Towards Greener Pastures - Pathogens and Pasture Pests

In the past two centuries the New Zealand environment has changed dramatically under the impact of European colonisation and the establishment of intensive agricultural systems. This massive environmental shift has led to major changes in the abundance of our native fauna. Many previously abundant species have been reduced to low numbers or even extinction, but a few endemic species have managed to bridge the gap between the old and new environments and succeed in the new agricultural conditions. The New Zealand grass grub, Costelytra zealandica White, (Coleoptera: Scarabaeidae) and porina (Wiseana spp., Lepidoptera: Hepialidae) have been particularly successful in colonising the new ryegrass/clover grasslands which dominate our current agricultural systems. The grass grub is the only one of more than 100 endemic scarabs (Given, 1952) that has extensively colonised new habitats and become a pest of grasslands in nearly all parts of New Zealand. While several species of Wiseana can reach high numbers in pasture there are also a number of non-pest species (Dugdale, 1994). In improved pastures both species reach densities unknown in the native habitat. Grass grub populations frequently reach more than 500 larvae/ m in improved grasslands, while in the native tussock densities are usually no more than one tenth of this number (e.g., Merton, 1980). Similar extremes occur with porina populations. Insect populations are frequently regulated by density-dependent or delayed-density-dependent T.C. BOURNER, T.R. GLARE, M. O’CALLAGHAN and T.A. JACKSON