Keep an eye on diamondback moth

Reports suggest that densities of diamondback moth (PDF 920.1 KB) (DBM), Plutella xylostella, in canola remain encouragingly low in most areas but are expected to increase in coming weeks. We recommend that regular monitoring is continued until late crop maturity. Based on reports, fortnightly monitoring intervals should be sufficient at present, increasing to weekly intervals if densities approach recommended thresholds.

On Lower Eyre Peninsula, strong moth activity has been consistently observed in traps around the coastal district of Mt Hope over the past few weeks but is not yet reflected in larval densities. It is likely that significant rainfall in the region during August caused high mortality of larvae at that time (heavy rain often drowns larvae), and moths now being observed may represent recent the emergence of local survivors.

Larval densities being observed include 10 per 10 sweeps around Mt Hope and Tumby Bay and 1-3 per 10 sweeps around Yeelanna and Karkoo. Several long season variety canola crops near Wharminda, sown in December 2017 and only now at bolting stage, have been more heavily attacked by DBM. Interestingly, larval numbers recently declined in one of these crops left untreated, implying that natural enemies may have naturally regulated pest numbers. On upper Yorke Peninsula, around 10 in 10 sweeps are being observed around Kadina, and low activity being is being reported in the SA Mallee around Lameroo.

Despite a lack of green bridge early this season (PestFacts Issue 1, 2018), DBM numbers can still build up in spring, particularly where conditions are relatively warm and dry. Warmer temperatures increase DBM development rates. The entire lifecycle is completed in about 3 weeks at constant temperatures around 20oC, decreasing to 15 days at 24oC.

When making threshold-based spray decisions, take into account crop stage, crop stress levels and forecast weather. Growers and advisors are encouraged to become familiar with the new GRDC resistance management strategy for diamondback moth. For more information on thresholds and control, refer to PestFacts Issue 6, 2018, Diamondback moth PestNote, and the GRDC diamondback moth Fact Sheet.

Diamond back moth larva (K. Perry)
Diamond back moth larva (K. Perry)

Sources: Brad Foster (Bawdens Rural, Tumby Bay), Chris Davey (YP Ag), Nigel Myers (Landmark Cummins), Tim Richardson (Landmark Cummins).

Page Last Reviewed: 14 Sep 2018
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