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Yellowheaded pasture cockchafers

Agronomist, Julian Minehan (Landmark), has reported significant feeding damage to several pasture paddocks around Goulburn, in the Southern Tablelands of NSW. Yellowheaded pasture cockchafers (Sericesthis geminate) have been found in high numbers upon digging in the affected areas. Julian says there are large bare patches within the paddocks and that the damaged pasture can be rolled back like a carpet in some places.

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

There is evidence that indicates heavy grazing during spring reduces grub density the following autumn. It is thought the grazing removes the dense growth under which the female cockchafers prefer to lay their eggs. However, be aware that overstocking during the season can reduce the plant’s capacity to replace severed roots and confound the pest problem.

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. Julian says BioGreen Granules have been applied to several paddocks and have provided some level of control for the yellowheaded pasture cockchafer in previous years.

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 (e.g. phalaris and cocksfoot) have been shown to reduce cockchafer numbers. Rotating pastures with a cereal, particularly oats, is a viable cultural control option. Experiments conducted in WA have shown that seed treating with chlorpyrifos powder (0.5kg/125kg seed) can be effective, however this is only recommended in severe cases. The recent wet conditions experienced in the Southern Tablelands may cause many larvae and pupae to die from drowning or disease.

Predatory invertebrates and insectivorous birds also play an important role in keeping cockchafer populations in check. However, large-scale land clearing has destroyed many nesting and roosting sites of non-migratory birds and has reduced food sources for parasitic wasps. To improve biological control, existing on-farm native vegetation should be preserved, and more breeding habitats for these birds and parasitic insects should be created. Julian reports that birds have been frequently seen feeding on cockchafer grubs in the affected paddocks.

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