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Blackheaded pasture cockchafers

Agronomist, Gerard O’Brien (Western AgSupplies), reports that high numbers of blackheaded pasture cockchafers (Acrossidius tasmaniae) have been observed in some pasture paddocks around Skipton, in the Western district of Victoria. Gerard says that at least six newly sown pasture paddocks were affected, although the number of cockchafers is patchy within each paddock. In all cases, these paddocks, which have not yet suffered any significant feeding damage, were pasture last year. There have also been reports of pasture paddocks near Ballarat, in the Western district of Victoria, that have very high numbers of blackheaded pasture cockchafers present.

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. Gerard became aware of the problem after observing little mounds of dirt surrounding small holes on the ground surface, which is typical of blackheaded pasture cockchafer activity. 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 problems with cockchafers last year, it does not mean that you won’t have 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 that spraying is not warranted at this stage. These paddocks will be monitored closely over the next few weeks as seedlings continue to emerge. Gerard says that when chemicals are needed, alpha-cypermethrin generally achieves adequate control. 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.

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