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Cockchafers

Agronomist, Ed Gebhardt (AGnVET Services), has reported cockchafer grubs attacking two wheat paddocks north of Trundle, in the Central West Slopes and Plains district of New South Wales. They appear to be yellowheaded cockchafers (Sericesthis spp.), which are widespread pests of cereals, particularly in southern and central NSW. Ed reports numerous large patches within each paddock that are showing obvious signs of feeding damage; plants have stunted growth, are turning yellow or have been killed completely. The damage was first noticed in early July but has escalated recently. The paddocks, which are now at the mid-late tillering stage, are about 3km apart and were both sown to pasture in 2012.

Yellowheaded cockchafer grubs are ‘C’ shaped, creamy-grey in colour and have a yellow head capsule. 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 beetles about 10-15 mm in length. Yellowheaded cockchafers are primarily root feeders and do not come to the surface to feed. They mostly attack cereals, but will also cause economic damage to pastures.

Cereal crops sown into long term pasture paddocks are most vulnerable to attack. Growers are advised to monitor susceptible paddocks prior to sowing and throughout winter. Inspect paddocks by digging to a depth of 10-20 cm with a spade and counting the number of larvae present. This should be repeated 10-20 times to get an estimate of larval numbers. Four larvae per spade square is roughly equivalent to 100 larvae per m2. 30 larvae per m2 can potentially reduce yields by up to 1400 kg/ha. Ed says cockchafer grubs were heavily concentrated under plants within the sowing row, but much harder to find within the inter-rows. Upon digging in the affected areas, Ed estimates finding 6-8 grubs in about 50 cm2.

Control of yellowheaded cockchafers at this time of year is difficult. Insecticides are largely ineffective because of their subterranean feeding habits, and there are currently no synthetic insecticides registered. Effective management strategies can be implemented in autumn. Cultivating paddocks prior to sowing a crop will reduce pest populations as it kills grubs, and exposes them to predation by insectivorous birds and invertebrates. Re-sowing bare areas using a higher seeding rate will assist plant establishment. Another option is to sow less-susceptible plants (e.g. pulse crops) if practical.

In pasture situations, intensively grazing in spring, summer and autumn will make eggs and grubs in the topsoil more susceptible to desiccation and predation. When damage is noticed in late autumn-early winter stock should be removed, particularly from ryegrass dominant pastures, and the paddock spelled until late winter. This will help prevent the ryegrass being uprooted by grazing animals and maintain maximum leaf area needed to re-establish root growth. To help facilitate biological control, existing on-farm native vegetation should be preserved, and more breeding habitats for birds and parasitic insects should be created.

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