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

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 rootfeeding scarab larvae on the West Coast initially attributed to grass grub was found to be caused by manuka beetles Pyronota spp Winter surveys during 20082012 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 phenolbaited pheromone traps was established around this epicenter during the grass grub flight season with traps spaced at approximately 05 km intervals Traps within the identified zone of grass grub infestation caught 15 beetles per night Single male beetles were trapped up to 75 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 newlyflipped land of Cape Foulwind may also be suitable for grass grub and should be regularly inspected to anticipate and prevent outbreaks