Inter-tree variation and yearly fluctuation in the susceptibility of Sakhalin spruce Picea glehnii, to shoot-boring sawfly Pleroneura piceae

Abstract

This study evaluated the effect of inter-tree variation in the bud phenology of Picea glehnii on susceptibility to the shoot-boring sawfly. Pleroneura piceae, and found that individual susceptibility fluctuates from year to year. The mechanism for the fluctuation between 1994 and 1997 is discussed. Inter-tree difference in the time of bud swelling is probably genetically based, since most of the trees that began to swell early in 1995 also swelled early in 1997, and those that began to swell late also did so in both years. Damage severity of each tree was evaluated by damage ratio: proportion of the number of damaged current shoots on the previous year's leader shoot. The rank of the bud swelling phenology of a tree was positively correlated to the rank of the damage ratio. This means that genetically based differences in phenology could explain why some trees are subjected to higher levels of herbivory than others. There was year-to-year variation in the damage severity for each tree. Nevertheless, no significant differences were found in the rank of the damage ratio between years. However the standard deviation of the damage ratios of each tree was highest for trees of intermediate rank. The skew of the frequency distribution of damage ratio was negatively correlated to the cumulated daily mean temperature in spring, which means that the spruce is more susceptible to the sawfly in warm springs than in cool springs. The mean growth rate of the lightly damaged trees increased constantly, while that of the heavily damaged trees seemed to reach a limit and then became lower than that of the lightly damaged trees.