Diamondback moth alert for canola crops
While growers have been focused on the threat posed by green peach aphids, diamondback moth (DBM) also has the potential to cause significant losses in canola this season. Moths have been sighted in large numbers through South Australia across rainfall zones, and the September weather forecast could favour the moth’s development.
SARDI entomology leader Greg Baker has received numerous reports of DBM infestations.
“In most years in most canola cropping districts of South Australia, DBM is not common, it’s one of those pests that fortunately we don’t have to deal with very often. However this year, the green-bridge and above average temperatures in late autumn that favoured the green peach aphid were equally suitable for DBM to be present and colonising canola early,” he said.
“As we now come out of winter, having had dry weather for much of August and temperatures starting to climb, we’re now in circumstances that are again quite favourable for DBM. We’re starting to receive agronomist reports that DBM are easy to find in many canola crops from all around South Australia, and I expect there will also be reports in Victoria and southern NSW.”
Growers are advised to sweep for DBM, and follow threshold guidelines (Table 1) in planning a control strategy.
“We advise growers work within the threshold guidelines before considering action to address DBM populations. Respond when needed rather than going in early, because DBM numbers can naturally decline from beneficial attack and disease. In fact we’re receiving reports from agronomists of DBM numbers in some crops doing just that in the past 2 weeks. Also it is important to recognize that spraying when it’s not required unnecessarily increases the risk of resistance development,” Mr Baker said.
Resistance is a major concern in DBM treatment, with only three products registered – two new synthetics (Affirm® and Success® Neo) and the biological insecticide Bacillus thuringiensis (Bt).
“While trials in WA have shown that Bt products work reasonably well in moderate infestations, growers are not confident in their effectiveness in more severe infestations,” Mr Baker said.
“The two synthetic products are equally effective, though Success® Neo is significantly more expensive. This results in one chemistry, Affirm®, being predominantly used to treat DBM, which is compromising the long-term effectiveness of that one product.”
While research to identify new chemistries is currently underway (see ‘GRDC Projects’), this means that growers should be careful when applying Affirm®, and not spray unless it is definitely required.
DBM have been shown to be susceptible to many beneficial insects including a number of parasitic wasps, damsel bugs, ladybirds and hoverflies. Fungal infection can reduce DBM numbers, and forecasts for light rain and warm temperatures could potentially favour fungal activity. Growers should look for signs of parasitic or diseased pupae, which are usually on the underside of leaves and may appear white and brittle, and consider this when testing thresholds. For instance, if an infestation is just on the edge of the threshold but a lot of beneficial insects and parasitized pupae are observed, spraying may not be required.
08 8303 9544,
GRDC Fact Sheet: Diamondback moth.
GRDC Project Code DAS00094, UA00146, UWA00165, UNE00016