Blackheaded pasture cockchafers

Agronomist, John Jervois (Tarcutta Rural), has reported high numbers of blackheaded pasture cockchafer larvae in a recently sown pasture paddock near Tarcutta, in the South West Slopes district of New South Wales. The paddock was a clover based annual pasture in 2012. Fortunately, the larvae were identified prior to sowing and a well-timed insecticide spray pre-emergence provided good control. John says that parts of the paddock were cultivated to a greater depth and provided additional control of cockchafer larvae. To date, there has been no significant feeding damage to emerging pasture seedlings.

Blackhead pasture cockchafers are pests of pastures and cereals across New South Wales and Victoria. They appear to be most problematic in areas where the annual rainfall exceeds about 480 mm. The larvae are creamy-grey in colour with a hardened black head capsule. They have soft bodies and six legs. Fully-grown larvae are 15-20 mm long and tend to curl into a C-shape when exposed. Adult cockchafer beetles are approximately 10 mm long, dark brown to black in colour.

Blackheaded pasture cockchafers are the only cockchafer species that comes to the surface to feed. The larvae typically surface at night in response to rainfall and heavy dews. Small mounds of dirt surrounding holes on the soil surface are often the first sign of their activity. Other indicators are bare patches that appear in pastures from mid-autumn to late winter. Heavily infested areas may feel spongy underfoot. 

Most feeding occurs in May-June, when the rate of pasture growth is slowing down due to the cold weather. If paddocks receive good rains and pasture is plentiful, blackheaded pasture cockchafers may only constitute a minor problem. Inspect susceptible 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. Established thresholds suggest control is warranted if densities exceed 150-200 per m² in pastures, and 100 per m² in cereals.

If cockchafers are a continual problem, consider sowing tolerant pasture species such as phalaris and cocksfoot. Avoid overgrazing pastures during late spring and early summer as these areas will be favoured for egg-laying by female beetles. Cultivating before sowing, or sowing with soil disturbance, can expose larvae. Several chemicals are registered for control of blackheaded pasture cockchafers. Previous reports suggest that when chemicals are needed, alpha-cypermethrin achieves adequate control.