RESISTANCE BY TRANSLOCATED SITKA SPRUCE TO DAMAGE BY PISSODES STROBI (COLEOPTERA, CURCULIONIDAE) RELATED TO TREE PHENOLOGY

Phenological asynchrony between white pine weevil, Pissodes strobi (Peck), and its host trees may largely explain why certain trees resist insect damage. In trials with Sitka spruce clones, adult P. strobi and their egg progeny were found mainly on trees whose seasonal development was best synchronized to successful insect reproduction. The least damaged trees started spring development first, suggesting that these trees developed beyond the optimal stage for weevil oviposition before enough spring heat units accumulated for the insect to become gravid. Seasonal variation in resistance was demonstrated by advancing or retarding the period when weevil oviposition occurs: when oviposition was advanced, normally resistant trees became more susceptible; and when oviposition was retarded, susceptible trees became resistant. Resistance thus may be seasonally transient rather than permanent. Resistance coincided with high production of wound resin. The observed variation in resistance of translocated trees at different planting sites reported in the literature may thus reflect changes in tree phenology under different environmental conditions. This asynchrony in phenology may be used to regulate P. strobi damage. Because damage resistance may be largely site specific, it is important to match the tree to the planting site.