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Diamondback moth larvae lurking

DBM populations are smaller and less advanced than this time last year, but in some areas activity is steadily increasing

Diamondback moth larvae and feeding damage (Source: cesar)

 

Where have they been reported?

In Victoria’s western Mallee (Underbool), moderate numbers of diamondback moth (DBM) (Plutella xylostella) caterpillars have begun to appear in canola during the last week of September. Most were quite small and probably below threshold, however, the rapidly warming conditions are expected to boost the development of this generation. In the Mallee, near Ouyen, 1-2 larvae per ten sweeps were observed in canola, but were well below threshold.

About diamondback moths

DBM tend to be most abundant during spring and summer. The moths are active at dusk and throughout the night, but usually do not fly far within a crop. Outside the crop, however, DBM can migrate long distances on prevailing winds, especially when their host plant has died. DBM can significantly impact canola yields and current control measures are variable.

Populations are likely to continue to increase over coming days and weeks now temperatures have increased. DBM development rate is closely linked to temperature; at 28°C, 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.

SARDI Entomologist, Kym Perry, believes the current low numbers are at least partly the result of low populations of DBM in wild hosts in autumn prior to sowing. Approximately 50% of sentinel crops in SA field surveys now contain DBM, but larval populations remain low and well below threshold levels across all districts. 

For comprehensive information on DBM in canola crops, including their occurrence, lifecycle, behaviour and management strategies, go to diamondback moth within the new PestNote series.

Thresholds and monitoring

Canola crops can tolerate considerable leaf damage before yield is affected. See GRDC Diamondback moth Fact Sheet for more information. Kym Perry and fellow entomologist, Greg Baker (SARDI) advise that, during flowering, DBM can cause significant yield loss if they feed on flowers within the terminal spike. There are no reliable thresholds for this early stage, but thresholds would probably be lower than the original ones (i.e. less than 30 per 10 sweeps). However, it is worth keeping in mind that 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.

Build-up of DBM populations within canola crops is very likely over the next few weeks, however, Kym suggests that crops within 3-4 weeks of harvest are considered at low risk of economic damage from DBM. Therefore, monitoring canola crops fortnightly is likely to be adequate at this stage, but if numbers do approach threshold levels, monitoring frequency should increase to weekly until harvest. 

Our advice

Sampling crops at several locations is important to determine whether numbers of DBM are increasing or decreasing before any spray decision is made.

If control is needed, keep in mind that moderate to high levels of resistance is 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; these 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.

 

Sources of advise and field reports of diamondback moth

Greg Baker - Researcher, SARDI (South Australia)

Brad Bennett – Agronomist, AGRIvision Consultants (Victorian Mallee)

James Mckee – Grower (Victorian Mallee)

Kym Perry – Researcher, SARDI (South Australia)

Matthew Whitney – Agronomist, Dodgshun Medlin (Victorian Mallee)

 

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