Lucerne root saponins a feeding deterrent for the grass grub, Costelytra zealandica (Coleoptera: Scarabaeidae)

Abstract

A crude extract of lucerne root (Medicago sativa ’Wairau') was found to contain a strong feeding deterrent for 3rd‐instar Costelytra zealandica (White) larvae when tested at fresh‐root concentration. Purified saponins obtained from such an extract by standard procedures markedly reduced feeding, and had an ED₅₀ of 0.019%. A commercial preparation of naturally derived saponins was also an effective feeding deterrent, with an ED50 of 0.18%. Considerable variation was found in the saponin levels present in the root of 13 cultivars of lucerne. A role for saponins in the resistance of lucerne to grass grub attack seems probable, but the mechanism of that role remains uncertain.