Relationship between leader morphology and resistance or susceptibility of Sitka spruce to the white pine weevil
Sitka spruce, Piceasitchensis (Bong.) Carr., from 27 provenances and five sites in British Columbia, were examined for traits of leader morphology that might be involved in resistance to the white pine weevil, Pissodesstrobi (Peck). Trees from the Usk Ferry provenance at Head Bay and the Kitwanga provenance at Nass River both had low incidence of weevilling, and had needles pressed more closely against the stems by up to 20° than trees from other provenances. At Fair Harbour, trees from the two most resistant provenances, Haney and Cedarvale, had inner resin ducts that were approximately twice the diameter of those in trees from the most susceptible provenances. Resistant trees from these two provenances also had significantly more outer resin ducts than those from any other provenance planted at any of the five sites. Values ranged from 28 outer ducts per centimetre for Haney trees, to <1 per centimetre for very susceptible trees from the Masset Sound provenance in the Queen Charlotte Islands, where the absence of weevils could have eliminated any selection for trees with a high density of resin ducts. We hypothesize that trees from the Haney and Cedarvale provenances may have large numbers of outer resin ducts as an important resistance mechanism, which could be used as a selection criterion for resistance. Resistant trees from other provenances such as Big Qualicum, Kitwanga, and Green Timbers, which have fewer outer resin ducts than those from Haney and Cedarvale, must rely heavily on mechanisms other than those associated with the resin system morphology.