Population dynamics of sugarcane soldier fly in the Mackay region

Populations of soldier fly, potential predators, other soil fauna and various abiotic factors were studied in eleven canefields and 2 grasslands during 1998A-2001, corresponding to the plant crop to young third ratoon of the canefield sites. An additional canefield with a heavy soldier fly infestation was studied in 2001. Numbers of soldier fly larvae were higher in plant crops of sugarcane that had been fallowed and planted by June compared with fields planted later after a long break. Trash blanketing seemed to favour higher earthworm numbers in soil cores and was associated with increased numbers of some predatory groups such as staphylinid beetles in soil cores or pitfall traps. However, results from pitfall trapping must be interpreted with caution, as catches are influenced not only by absolute numbers of the faunal groups but also by surface activity, which may change depending on trash cover. No relationship was seen between these other fauna and soldier fly population dynamics. Soldier fly remained at sub-economic levels in most of the fields studied. Disappearance of eggs and early-stage larvae, that is, the difference between the number of larvae expected to be present in spring as calculated from empty pupal cases (an indicator of adult soldier fly emergence) and the number of larvae actually measured, was the key mortality that determined the change in soldier fly numbers between generations. Key factors that influenced population dynamics presumably acted during this period. However, the actual factors responsible for regulating numbers were not identified.