Blackheaded pasture cockchafers

Philip Hawker reports that blackheaded pasture cockchafers (Acrossidius tasmaniae) have been observed in some pasture paddocks around Derrinallum, in the Western district of Victoria. Vicky French (Elders) says blackheaded pasture cockchafers were observed earlier in the season in pastures around Mortlake. The affected paddocks had a history of cockchafer activity. Considering earlier numbers, Vicky is surprised that the damage and population sizes thus far have been relatively minimal.

Blackhead pasture cockchafers are pests of pastures in Tasmania, SA, NSW and Victoria. The grub is creamy-grey in colour with a black head. When fully grown in winter they are about 15mm long. Other cockchafer grubs often have similar colouring on their bodies with yellow or red heads. The grubs live in the soil until mid to late summer, where they emerge as shiny black beetles about 10-12mm in length.

The grubs come to the surface at night in response to rains and heavy dews. They feed on clovers, grasses and some weeds, chewing plant material in their tunnels throughout the day. Blackheaded pasture cockchafer activity results in little mounds of dirt surrounding small holes on the ground surface. Another indicator is bare patches that appear in pasture from mid-autumn to late winter.

To check for cockchafers, dig in the affected areas or look on the soil surface for tunnel entrances. Be aware that if you did not have cockchafer trouble last year, it does not mean that you won’t have it this year. Adult beetles achieve long distance dispersal over summer by flying, usually at dusk on warm evenings.

Several chemicals are registered for the control of the blackheaded pasture cockchafer grub and have varying withholding periods. However, reports indicate spraying is not warranted at this stage. If cockchafers are a continual problem, consider sowing tolerant pasture species such as phalaris and cocksfoot. Avoid short, open pastures and a high clover content, which favour pasture cockchafers.