Genetic differences in susceptibility of Japanese chestnut cultivars to infestation by chestnut weevil (Cuculio sikkimensis)

Chestnut weevil (Cuculio sp.) is one of the most nefarious insect pests, adversely affecting chestnut cultivation. Despite the economic importance of chestnut, genetic differences of susceptibility to weevil infestation among cultivars has not been clarified. We estimated the environmental variance for the infestation rate of chestnut weevil (IRCW) using seven cultivars/selections with two single-tree replicates for 11 years. Analysis of variance using root-squared-transformed values of the IRCW revealed significant effects of genotype, year, the genotype × year interactions, and tree within genotype. Year variance consisted of 66% of the total variance, while genetic variance and genotype × year interaction account for a certain percentage (11.9% and 12.9%, respectively). Linear regression analysis of the IRCW and nut harvesting date (NHD) revealed their close association. Because NHD is a high heritability trait, the genetic effect of IRCW would be mostly explained by the difference in NHD among cultivars. Interestingly, ‘Riheiguri,’ a Japanese–Chinese hybrid cultivar, has a lower IRCW than others even after adjustment for the effect of NHD. A 2 years evaluation of IRCW with five single-tree replicates confirmed that the IRCW of ‘Riheiguri’ was lower than ‘Tsukuba,’ a Japanese cultivar with almost the same NHD. Our results demonstrated that the year variance was the largest variance component of the IRCW, whereas the genetic difference was closely associated with the difference in NHD, but was also related to factor(s) other than NHD. Elucidating this unknown factor may open the possibility of breeding tolerant cultivars for the chestnut weevil.