Evaluating possible intraspecific variation in resistance of western hemlock (Pinaceae) to Adelges tsugae (Hemiptera: Adelgidae)

Western hemlock, Tsuga heterophylla (Rafinesque) Sargent (Pinaceae), trees growing in a clonal, commercial seed orchard on Whidbey Island, Washington, United States of America were assessed for Adelges tsugae (Annand) (Hemiptera: Adelgidae) infestation levels to determine whether there was any evidence of resistance to this herbivore. Join-count statistics revealed that A. tsugae infested trees were not clumped, but instead were randomly distributed throughout the seed orchard. Chi-square analyses suggested that there is a genetic basis for different levels of A. tsugae infestation among the genets represented in the orchard. However, since none of the genets were completely free of A. tsugae infestation, differences among genets seem to be related more to susceptibility or tolerance than complete resistance. Among the 17 genets that had five or more ramets in the seed orchard, only one had more than one ramet free of A. tsugae. That genet had four of five ramets that were free of A. tsugae. However, it is not possible to say whether that was due to chance or inherent tree characteristics that make it less susceptible to A. tsugae infestation.