Development of resistant pasture and methods of pasture management for grass grub (Costelytra zealandica(White)) control

Abstract

Abstract In the field, larval resistance in Holcus lanatus and Phalaris tuberosa was identified and resistance in Lotus pedunculatus, Medicago sativa, and Poa pratensis confirmed. Trifoliate legumes were extremely favourable host plants for larvae in comparison with grasses, and larval populations particularly during the early larval stages were sensitive to differences in the legume: grass ratio of pastures. For the range of plants and cultivars studied in the laboratory, larvae were able to differentiate between legume and grass roots, but could not distinguish between roots of legumes which are resistant and susceptible host plants. The performance of larvae under laboratory and field conditions, in terms of growth and density under well-balanced grass-legume mixtures closely paralleled that under the pure legume. The addition of L. pedunculatus to Trifolium repens markedly decreased larval performance. Larval weight-density relationships were studied and enabled calculation of critical weights of larvae of different ages required to ensure population survival. The feasibility of utilising resistant pastures was assessed; the concept of grass grub control by manipulating the botanical composition of existing pastures is proposed.