Underground grassgrubs

District agronomist, Nathan Ferguson (NSW DPI), has reported significant feeding damage to a pasture paddock, near Tumbarumba, in the South West Slopes district of New South Wales. Very large numbers of caterpillars have been found after digging in the soil within the affected parts of the paddock. These have been identified as underground grassgrubs (Oncopera fasciculata), although other very closely related species (e.g. winter corbies) cannot be discounted.

Extensive areas of the paddock have been attacked, with entire plants dead or dying. The caterpillars were more prevalent directly under cocksfoot, which appeared to be the most severely damaged plants. Nathan said up to 20 caterpillars could be found per shovel of soil. There was not as much feeding damage to rye grass. The presence of vertical, silk-lined tunnels were prevalent across the paddock. Nathan says the caterpillars were mostly found within these tunnels between 50 and 200 mm below the soil surface.

Underground grass grub larvae grow up to 65 mm long and have a greyish-green body with distinct darker patches around the body hairs. 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. 

Excess plant growth during summer favours egg laying and the build up of larval numbers. Heavy grazing in late spring or summer will reduce populations as this exposes young caterpillars to desiccation and trampling by stock. There are several insecticides registered in New South Wales, and sprays are most effective if applied in autumn, winter or early spring.

Growers and advisers should begin planning for other caterpillar issues that often occur in late winter and spring. Many species, such as diamondback moth (canola and other brassicaceous crops), armyworms (cereals & pastures) and native budworm (most broadacre crops) will start moving into crops as they mature and temperatures increase. Spend some time learning about when and where these species may occur in your region, and how to monitor effectively. Early detection can greatly assist in selecting the most appropriate control method(s) before economic losses occur.