Diamondback moth caterpillars: early signs are ominous

Early DBM moth catches are a reminder that it’s not too early to monitor. In SA, indicators suggest that this is a moderate to high risk season.

Where have they been reported?

Traps baited with pheromones of diamondback moth (DBM – Plutella xylostella) in the Ouyen area of the Victorian Mallee have revealed an initial surge in moth catches.

This provides an early reminder of the need to monitor brassica crops for this pest.

SARDI researcher, Kym Perry, has reported that there is an above average risk this year for DBM heading into early spring. This assessment is based on the relatively early arrival of DBM in SA canola crops this season. Kym has been undertaking research on DBM for the past three years. 

Kym believes that the outlook for DBM is similar to that of 2014. In that year, there were extensive spring outbreaks in areas of SA, northern Victoria and NSW.

As in 2014, March rainfall in many regions this year created an abundance of brassica weeds in autumn prior to sowing and during early crop establishment. Follow up surveys in SA revealed a widespread DBM presence. Subsequent monitoring of sentinel canola fields in SA has shown that at a high proportion of crops have been colonised by DBM during May and June. Moderate moth activity, localised larval infestations and some damage have been observed in some areas of SA, in the Victorian Mallee and in North East Victoria.

About diamondback moth

Diamondback moth caterpillars are typically most abundant during spring and summer where they can cause extensive damage to canola foliage and seed.

As temperatures warm from late August, the pace of larval development and population growth will increase considerably.

Consider this: at 12°C the lifecycle takes more than 100 days whereas at 28°C the DBM lifecycle takes only 14 days.

Accordingly, population growth of DBM in winter will be slow due to the cool to moderate temperatures, but will escalate in September. 

In many cases, DBM build-up will be limited by beneficial species, many of the same species that control aphids, along with Zoopthora fungal infections, particularly where wet weather conditions persist.

For detailed information about this pest, and for comprehensive advice on preventative management strategies,go to our diamondback moth PestNote.

Our advice

We recommend starting to monitor for DBM larvae in canola crops particularly those sown in the vicinity of volunteer canola and weed brassicas. Infested crops should be regularly monitored. This will be especially important once conditions become warm and/or dry.

Spraying at this time of the year for DBM is unlikely to be warranted.

If early DBM control is necessary, Bacillus thuringiensis may be an option for growers that are practicing a more biologically-based form of IPM. Alternatively, Affirm® and Success Neo® are registered for DBM control in canola, and are reasonably effective and less disruptive to beneficial insects. Keep in mind that moderate to high levels of resistance is widespread in Australian populations.

Aphids may also be present in canola crops. Given the DBM risk, any aphid sprays should aim to conserve DBM beneficials early in spring.

Be careful not to confuse DBM caterpillars with other insects. In particular, the green larvae of the hoverfly can appear similar. Hover fly larvae are an important predator of aphids, and can best be distinguished by the absence of an obvious head capsule. In contrast DBM larvae have a small brown head capsule and wriggle frenetically when disturbed.


Sources of field reports of diamondback moth

Brad Bennett  – Consultant, AGRIvision Consultants (Victorian Mallee)

Siobhan de Little – Researcher, cesar

Kym Perry – Researcher, SARDI (South Australia)

Cover image: Photo by Andrew Weeks, Cesar Australia

What is Pestfacts south-eastern?

PestFacts south-eastern keeps growers and advisers informed about invertebrate pests and beneficials in broadacre crops and pastures during the winter-cropping season in Victoria and southern New South Wales.


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