Blackheaded pasture cockchafers

Agronomist, Todd Jones (Elders), has reported blackheaded pasture cockchafers (Acrossidius tasmaniae) attacking several cereal crops around Narrabri, in the North West Slopes and Plains district of NSW. Todd reports that the distribution across the affected paddocks is fairly even, with approximately 10 grubs observed per square metre. Significant feeding damage has occurred to oat and barley seedlings, which are at the 3-5 leaf stage.

Blackhead pasture cockchafers are wide-spread pests in south-eastern Australia. The grub is creamy-grey in colour with a black head. When fully grown in winter they are about 15 mm long. Other cockchafer grubs often have similar colouring on their bodies with yellow or red heads. The grubs live in the soil until mid to late summer, where they emerge as shiny black beetles about 10-12 mm in length.

Blackheaded pasture cockchafers come to the surface at night in response to rains and heavy dews. They damage pasture and cereal crops in late autumn and early winter, chewing plant material in their tunnels throughout the day. Todd says a growing number of seedlings are being ring-barked, with clear evidence of tunnelling on the soil surface, particularly around the crown of the plants. Todd also reports that all the affected crops have been sown into paddocks with a long pasture history.

To check for cockchafers, dig in the affected areas or look on the soil surface for tunnel entrances. Be aware that if you did not have problems with cockchafers last year, it does not mean that you won’t have this year. Adult beetles achieve long distance dispersal over summer by flying, usually at dusk on warm evenings.

Several chemicals are registered for the control of blackheaded pasture cockchafers. Todd reports that insecticide sprays of either alpha-cypermethrin or methidathion are likely to be required. Non-chemical options are also available, but will vary depending on the crop. Agronomist, Mick Duncan (Northern Agriculture P/L) says that in pastures, a light disturbance with shallow chisel ploughing can provide adequate control. This directly kills grubs, increases the likelihood of attack by natural enemies (e.g. predatory birds) and distributes pasture seeds to assist in re-vegetation. Avoiding short, open pastures with a high clover content will also lessen the problem.

For further information and control options, refer to PestFacts Issue No. 1.