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The main stages of the grass grub's life cycle available to starlings are third-instal' larvae (autumn and winter) and adults (spring). Starlings were observed to concentrate their probing within localised areas of high grass grub density during autumn and winter, but the birds appeared to probe randomly for grubs within these areas. Average starling numbers were 1711.5-2915.4 starling-hours per ha per day during autumn and winter in 0.25-to 0.8-ha plots in an area of irrigated pastureland at Winchmore containing an isolated grass grub infestation near a large starling roost. Exclusion studies in this area revealed that starlings inflicted mortalities of 40-60% on medium (400-600 per m-) and high C> 1000 per m 2) third-instal' grass grub populations between March and July and that this mortality prevented medium grass grub populations from increasing. Observational studies at Winchmore showed that starling feeding effort in 0.25-to 0.40-ha areas increased markedly when local grass grub density exceeded a threshold of approximately 150-200 per m 2 in the top 2.5-3.0 em of turf (a total population density of about 300-380 per m 2) • In three other study areas, which were more typical of Canterbury pastureland, average starling numbers during autumn and winter ranged from zero to 152.1 starling-hours per ha per day and exclusion of starlings did not significantly influence grass grub mortality. Starling predation on grass grub adults apparently has a negligible effect on grass grub populations. In the area of high predation at Winchmore, starling predation caused strongly density dependent mortality of grass grub populations over the range of densities from approximately 250-750 third-instal' larvae per m>. This predation, in combination with irrigation, high stocking rates, and grazing management which encouraged starlings to feed in infested paddocks, gave effective biological control of grass grub by preventing the population density from increasing above a level which caused moderate pasture damage in autumn but did not affect spring pasture production. Biological control of grass grubs by starlings may be possible only in localised areas with very high starling numbers.

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