Management of bronzed field beetle - a pest of canola in southern Western Australia

Page last updated: Tuesday, 1 March 2022 - 7:57am

Damage

Bronzed field beetle larvae are most destructive when they have reached at least half size (5-6mm), before the crop emerges.

Several hundred larvae of this size per square metre will cause serious damage to the young crop by chewing the stems at ground level, causing the plants to fall and die.

Older plants may be ringbarked and leaves in contact with the ground may be eaten, but the most serious damage is to young plants and is easily missed.

Larvae can cause substantial economic losses. For example, where larvae were left uncontrolled and plant numbers fell to less than 10 per square metre, the yield was only 0.6 tonnes per hectare (t/ha). Where larvae were controlled and plant numbers remained at between 30-40 per square metre, a yield of 2.8t/ha was achieved.

Bronzed field beetle larval damage to canola crop
Bronzed field beetle larval damage to canola crop
Ringbarking of canola stems and chewing of leaves by bronzed field beetle larvae
Ringbarking of canola stems and chewing of leaves by bronzed field beetle larvae

Monitoring

Full-size or even half-size larvae may be seen in the field, but it is difficult to estimate their numbers. They usually hide under stubble, clods of dirt or in cracks in the soil during the day. They move and feed most actively during the night, but also in the evening and morning when conditions are damp and cool.

In areas where this pest is known to cause damage, paddocks containing a lot of surface plant material such as crop stubble or heavy pasture growth should be checked for adults before autumn rains or seeding of canola.

Check for adults by:

  • Inspecting small areas (such as one tenth of one square metre) many times within a paddock to estimate numbers.
  • Carpet squares placed on the ground for a few days will attract four times the number of adults compared to uncovered areas.

Check crops every few days during and after crop emergence to determine whether pests are causing damage and plant loss.

Plant counts in marked stations provide valuable information on pest damage levels when pests are difficult to detect.

When plants are falling to low levels, make a thorough search to determine if one or more pests are responsible.

Several other species including mites, weevils, slugs and European earwig may also cause a thinning of the crop.

Threshold

15 or more adults per square metre may produce up to 1500 larvae.

Management options

Stubble reduction

Bronzed field beetle larvae have caused damage to crops sown into paddocks with abundant surface plant material using zero or minimum tillage methods. A reduction in the number of grazing animals has contributed to the amount of surface material in many paddocks.

Department of Agriculture and Food, Western Australia experiments have demonstrated very clearly that this pest needs surface plant residues for shelter and for breeding. The presence of surface crop residues was compared to the removal of such residues by light raking in February before eggs were laid. The canola was seeded with zero tillage methods.

By day, adults sheltered in stubble plots, while at night they moved freely between stubble and raked plots.

However, there was almost no breeding in raked plots. In contrast, there were 1500 larvae per square metre in stubble plots and most plants were killed.

The removal of much of the surface plant material, where it is considered to be a suitable practice, will reduce or eliminate this pest. This may be achieved through cutting and baling, grazing or burning.

Cultivation before seeding will also create a hostile environment for the larvae and few are likely to survive.

Increase seeding rate as seed dressings are ineffective

Increasing the seeding rate will result in a much greater plant density. Although insecticide seed dressings have been effective against other false wireworms, they have not provided control of this pest.

Chemical control

Trials have shown that using alphacypermethrin (100 grams per litre EC) at 400 millilitres per hectare has provided a reasonable level of control of this pest. Poor control occurs if there is heavy stubble.

Acknowledgements

The support of Grains Research Development Corporation in this work is gratefully acknowledged. 

Contact information

Management of bronzed field beetle - a pest of canola in southern Western Australia

Author

Svetlana Micic