Secondary metabolites as stimulants and antifeedants of Salix integra for the leaf beetle Plagiodera versicolora.

Plagiodera versicolora, a willow beetle living on S. sachalinensis, is found on S. integra during early June in Hokkaido Island, Japan. This insect selects several species of willows (Salix), including S. integra as host plant in Honshu Island of Japan. To determine the reasons for the limited distribution of this beetle on the willows of Hokkaido, the feeding preference of the insect to leaves of S. integra and its constituents was performed. Feeding-bioassay guided fractionation of an 80% aqueous acetone extract of fresh leaves of Salix integra to Plagiodera versicolora resulted in isolation of feeding stimulant and antifeeding constituents. Chlorogenic acid (1) and 3,5-dicaffeoyl quinic acid (2) were identified as antifeedants and 1,2-di[(9Z,12Z,15Z)-octadeca-9,12,15-trienoyl]-3-beta-D-galactopyranosyl-sn-glycerol (MGDG, 3) as feeding stimulants. The feeding test was performed by an agar disk method. The treated agar disks contained sucrose and test sample in different doses. The antifeeding activities of 1 and 2 and stimulant activity of 3 may be one of the reasons for the limited presence of P. versicolora on S. integra in Hokkaido.