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New Zealand Plant Protection 66 (2013): 376

Grass grub distribution on the upper West Coast defined by soil sampling and pheromone trapping

R.J. Townsend, J.E. Dunbar, and T.A. Jackson

ABSTRACT

The New Zealand grass grub (Costelytra zealandica) is distributed throughout the South Island but, surprisingly, has not been recorded west of Reefton. In 2006, pasture damage from root-feeding scarab larvae on the West Coast, initially attributed to grass grub, was found to be caused by manuka beetles, Pyronota spp. Winter surveys during 2008-2012 between Karamea and Hokitika confirmed that most damage patches were caused by manuka beetle larvae, but there was a small, localised population of C. zealandica associated with Westport airport and golf course. In 2012 a network of phenol-baited pheromone traps was established around this epicenter during the grass grub flight season, with traps spaced at approximately 0.5 km intervals. Traps within the identified zone of grass grub infestation caught 1-5 beetles per night. Single male beetles were trapped up to 7.5 km from the epicenter but with no evidence of established populations from larval sampling. It is likely that the localised grass grub population became established after an accidental introduction of insects with soil or plant material to the modified, and drained, airport and golf course environments and may act as an infestation source for other areas. Pastures on the nearby, newly-flipped land of Cape Foulwind may also be suitable for grass grub and should be regularly inspected to anticipate and prevent outbreaks.

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