Pasture Pests--Are They the Real Problem?

The New Zealand grass grub, Costelytra zealandica (White)(Coleoptera: Scarabaeidae) and species of the porina complex, Wiseana spp. (Lepidoptera: Hepialidae), (hereafter referred to as ‘porina’) are endemic New Zealand insects whose larvae have a long history as significant, intransigent, agricultural pests. Both affect pasture production and plant composition in most regions of the country. Grass grubs are root feeders while porina caterpillars, although dwelling in permanent subterranean burrows, emerge at night to feed on above ground plant foliage. Both find rye-grass and white clover, the basis of most New Zealand pastures, very favourable food plants. The life histories and larval development of both are well understood and the onset of damage caused by both insects is related to development. Pasture damage as a result of their feeding is generally first noticed by farmers in late autumn and becomes more severe through winter as their body sizes increase and plant growth slows. Damage mitigation has historically centred on chemical insecticides, particularly organochlorine insecticides in the 1950s and later of organophosphate insecticides, and resulted in the entrenchment of palliative applications of insecticide when damage by either of these pests was ob-served as a consequence of apparent effectiveness, ease of application and, initially at least, low cost. This entrenchment is still present in contemporary farming