Foliar terpenoids in Tsuga species and the fecundity of scale insects

The relationship between the reproductive success of two Japanese scale insects, Fiorinia externa Ferris and Nuculaspis tsugae (Marlatt) (Homoptera: Diaspididae) and the concentrations of 15 terpenoids in needles of Tsuga sieboldii, the Japanese host, and T. canadensis, the North American host, was investigated during 1981 and 1982 in a field plot of 8-year-old trees in New Haven, CT, USA. Both scales produced significantly more eggs per female on T. sieboldii than on T. canadensis. Stepwise multiple regression analyses indicated that the variation in fecundity within both scales was strongly associated with variation in the terpenoid profile between tree species.General patterns of phytochemical variation between the two Tsuga species based on differences in the concentration of terpenoids having similar chemical structures were revealed by the multivariate statistical technique, principal components analysis. The volatile leaf oil profile of T. sieboldii was relatively richer in terpene alcohols, while that of T. canadensis was relatively richer in terpene hydrocarbons and terpene acetates. The individual terpenoids were then assigned to one of five groups based on chemical structure and regression analyses were repeated; fecundity of both scales increased with increasing concentration of terpenoid alcohols. Fecundity of F. externa was negatively associated with the relative concentration of acyclic terpenes but the opposite was true for N. tsugae. Analysis of foliar terpenoids may provide a basis for predicting the relative susceptibility of Tsuga species to attack by F. externa and N. tsugae.