Pasture cockchafers

Julian Mineham (Landmark) reports that blackheaded pasture cockchafers (Acrossidius tasmaniae) have been observed in some established and newly-sown pasture paddocks around Goulburn, in the Southern Tablelands of NSW. The damage has been severe in patches, with some areas being sprayed with synthetic pyrethroids.

The blackhead pasture cockchafer grub is creamy-grey in colour with a black head. When fully grown in winter they are about 15mm long. The grubs live in the soil until mid to late summer, where they emerge as shiny black beetles about 10-12mm in length. They come to the surface at night in response to rains and heavy dews, feeding on clovers, grasses and some weeds.

Julian says a high number of adult beetles were observed in late summer, particularly around trees. Based on these observations and the damage caused to date, Julian expects large numbers and potential problems in spring. Monitoring will be critical. 

Julian has also observed some redheaded pasture cockchafers (Adoryphorus couloni) in pastures. Redheaded pasture cockchafer adults are stout, shiny black beetles about 15mm long. Larvae are soft, whitish grubs with three pairs of yellowish legs and a hard, reddish brown head capsule. Newly hatched larvae are only 5mm long but when mature grubs reach up to 30mm in length.

Unlike the blackheaded pasture cockchafer, which feeds above the ground, there are no synthetic insecticides that give effective control of redheaded cockchafers because of their subterranean feeding habits.

A biological insecticide (BioGreen Granules) is one option available for the control of the redheaded cockchafer. BioGreen is a strain of the native soil fungus Metarhizium anisopliae, which is mixed with seed when sowing pasture. Julian says some growers in the Southern Tablelands have used BioGreen as a preventative strategy over several years. Remember, it is not a quick remedial strategy for one season.

Consultant, Simon Mock (Clovercrest Consulting) has recently observed feeding damage and the presence of a large number of dead beetles within canola crops in the Wimmera district of Victoria. The beetles have been identified by Senior Technical Officer, Ken Henry (SARDI), as the little pasture cockchafer (Australaphodieus frenchi).

Simon says between 10-20 beetles/m^² could be seen on the soil surface following a spray – and subsequent rainfall –  with a synthetic pyrethroid. Similar observations have been found over the last few years. The affected canola crops have been sown into paddocks with a pasture history. No other pests have been found attacking the canola seedlings.

The little pasture cockchafer is not regarded as a pest, although only a small amount is known about their biology and feeding habits. They belong to the Family Scarabaeidae and are found in NSW, SA, Tasmania, Victoria and WA. Adults and larvae are believed to be coprophagous: consuming and redigesting animal dung. They are not reported to feed on living plant material. These paddocks will be closely monitored to better understand the situation and ascertain the pest status of the little pasture cockchafer.  

To check for cockchafers, dig in the affected areas or look on the soil surface for tunnel entrances. Non-chemical control practices, such as sowing tolerant pasture species have been shown to reduce cockchafer numbers. Rotating pastures with a cereal, particularly oats, is a viable cultural control option. Predatory invertebrates and insectivorous birds also play an important role in keeping cockchafer populations in check.