – 150 larvae per 10 sweeps are common. Very young caterpillars (1-5 mm) have dominated a mixture of all stages, including adults. Some crops have populations well over threshold and are being sprayed. In some crops, large numbers of small grubs have yet to cause much detectable damage. Crops are generally at pod set or in a few cases, late flowering. There has been some use of synthetic pyrethroids; but these have proven unsuccessful, and have been repeat sprayed, often with Affirm®.

Diamondback moth habits and behaviour

DBM tend to be most abundant during spring and summer. DBM outbreaks have become more frequent and severe in some regions, particularly in seasons with mild winters. The moths are active at dusk and throughout the night, but usually do not fly far within a crop. However, outside the crop, they can migrate long distances on prevailing winds, especially when their host plant has died. DBM can have significant effects on canola yields and current control measures are variable.

Populations are likely to continue to increase over coming days and weeks as temperatures rise. We recommend frequent monitoring of canola crops (at least once per week) from now until late spring. DBM development rate is closely linked to temperature; at 28 degrees, the lifecycle only takes about 14 days. Heavy rain events can cause significant larval mortality by drowning; therefore, we recommend re-assessing spray decisions after any forecast rain.

A current study of parasitism in DBM larvae in crops of SA, Victoria, NSW and Queensland by Kym Perry (Researcher - SARDI & University of Adelaide) over recent weeks has revealed low to moderate parasitism, ranging from 0% to 30%, with most in the 5% to 10% range. Rates of parasitism tend to increase as the season progresses.

Monitoring and control options

Sampling crops at several locations is important to determine whether numbers are increasing or decreasing before any spray decision is made. If control is needed, keep in mind that moderate to high levels of resistance are widespread in Australian populations. Avoid pyrethroids and organophosphates, either alone or in mixtures, because they are ineffective for DBM control and will kill beneficial insects, potentially leading to other problems (e.g. DBM or aphid resurgence).

Bacillus thuringiensis products and two new synthetic insecticides, Affirm® and Success Neo®, are registered for DBM control in canola, and are less disruptive to beneficial insects. No single insecticide application will completely eliminate the DBM population. In years when populations are large, a two-spray strategy is recommended within seven days to control DBM.

Our advice

Canola can tolerate considerable leaf damage before crop yield is affected. A threshold of around 100 larvae per 10 sweeps is our recommendation for post flowering crops, even in moisture stressed situations. The original work on DBM thresholds was undertaken with cheaper broad-spectrum insecticides; the only effective chemical options now available are more expensive, placing downward pressure on thresholds. See GRDC Diamondback moth Fact Sheet for more information.

We recommend avoiding the use of synthetic pyrethroids and organophosphates for DBM control in canola crops because of widespread resistance. Partial control of DBM may be possible, but rapid reinvasion is likely because of the removal of beneficials.

Description

DBM larvae grow to 12mm long, are pale yellowish-green and tapered at both ends. The larvae go through four growth stages and have a dark head in the first two stages. Mature larvae spin gauze-like cocoons, usually on the underside of leaves. The pupa is visible inside the cocoon and starts off green in colour, turning brown before emerging as an adult moth. The AusVeg VegeNotes has excellent images of DBM growth stages.

 

* Sources of field reports of diamondback moths

Brett Atkin – Agronomist, Elders (Victorian Mallee)

Simon Craig – Agronomist, BCG (Victorian Mallee)

Andrew McMahen – Agronomist, Landmark (Victorian Mallee)