Starling (Sturnus vulgarisL.) predation on grass grub (Costelytra zealandica(White), Melolonthinae) populations in Canterbury

Abstract

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.

Predation by the rook,Corvus frugilegusL.,on larvae of the grass grub,Costelytra zealandica(White),in Hawke’s Bay, New Zealand

“…The third assumption (that the vertical distribution of the grubs did not change between early and late April) is supported by the observations of East & Pottinger (1975), who found in 2 successive years at Winchmore that the proportion of grubs in the top 2.5 ern was stable from late March to early June. Our own samples from Field I showed that the grubs' vertical distribution did not change between late April and late May ( Table 2).…”

“…1), some 15-20% less than that recorded in irrigated pastures in Canterbury (East & Pottinger 1975). The vertical distribution of the grubs was not influenced by density (Fig.…”

“…3) and reduced numbers in these areas from high (7oo-9OO/m') to intermediate (420-6oo/m') levels. East & Pottinger (1975) found at Winchmore that 400-600 third instar larvae per m' reduced dry matter production from autumn pasture by about 50%. Where there was no starling (Sturn us vulgaris) predation, grub populations normally increased by 1.5 to 2.0 times in the next generation.…”

“…Rooks eat the grubs in winter (Porter 1979). Some farmers believe that they help to control the pest, as starlings do in specific areas in Canterbury (East & Pottinger 1975). Our aim was to test this belief.…”

Incidence and transmission of a disease of grass grub(Costelytra zealandica)in Canterbury

“…The first, an annual seasonal decline of aggregate numbers (Kain 1975), has been attributed to a combination of regulatory mechanisms including milky disease, rickettsia and other unspecified diseases (e. g. , Fowler 1974), protozoan infection (Miln et al 1979), predation (East 1972), and combat mortality (Kain 1975).…”