SAMPLING STATISTICS AND POPULATION DYNAMICS OF GREYBACK CANEGRUBS IN SUGARCANE FIELDS: A CASE STUDY

DENSITIES OF greyback canegrubs were recorded over five consecutive years (2003–2007) in three regions of North Queensland (Mulgrave, Innisfail-Tully and Herbert). Data were collected by digging up 20 cane plants per field during March–April each year. Grubs were transferred to the laboratory and kept under constant temperature until they either emerged as beetles or died in the larval or the pupal stage. Results showed that grub densities were very low in all districts when the study commenced in 2003 and remained constant for the three following years in the Herbert but increased in the other two regions. Grub numbers eventually declined in the Mulgrave and Innisfail–Tully in 2006 and this was associated with an increase in insecticide application in those two districts. Variable grub mortality levels were recorded due to Adelina, Metarhizium anisopliae and milky diseases, with Adelina being the most abundant in the three regions. Other factors responsible for significant mortality in the laboratory remain unknown and more work is required to identify them and quantify their impact on grub population dynamics. We used monitoring results to determine the optimum sample size required for a precise estimation of the mean and this information will be used to improve future monitoring work.