Levels of isoflavonoids as indicators of resistance of subterranean clover trifoliates to redlegged earth mite Halotydeus destructor

The extracts, obtained with methanol from crushed trifoliates and from the surface layer of the trifoliates, contained isoflavones in both the free and bound forms as the major components, compounds known to exhibit feeding deterrence towards the redlegged earth mite. The levels of these compounds were quantified and showed considerable variation in extracts of one susceptible variety and four varieties with differing levels of resistance. The relative resistance of the five varieties established from field observations was compared with the amounts of isoflavone derivatives obtained in the two extracts. A better ranking of resistance was found with levels of isoflavones on the leaf surface than with the total amount of isoflavones in the trifoliates, These results suggest a simple method for determining the relative resistance of subterranean clover to the mite.