Blackheaded pasture cockchafer; almost run its race

As with other scarabs, the blackheaded pasture cockchafer appears to be particularly active this season

Where have they been reported?

High populations of blackheaded pasture cockchafer (Acrossidius tasmaniae) larvae have been identified in an established lucerne stand in the NSW Riverina area of Walla Walla. The larvae were found with high populations of adult African black beetle, (Heteronychus arator) which are together thought to have caused severe damage with large patches of pasture destroyed.  According to several growers, severe pasture damage caused by blackheaded pasture cockchafer has also been observed in several areas of the Strathbogie ranges, in Victoria’s North East. This scale of damage has not been seen in many years.

About blackheaded pasture cockchafers

Larvae are white or greyish-white in colour with a hard, dark brown to black head capsule. Compared with other scarab species, their head capsules are distinctively darker and bodies slenderer. They have soft bodies, six legs and tend to curl into a ‘C’-shape when exposed or handled. Fully-grown larvae are 15 to 20 mm in length.

Blackheaded pasture cockchafer (left), the only cockchafer species that comes to the soil surface to feed, leaving small piles of soil excavated from their tunnels (right)(Source: cesar)

 

Blackheaded pasture cockchafer larvae come to the soil surface at night after rainfall or heavy dews, harvest plant foliage (clovers, grasses, cereals and some weeds) and drag it into their tunnels. They feed on the 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. Most feeding damage occurs in May and June, when the rate of pasture growth is slowing down due to the cold weather.

This year’s wet autumn and winter experienced in the NSW Riverina and Victoria’s North East have undoubtedly favoured survival, tunnelling and foraging of blackheaded cockchafer larvae.

Our advice

Blackheaded pasture cockchafer larvae mature and stop feeding (and stay below ground) from mid winter onwards. For best results, insecticide sprays generally need to be applied before the onset of established cold, wet winter conditions in late June and early July. If paddocks receive good rains and pasture is plentiful, blackheaded pasture cockchafers may only constitute a minor problem.

Look for evidence of scarab tunnels (small mounds of soil). Monitor pastures and emerging cereal crops by digging to a depth of 10-20 cm with a spade, counting and repeating about 10 times across the paddock. When densities exceed 150 per m² in pastures, and 100 per m² in cereals, chemical control should be considered.

Several chemicals are registered for control of blackheaded pasture cockchafers.  Chemicals are 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).

Click here for further information on blackheaded pasture cockchafer.

Sources of field reports of blackheaded pasture cockchafers

Rebecca Bingley – Agronomist, Landmark (Riverina NSW)