RLEM - noteworthy developments

The late season for canola has resulted in some unexpected damage to established crops. Another incidence of insecticide resistance in South Australia, this time closer to Victoria, emphasises the importance of resistance management.

Unseasonal damage

We reported earlier this year that the slow start to the growing season in many south-eastern regions may increase the vulnerability of young crops to redlegged earth mite (Halotydeus destructor, RLEM) damage. Generally, the slow growth of fragile seedlings simply extends their exposure to mite feeding before they are able to develop adequate leaf and root resources. Canola plants that are beyond the rosette stage are generally able to outgrow mite damage.

So it has come as some surprise that some crops at the rosette to cabbaging growth phase in northern Victoria have succumbed to serious damage by RLEM and possibly blue oat mite (Penthaleus major, BOM). The crops were near Elmore in the North Central district and near Devenish in the North East district. Severe leaf damage caused by intense albeit patchy feeding threatened plant viability and survival. At this stage of plant growth, the crops were presumably no longer under the protection of any insecticide seed dressing.

 

Mite damage to leaves of a cabbaging canola crop near Elmore (Image credit: Greg Toomey)

This demonstrates the importance of monitoring crops for mite activity when plant growth is so slow, especially when crops are most vulnerable but even after plant establishment. Damage manifests itself in silver or white patches, which can be sporadic. If mite activity is suspected, it is important that the correct species is identified. For assistance with identification download the GRDC Back Pocket Guide - Crop Mites.

Insecticide resistance closer to Victoria

More evidence of the easterly spread of insecticide resistance in RLEM has recently emerged from a GRDC-funded resistance screening project. cesar researcher Dr Aston Arthur has identified resistant mites found on a property near Bordertown, SA - close to the Victorian/SA border. The mites were taken from a pasture paddock where organophosphate sprays were used but failed to provide adequate control of mites. Dr Arthur has now found that the mites had acquired moderate resistance to chlorpyrifos and low levels of resistance to dimethoate. Genetic tests for resistance to synthetic pyrethroids are underway.

Resistance to synthetic pyrethroid and organophosphate chemicals has been detected previously in RLEM populations in Western Australia and in South Australia in areas 50 km north west of this site. The spread of resistance is likely to be slowed if techniques described in the recently updated resistance management strategy for RLEM are applied.

Download a copy of the new strategy here. The strategy is also available at the GRDC and the IPMGuidelinesforGrains websites. For further information on redlegged earth mite management visit redlegged earth mites within our PestNote series. 

 

Field observations

Bruce Larcombe – Larcombe Agronomy (Victorian North East)

Greg Toomey – Agronomist, Landmark (Victorian Northern Country)