Pasture scarab beetles

District agronomist, Rebecca Byrne (I&I NSW), has reported finding pasture scarab beetle larvae (Family: Scarabaeidae) causing damage to sorghum crops near Moree, in the North West Slopes and Plains district of New South Wales. In one paddock, which was previously sown to couch pasture, the sorghum is showing signs of damage and Rebecca says 2-3 grubs could be found beneath each plant. Although their identification has not been confirmed, the grubs are similar in appearance to African black beetle (Heteronychus arator) larvae. African black beetle larvae have recently been observed in high numbers across many paddocks around Armidale, in the Northern Tablelands district of New South Wales.

The African black beetle is a soil dwelling insect that favours pastures, particularly newly sown ryegrass and also summer-dormant perennial grasses. They will also attack cereals. Adults are 12-14 mm long, cylindrical and glossy black in colour. Larvae are ‘C’ shaped with a brown head, 3 pairs of legs and can grow up to 25 mm long. Most damage is caused by adults, which emerge in autumn to early winter and chew plants at or just beneath ground level. They may chew right through the stem or cause ‘ring barking’ on larger plants. Larvae are generally present from late spring to mid-summer and feed underground on the roots of perennial grasses. As with most other soil-dwelling pests, the African black beetle is difficult to control and insecticide sprays are often ineffective. Increasing seeding rates is a useful option in paddocks where the pest is anticipated to cause damage. If chemical control is required, spraying just prior to rain or late in the day may be better as beetles often come to the surface at night.

Grower, Michael Geddes, has reported damage to the crowns of wheat plants in a crop southwest of Donald, in the Wimmera district of Victoria. Some stems had been severed enough to cause plants to fall over. Michael says small burrows, approximately 8 mm in diameter, were also found near affected plants. No pests could be found after searching, however it is possible that adult African black beetles are the culprit.

Grubs that appear very similar to African black beetle larvae have also been found in a semi-improved pasture paddock near Armidale. They have been identified by technical officer, Gabriella Caon (SARDI), as Christmas beetles (Anoplognathus sp.), which are regarded as only a minor pest. Christmas beetle larvae live and develop in the soil. They feed on the roots of pastures, some crops, native grasses and lawns. Adult Christmas beetles typically feed on eucalyptus leaves, and heavy defoliation of eucalypts is possible when weather conditions promote the formation of feeding ‘swarms’. Many species are brightly coloured beetles and are attracted to lights at night.