POPULATION STUDIES OF GRASS GRUB (Costelytra zealandica) AND BLACK BEETLE (Heteronychus arator) (COLEOPTERA: SCARABAEIDAE)

Population studies of grass grub and black beetle have revealed simple patterns of population determination. Populations fluctuate in response to variations in soil moisture and temperature. The upper limit to population growth is set by density dependent factors (larval mortality of grass grub and variation in natality of black beetle). Parasites and predators appear to be generally unimportant, but disease organisms may be potentially useful in the management of grass grub and black beetle populations. Black beetle outbreaks occur sporadically following warm springs whereas grass grub tends to be a more persistent problem. In areas such as the Waikato and Bay of Plenty where both species can occur as pasture pests, years in which the weather is favourable for black beetle population growth are unfavourable for grass grub. Population models have been developed which accurately describe the general patterns of grass grub and black beetle population change and can account for the observed effects of varying the application time of transient soil insecticides.