Grass grub ( <i>Costelytra zealandica</i> ) population collapse in the northern North Island

Abstract

Abstract Annual surveys of grass grub populations in early autumn from 1979 to 1982 at Tihoi near Lake Taupo and at Waingaro in the western Waikato have revealed that populations in old pastures (at least 7 years old) increased to a peak by 1979 or 1980 and subsequently declined naturally to low levels. Populations in newly sown pastures increased to a peak in the second or third autumn after sowing and then declined. Application of lindane at Tihoi in 1979 reduced grass grub numbers by an average of 840/0 in the year after treatment, but 3 of the 5 treated populations have since increased to higher levels than the untreated. A life table study of the 1981‐82 grass grub generation at Tihoi showed that variation in summer mortality was responsible for differences in population trend. Weather, pasture composition, gcazing management, depletion of the food supply, and predation did not appear to be major causes of summer mortality. This mortality was not significantly related to population density on a continuous scale but was much lower in increasing populations than in populations which had declined for 1‐3 generations after reaching a peak.