Black Soil Scarab Damage to Winter Cereals

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This winter we’ve seen several instances of severe scarab damage to winter cereals (wheat, barley and oats) , including on crops at Jandowae and Felton on the Darling Downs. The photograph below shows that severe damage is mostly confined to areas of paddocks which were previously pasture – and this is where the highest scarab populations were found.

Scarab damage in barley crop at Jandowae.  The severely damaged crop area on the right was under pasture prior to planting. The area on the left was previously cropped and cultivated.  

The insect responsible for the damage is the juvenile (larval) stage of a scarab beetle. Larvae collected from one of the affected fields have been identified as most likely black soil scarab (Othnonius batesii). Larvae are currently being reared to adulthood to confirm the identity.  The adults (yet to emerge) are very distinctive

Third (and final) instar black soil scarab larva (30 mm) found under damaged barley plants at Jandowae.  Note the pale head capsule of this species. 

 Damage to crops was significant because the larvae were large from crop establishment onwards. Black soil scarabs have a 2 year larval stage. The eggs would have been laid in the pasture in spring/summer of 2009/10, and the small larvae developed whilst feeding on the roots of the pasture grasses. By autumn/winter 2010, the larvae were large (up to 30 mm), and feeding voraciously. When the pasture was removed and the winter cereal planted, the scarab larvae started feeding on the emerging cereal plants.  Larvae in the field are currently estimated as 10-12 months old and at the early 3rd instar stage. They are likely to pupate in mid to late summer and emerge as adults in the spring of 2011.

 

 

The scarab larvae feed on the below-ground parts of the cereal seedlings, limiting root growth or severing roots completely. As a result, seedling cereals are moisture stressed and unthrifty.

Because scarab larvae live entirely below ground, once the crop is planted and the damage evident, there is no control option available to prevent further damage to the crop. It is simply impossible to contact the larvae with insecticide and large larve would be difficult to control even if they could be reached.

Black soil scarab adult (17 mm).  Note the two-tone colour scheme – dark head and thorak, and light brown wing covers.  Image by CSIRO.

Scarab damage to winter cereals is uncommon, but these cases highlight the importance of checking for soil insects prior to sowing, particularly in high risk situations such as when coming out of a pasture into a crop.  Seasonal conditions (the onset of wetter than average seasons) may also have contributed to an increase in black scarab numbers.

Anyone with suspected scarab activity can contact the entomology team via this blog.

Further information on scarabs, and soil insect monitoring strategies can be found at the QDAFF website.

Acknowledgements: Dr Peter Allsop from BSES for the identification of the scarab larvae. Steve Henning for photographs of the scarab larva and the damaged barley crop.

Article by Hugh Brier and Melina Miles