Effects of pasture defoliation in summer on grass grub (Costelytra zelandica) populations

Abstract

Life table studies of grass grub populations at Rukuhia in plots set-stocked with sheep under hard grazing (pasture height maintained at 2–4 cm) and lax grazing (average pasture height 15–20 cm) revealed significantly higher summer (P <0.01) and total generation (P <0.05) mortalities and a markedly greater population decline in the former treatment. This difference appeared to be caused mainly by the effects of summer pasture cover on soil temperature. These studies and mowing experiments on Horotiu sandy loam and Taupo ash soils showed that the reduction of summer pasture cover to a height of 5 cm (approximately 1500–2000 kg/ha DM) or less can result in a 50–80% reduction in autumn grass grub populations, compared with more laxly defoliated or uncut pasture. These higher summer mortalities under short pasture occurred when defoliation coincided with periods of 4 weeks or more when soil moisture was low enough to suppress pasture regrowth and to allow soil temperature to reach levels lethal to second- and third-instar grass grubs (32.5–35.0°C). Irrigation, shading, and summer rainfall all prevented soil temperature from reaching the lethal range, and resulted in summer defoliation having little or no effect on grass grub numbers. Possible causal factors and practical implications of these observations are discussed.

Keywords:

• Coleoptera
• Scarabaeidae
• Costelytra zealandica
• cultural control
• pasture defoliation
• grazing management
• population trends