Role of foliar phenolics in host plant resistance of Malus taxa to adult Japanese beetles

Japanese beetles (JB),Popillia japonica Newman, are destructive, highly polyphagous herbivores that show a general preference for Rosaceous plants. Choice and no-choice feeding assays were conducted to determine the level of resistance among 10 taxa of Malus spp. Mill. Under no-choice conditions, M. baccata (L.) Borkh. 'Jackii', M. x 'Hargozam' Harvest Gold and M. transitoria (Balatin) Schneider 'Schmitcutleaf' Golden Raindrops were highly resistant, with <2 cm(2) leaf area consumed in 24 hours. M. x 'Radiant' was highly susceptible, with 7.6 cm(2) consumed, and the remaining six cultivars were intermediate, Under choice conditions, eight tars were resistant with <10% defoliation, M. x 'Red Splendor' was intermediate with 26%, and M. x 'Radiant' was susceptible with 73% defoliation. Feeding responses to eight individual phenolics were tested in artificial diets over a range from 0 to 100 mM. Phloridzin, phloretin, naringenin, and catechin were sill feeding deterrents; whereas quercetin and rutin were feeding stimulants. Chlorogenic acid stimulated feeding at low concentrations and deterred feeding at higher concentrations (i.e., a peak response). Kaempferol had no effect. Analysis of endogenous foliar phenolics showed considerable variation in concentrations among taxa. Stepwise multiple regression analysis identified phloridzin as the only endogenous phenolic that was significantly related to resistance under both choice and no-choice feeding conditions.