The grass grub (Costelytra zealandica) is a major pest of Canterbury pastures. Grass grub numbers are low in young pastures and then commonly rise to a peak 4-6 years from sowing, before declining. Grass grub numbers in older pastures fluctuate but rarely reach the same levels as the early peak. Biological control agents such as bird and invertebrate predators, parasites and diseases cause mortality in grass grub populations; the effect of predators and parasites is limited. Pathogens are common in grass grub populations. Amber disease, caused by the bacteria Serratia spp., was the disease most frequently found in population surveys in Canterbury, while milky disease (Bacillus popilliae) and protozoan diseases were less common. The level of amber disease was reduced in pastures where insecticide had been applied. Inundative applications of entomophagous nematodes and the bacterium Serratia entomophila have been tested against grass grub. A bacterial productbased on S. entomophila is now being marketed for grass grub control. Keywords grass grub, biological control, pathogens
