Cockchafers

Cockchafer larvae are active in the NSW South West Slopes and have begun causing crop loss.

 

Where have they been reported?

Cockchafers have been reported from several paddocks north east of Junee and east of Wagga Wagga, in the South West Slopes district of NSW. It appears that both redheaded pasture cockchafers and yellowheaded cockchafers are present. High densities have been observed across oat, triticale and oat/vetch crops. Substantial plant loss has recently occurred in one paddock sown to oats and vetch. In this paddock, it is estimated that 30% of the paddock is impacted, with most vetch plants completely missing, and oat seedlings also suffering from cockchafer attack.

Cockchafer habits and behaviour

Redheaded pasture cockchafers (Adoryphorus coulonii) and yellowheaded cockchafers (Sericesthis spp.) are primarily root feeders and most damaging to shallow-rooted plants. Notably, they do not come to the surface to feed like some other scarab beetles. Redheaded pasture cockchafers appear to be problematic in areas where the annual rainfall exceeds about 500mm. They tend to be more prolific on lighter sandy loam soils, attacking pastures and cereals. Yellowheaded cockchafers are widespread pests of cereals, and will occasionally cause damage to pastures.

In autumn, increased soil moisture stimulates cockchafer larvae to move closer to the soil surface to feed on plant roots or decaying organic matter. Larvae prune or completely sever roots, with damaged plants sometimes dying or showing signs of reduced growth. In severe cases, pasture becomes patchy and can be rolled back like a carpet. In cereals, damage may result in bare patches and require re-sowing. Damage is typically most serious from March to June. Low soil temperatures over the winter period slow down feeding activity, and many larvae begin to pupate.

Our advice

Growers are advised to monitor susceptible paddocks prior to sowing and throughout winter. Cereal crops sown into long term pasture paddocks are most vulnerable to attack. 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 by flying, usually at dusk on warm evenings in late spring-early summer. Inspect paddocks by digging to a depth of 10-20 cm with a spade and counting the number of larvae present. This should be repeated 10-20 times to get an estimate of larval numbers.

Control options

Control of yellowheaded and redheaded pasture cockchafers is complicated. Insecticides are largely ineffective because of their subterranean feeding habits, and there are currently no synthetic insecticides registered. Effective management strategies can be implemented in autumn. Cultivating affected areas prior to sowing can help reduce pest populations. Re-sowing bare areas using a higher seeding rate will assist plant establishment. Another option is to sow less-susceptible plants (e.g. pulse crops) if practical.

If damage is noticed in pastures in late autumn/early winter, stock should be removed, particularly from ryegrass dominant pastures, and the paddock spelled until late winter. This will help prevent pasture grasses being uprooted by grazing animals and maintain maximum leaf area needed to re-establish root growth.

Description

Cockchafer grubs are ‘C’ shaped and creamy-grey in colour. When fully grown yellowheaded cockchafers are about 25-30 mm long and have a yellow head capsule. Redheaded pasture cockchafers are a similar size and have a red-brown coloured head capsule. Redheaded pasture cockchafer 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. Yellowheaded cockchafers grubs live in the soil until mid-to-late summer, where they emerge as yellow-reddish beetles about 10-15 mm in length.

 

* Source of field reports of cockchafers

Andrew Reardon - Agronomist, JJS Glass & Co (NSW South West Slopes)

Glen Smith - Consultant, 3D-Ag (NSW South West Slopes)