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Redheaded pasture cockchafers

Agronomist, Mick Duncan (Northern Agriculture), has reported feeding damage to several pasture paddocks, very likely to be caused by redheaded pasture cockchafers. The paddocks are located near Walcha and east of Armidale, in the Northern Tablelands district of New South Wales. In one paddock, Mick reported finding approximately15 grubs per square metre. The affected paddocks are predominantly pastures containing ryegrass, cocksfoot and fescue. The grubs, which are typically between 20-30 mm in length, have mostly pruned the roots of ryegrass plants. The feeding damage has been compounded by the presence of large numbers of birds that are turning over grasses looking for grubs.

Redheaded pasture cockchafer (Adoryphorus coulonii) grubs are ‘C’ shaped, have a red-brown head capsule and grow up to 30 mm in length. They are primarily root feeders and most damaging to shallow-rooted plants such as ryegrasses, sub-clovers and barley grass. Adults are stout, shiny black beetles and are approximately 15 mm long. Redheaded pasture cockchafers differ from other scarabs; they have a two-year lifecycle. In some instances this may mean damage only occurs every second year, however overlapping populations are often present meaning damage can still occur every year.

In autumn, increased soil moisture stimulates grubs to move closer to the soil surface to feed on plant roots. In severe cases where populations are high, pasture becomes patchy and can be rolled back like a carpet. Damage is typically most serious from March to June. Low soil temperatures over the winter period slow down feeding activity. Control of redheaded pasture cockchafers is complicated. Insecticides are largely ineffective because of their subterranean feeding habits. Re-sowing bare areas using a higher seeding rate is often the most effective strategy. Cultivating the affected areas prior to sowing a crop can also help reduce pest populations.

To check paddocks for cockchafers, dig in the affected areas. Crops sown into long term pasture paddocks are vulnerable to attack. 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 by flying, usually at dusk on warm evenings in late spring-early summer. Although predominantly an issue in pastures, redheaded pasture cockchafers will occasionally attack cereal crops.

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