ASSESSMENT OF GRASS GRUB (COSTELYTRA-ZEALANDICA (WHITE)) (COLEOPTERA,SCARABAEIDAE) DAMAGE IN RYEGRASS WHITE-CLOVER PASTURE IN SOUTHLAND

The effects of grass grub density on pasture quality and quantity unde r sheep grazing were examined at three sites near Riversdale, central Southland, New Zealand. Using insecticide for 2-3 years enabled pastur es with low levels of grass grub and those with natural levels to be c ompared. Grass grub reduced pasture production in autumn, winter, and spring by 6.4, 8.0, and 4.4% per 100 larvae/m2 in April-May, respectiv ely. Although grass grub caused occasional reductions in the proportio n of ryegrass (up to 20% of sward) and white clover (up to 7% of sward ), during spring-summer, the long-term effect on pasture composition w as minor. This information can be used to predict the value of pasture lost and the economics of adopting a particular control option.