Effects of grass grub (Costelytra zealandica white) larvae on pasture plants

Abstract

Perennial ryegrass Lolium perenne L.) was established in a pot and field trial with and without mineral fertiliser and 60 third-instar grass grubs (Costelytra zealandica White) per 30 cm², for 86 and 93 days respectively. Fertilisers stimulated shoot growth by 30% in the pot trial and 90% in the field trial. In both trials 60 grass grubs per 30 cm² induced a 10% depression in total shoot growth. Smaller shoot yields in treatments with grass grubs were associated with slightly lower levels of P, K, and N in the herbage. Root yields in pots were depressed by grass grubs at both levels of fertility, but in the field this depression occurred only in the fertilised treatment. There were no treatment differences in grass grub survival.