Blackheaded pasture cockchafers

Agronomist, Julian Minehan (Landmark), has reported very high numbers of cockchafer larvae in a lucerne paddock near Goulburn, in the Southern Tablelands district of New South Wales. More than 10 larvae could be found per shovel of soil, however Julian says despite the high numbers, there is no noticeable damage to lucerne plants. Entomologist, Kym Perry (SARDI), has confirmed the larvae as blackheaded pasture cockchafers (Aphodius tasmaniae). Julian says the paddock is basalt soil and has been sown to lucerne for the last 4 years. The presence of tunnel entrances can be easily seen, with the highest numbers of larvae observed along tree lines.

Blackhead pasture cockchafers are pests of pastures and cereals in Tasmania, South Australia, 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. They feed on clovers, grasses and some weeds, chewing plant material in their tunnels during the day. 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. Pastures and cereal crops should continue to be monitored. 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. If densities exceed 300 per m²^{, control should be considered. An average infestation of 300-400 larvae per m² can cause a 50-70% reduction in winter pasture production and a 40-50% loss of desirable pasture species production in spring.}

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 favored 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.