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Cockchafers and other Scarab grubs

High numbers of redheaded pasture cockchafer grubs (Adoryphorus coulonii) have been found causing significant damage to several pasture paddocks. Research consultant, Samantha Strano (cesar), says up to 20 large sized grubs were found per shovel in several paddocks near Drouin, in the West Gippsland district of Victoria. Damage was found in varying sized patches across the paddock, where the pasture was either dead or dying, and could be easily pulled up out of the ground.

Redheaded pasture cockchafer grubs are ‘C’ shaped, have a red-brown head capsule and grow up to 30 mm in length. They are primarily root feeders and most damaging to shallow-rooted plants such as ryegrasses, sub-clovers and barley grass. Adults are stout, shiny black beetles and are approximately 15 mm long. Redheaded pasture cockchafers differ from other scarabs; they have a two-year lifecycle. In some instances this may mean damage only occurs every second year, however overlapping populations are often present meaning damage can still occur every year.

Agronomist, David Strahorn (Furney’s CRT), has reported finding high numbers of grubs in a perennial pasture paddock south of Dubbo, in the Central West Slopes and Plains district of New South Wales. Although unconfirmed, these appear to be Argentinian scarabs (Cyclocephala signaticollis). David says he found 6-8 grubs per square foot, and there were several large, heavily damaged areas within the paddock.

The Argentinian scarab is an important pest of turf and some horticultural crops, but is not usually a problem in broad-acre crops or pastures. The larvae have a yellowish-brown head and are very similar in appearance to yellowheaded cockchafers. The larvae are likely to be most problematic from April-June. The Argentinian scarab was probably introduced in the 1940’s near Sydney, and although their current distribution is unknown, they have not previously been recorded outside of New South Wales.

Activity of scarab grubs and damage to pasture paddocks has also been reported by district agronomist, Jo Powells (I&I NSW). Jo reports finding yellowheaded cockchafer grubs (Sericesthis spp.) in a pasture paddock near Goulburn, in the Southern Tablelands district of New South Wales. Yellowheaded cockchafer larvae are ‘C’ shaped with a yellowish head capsule and grow up to 25-30 mm long. They mostly attack cereals, but will also cause economic damage to pastures. The larvae are generally present from autumn through to spring. Adults are yellow-reddish beetles that are 10-15 mm long. 

Control of scarab grubs is difficult as insecticides are largely ineffective due to their soil dwelling habits. The exception is blackheaded pasture cockchafers (Acrossidius tasmaniae), which do come to the surface to feed and can be controlled with some chemicals. There are reports that soil dwelling scarabs can be controlled with high rates of insecticides applied prior to irrigation or rain events, which causes the chemicals to leach into the soil. This approach is extremely problematic and is only ever likely to control a proportion of the population.

Rather than potentially wasting chemicals, control efforts should be focused towards other management practices. Increasing seeding rates is a useful option in paddocks where scarab grubs are anticipated to cause damage. If damage has already occurred, re-sowing bare areas using a higher seeding rate is an effective strategy. Cultivating affected areas prior to sowing a crop will help reduce pest populations as it exposes the grubs to predation by predatory invertebrates and birds. If faced with continual problems, growers should consider sowing pasture species such as phalaris, cocksfoot, tall fescue, lucerne or less palatable crops such as oats.

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