Yellowheaded cockchafers & underground grass grubs

Agronomist, Mick Duncan (Northern Agriculture), has reported yellowheaded cockchafers and underground grass grubs damaging an improved pasture paddock near Guyra, in the Northern Tablelands district of New South Wales. The cockchafers (likely to be Sericesthis spp.) were found to be chewing the plant roots, whereas the underground grass grubs (Oncopera fasciculata) were found feeding above the surface on the tall fescue and rye grass plants. Mick estimates the density of cockchafers to be >10 per m², in the top 10cm of soil. 

Yellowheaded pasture cockchafer larvae are “C” shaped, creamy-grey in colour and have a yellow head capsule. When fully grown they are about 25-30 mm long. The grubs live in the soil until mid-to-late summer, where they emerge as yellow-reddish beetles about 10-15 mm in length. There are no insecticides that provide effective control of yellowheaded cockchafers due to their subterranean feeding habits. Cultural control methods including pasture renovation, re-sowing bare areas using a higher seeding rate, and cultivating affected areas prior to sowing may be the best methods for control. Mick says the affected area is likely to be re-sown in order to accelerate re-vegetation.

Click here for images of yellowheaded cockchafers.

Underground grass grub larvae grow up to 65 mm long and have a greyish-green body with distinct darker patches around the setae. Adult moths are a mottled grey-brown colour and have a wingspan of 35-45 mm. Eggs are laid in long grass and larvae remain in the litter until autumn rainfall when they construct tunnels into the soil. Damage occurs when they come to the surface to feed, generally at night. Underground grass grubs feed on a variety of pastures and clovers, and some weeds.