Damage to ryegrass plants of different size by various densities of the pruinose scarab (Sericesthis geminata (Coleoptera))

Abstract

The effects of the number of pruinose scarab larvae (Sericesthis geminata) and the size of ryegrass plants on the damage to roots and reduction of foliage yield were examined in a factorial pot experiment. No significant interactions occurred. At each larval density the weight of roots eaten or damaged was linearly related to the weight of roots in the controls. The slight but significant reduction of relative damage to roots and foliage with increasing plant size is therefore thought to result from a greater capacity for regrowth in the plants with larger stembases. The number of larvae had a very highly significant effect on damage to roots, but did not influence foliage yield to a significant degree, the mean reduction of foliage being 36 per cent of the control yield. As in previous experiments the ryegrass plants appeared' to lose half their roots before foliage yield was affected. The mean damage to roots was 83 per cent of the controls.