Two of New Zealand's most important insect pests, grass grub and porina, are endemic species which have successfully colonised improved pastures. Population densities of these insects within this new environment are far greater than in the native plant systems in which they evolved. Within these high populations diseases have flourished, and high numbers of diseases are recorded from each of these pests. These include bacteria, fungi, nematodes, viruses and protozoa.
Simple grazing models with two herbivores are used to assess the effects of pasture pests on stability and productivity of continously-growing pastures. Algebraic and graphical methods are also presented for estimating losses from pasture pests at different stocking rates directly, from data on productivity/stocking rate relationships. Pests are considered as competing grazing herbivores and denuders of pasture area. Denuding pests have no effect on stability but grazing pests increase the likelihood of discontinuous stability.