Relationships between grazing animals and populations of the pasture insects Costelytra zealandica (White) and Inopus rubriceps (Macquart)

Abstract

Populations of grass grub (Costelytra zealandica (White)) and soldier fly (Inopus rubriceps (Macquart)) were studied in trials in which a range of stocking rate treatments were imposed. The stocking rates ranged from 8.9 to 53.1 dry ewes/ha in one trial and from 9.4 to 32.6 stock units/ha in another. Increasing a low stocking rate (e.g., from 8.9 to 22 dry ewes/ha) greatly reduced the grass grub population, but a similar stocking rate increase at a high stocking rate had little further effect upon the insect population. A similar effect of stocking rate on soldier fly populations was found. In the trial affected by grass grub, live-weight gam/ha and/dry sheep, and wool weight/ha, were highest at stocking rates of 30 dry ewes/ha or more. At such rates the insect population was reduced to a low level. For both ecological and productive reasons the manipulation of stocking rates per se through grazing management at critical times in the life cycle of these pasture insects, merits study as a means of their control.