Blackheaded pasture cockchafer (Acrossidius tasmaniae) impacts seedling cereals and pastures in Tasmania, Victoria, South Australia and New South Wales.
Insects and damage
Larvae live in the soil, are “C” shaped and white with a black head capsule. Adults beetles are most visible in summer, particularly after rain and may be attracted to lights.
Larvae feed at night, on seedlings and may drag severed leaves into their tunnels for consumption during the day.
Bare patches appear in pastures (particularly improved pastures) from mid-autumn to late winter. Heavily infested areas may feel spongy underfoot.
Small mounds of dirt surrounding holes on the soil surface are often the first sign of activity.
Birds foraging in the soil after cultivation are often an indication of larvae presence.
Inspect susceptible paddocks prior to sowing, or when signs of damage are evident.
Use a spade to dig an area the width of the spade and to a depth of 10-20 cm. Repeat 10-20 times at different locations across the paddock. Count the number of larvae present in each sample and average to get an estimate of larval density across the paddock. Adjust the density estimate to a square metre equivalent to compare with the threshold.
If defoliated, wilted, or dead seedlings are present, check to see if roots are damaged.
Natural enemies include birds, parasitic flies, predatory insects and diseases, but biological control generally does not prevent crop loss when larval populations are high.
Females are attracted to sparse, very short pastures (2-3 cm) caused by heavy grazing or hay cutting. Good pasture cover in summer (at least 5 cm height), may reduce the risk of females laying eggs.
Cultivation damages grubs and exposes them to predators and birds.
If densities exceed 30 per m², control is warranted. An average infestation of 30-40 larvae per m² can cause a 50-70% reduction in winter pasture production and a 40-50% loss of desirable pasture species production in spring.
Thresholds for winter cereals are the same as those suggested for pasture, 30 larvae per m².
Blackheaded cockchafers can be controlled by insecticides as they are surface feeders, unlike some of the other species that feed solely on plant roots.
Control of larvae after seedling emergence is more effective than prior to sowing. Insecticide is applied to seedling foliage, which is ingested by the cockchafer larva when it emerges to feed on the foliage.
Applying insecticides to control mature grubs is rarely successful. Larvae may feed beyond July/August if the winter is mild and the soil is warmer or drier than normal.
Cockchafers do not thrive in very sandy or very heavy clay soils and numbers are greatly reduced in moisture saturated soils. Eggs and larvae desiccate in dry conditions and many also die in very wet conditions. They are more frequently a pest in areas where the annual rainfall exceeds 480 mm.
Cockchafers will not inhabit just one farm but more likely to occur in regions – area wide management can contribute to information transfer between neighbours and more effective control strategies Industry publications provide up-to-date information about regional pest issues in field crops