Yellowheaded pasture cockchafers

Agronomist, Julian Minehan (Landmark), has observed bare patches within some established pasture paddocks around Goulburn, in the southern tablelands of NSW. Pasture cockchafer grubs have been found in high numbers upon digging in the affected areas. The species have been identified as the yellowheaded pasture cockchafer (Sericesthis geminata), also known as the pruinose scarab.

The yellowheaded pasture cockchafer grub is creamy-grey in colour with a yellow head. When fully grown in winter they are about 25-30mm long. The grubs live in the soil until mid to late summer, where they emerge as brownish yellow to reddish brown beetles about 10-15mm in length. Yellowheaded pasture cockchafers are primarily root feeders.

Control of the yellowheaded pasture cockchafer is more complicated than methods used for the blackheaded pasture cockchafer, which feeds above the ground. There are no synthetic insecticides that give effective control of yellowheaded cockchafers because of their subterranean feeding habits. Re-sowing areas made bare by cockchafer damage using a higher seeding rate is often the most effective strategy, although may not be appropriate at this time of year.

There is some evidence to suggest that heavy grazing during spring reduces grub density the following autumn. The heavy grazing removes the dense growth under which the female cockchafers prefer to lay their eggs. A biological insecticide (BioGreen Granules) is one option available for the control of soil-dwelling cockchafers. However, this product, which is a strain of the native soil fungus Metarhizium anisopliae, is currently only registered for the control of the redheaded pasture cockchafer.

To check for cockchafer grubs, dig in the affected areas or look on the soil surface for tunnel entrances.Non-chemical control practices, such as sowing non-preferred pasture species have been shown to reduce cockchafer numbers. Rotating pastures with a cereal, particularly oats, is a viable cultural control option. Remember that predatory invertebrates and insectivorous birds also play an important role in keeping cockchafer populations in check.