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Blackheaded pasture cockchafers

Grower, Brooke White, has reported blackheaded pasture cockchafers causing extensive damage to a number of clover-based pastures near Edenhope, in the Wimmera district of Victoria. Highest numbers have been found on heavy soil types and in areas that contained limited dry feed over summer. Brooke says a large number of cockchafer adults were observed over summer. Adult beetles are commonly seen flying at dusk on warm nights in January and February. During this time, adults achieve long distance dispersal and then lay their eggs in the soil of new paddocks. Thus, if paddocks did not have problems with cockchafers last year, it does not mean they won’t have this year.

Blackhead pasture cockchafers (Acrossidius tasmaniae) are a common pest of pastures and cereals in Victoria, New South Wales, Tasmania and South Australia. They appear to be most problematic in areas where the annual rainfall exceeds 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/or 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. Most feeding damage occurs in May and June, when the rate of pasture growth is slowing down due to the cold weather.

Pastures and emerging cereal crops should 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 at least 10 times to get an accurate estimate of numbers. When densities exceed 150 per m² in pastures, and 100 per m² in cereals, control should be considered.

Several chemicals are registered for control of blackheaded pasture cockchafers. Previous reports suggest that when chemicals are needed, alpha-cypermethrin achieves adequate control, and is best applied just before rain or when a heavy dew is expected (but allowing enough time for the spray to dry to prevent it being washed off the foliage). Once blackheaded pasture cockchafer larvae mature they stop feeding and stay below ground. For best results, insecticide sprays should be applied before the onset of cold wet winter conditions in late June-early July. If paddocks receive good rains and pasture is plentiful, blackheaded pasture cockchafers may only constitute a minor problem.

Cultivating before sowing, or sowing with soil disturbance, is recommended. This directly kills larvae, increases the likelihood of attack by natural enemies (e.g. predatory birds) and distributes pasture seeds to assist in re-vegetation. If cockchafers are a continual problem, consider sowing tolerant pasture species such as phalaris and cocksfoot.

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