Evidence for density-stabilising effects influencing the population dynamics of greyback canegrub in northern Queensland

The failure of suSCon® Blue on a number of farms in the Burdekin has led many farmers to examine adult control as a means of reducing populations of grey back canegrubs. Despite directing considerable effort towards controlling adults, the area of cane damaged by canegrubs has continued to increase. Three experiments were undertaken to examine why adult control has failed to reduce the area damaged by canegrubs. The experiments examined the relationship between adult and larval populations and the effect of grub density and plant size on the establishment of neonate larvae. Established larval populations in fields in the Herbert, Tully, Innisfail and Burdekin areas were relatively constant at between 2.5 and 4.2 grubs per stool and showed no relationship with the beetle populations in the vicinity of the fields. This was despite total beetle catches ranging between 69 and 92000 across the eight sites studied. Mortality of neonate larvae was affected by density dependent mortality factors. Mortality was highest under small plants and plants with large initial larval populations. These findings suggest that larval populations are regulated by density stabilising mechanisms, which limit the size of established larval populations. High larval population densities are able to establish from relatively low beetle populations, suggesting that beetle mortality is largely replaceable mortality. This accounts for the apparent failure of adult control strategies to reduce canegrub damage and emphasises the importance of targeting the larval stage as opposed to the adults for this species. High mortality of larvae induced by disease leads to lower numbers of beetles in the Herbert, Tully and Innisfail areas, compared to the Burdekin district.