Microbes, models and management of the New Zealand grass grub, Costelytra zealandica

[First paragraph]
Improved grasslands pose particular problems in pest management. The areas are extensive and the returns per hectare from grassland agriculture are much less than those from intensive cropping or horticulture, but pastures are usually sown to last for a number of years and have a much more stable ecology than in cropping or horticulture. For these reasons, biological controls and plant resistance have long been the preferred options for managing pasture pests. Within this context, the role of diseases in pasture pest population dynamics has received increasing attention, especially their ability to control pest outbreaks. Diseases are common within our major pasture pest species but their role in population regulation is often difficult to define. One of New Zealand’s major pasture pest species, the grass grub Costelytra zealandica, is widespread and often damaging throughout the country. The insect is an endemic species that has adapted to an introduced pasture system, dominated by exotic plant species (Lolium perene/Trifolium repens), where it can reach populations ten-fold higher than in its native habitat. Such high densities favour disease transmission and it is not surprising that a wide array of pathogens have been recorded from this insect (Glare et al., 1993). But the insect can still be highly damaging and can cause total loss of sown species within 3–4 years from sowing in grass grub prone regions. Probably the most important of these diseases, found throughout New Zealand, is amber disease caused by strains of the bacteria Serratia entomophila and S. proteamaculans. This is an unusual disease, controlled by a bacterial plasmid which has only been found in New Zealand bacterial isolates (Jackson et al., 2001).