Don P. Ostaff

Impact of old foliage removal, simulating defoliation by the balsam fir sawfly, on balsam fir tree growth and photosynthesis of current-year shoots

Abstract To investigate the effect of defoliation by the balsam fir sawfly (Neodiprion abietis) on current-year growth and photosynthesis, all 1-year-old and older foliage were removed by manual clipping before the start of the growing season from balsam fir seedlings cultivated at two levels of fertilization. Measurements carried out late in the same growing season showed that both defoliation and withholding fertilizer reduced current-year foliar dry weight, shoot length, needle size, and stem xylem radial width, as well as total foliar dry weight, and root dry weight. Defoliation increased the projected area to weight ratio, nitrogen concentration, net photosynthetic rate per unit of dry weight, and ratio of photosynthesis to internal CO2 concentration in current-year needles. In contrast, withholding fertilizer did not alter the projected area to weight ratio and decreased the nitrogen concentration, net photosynthetic rate per unit of dry weight, and ratio of photosynthesis to internal CO2 concentration. Both defoliation and withholding fertilizer also decreased the number of current-year needles produced in the growing season following treatment. The results indicate that (1) the reduction in total photosynthesizing biomass caused by removing old foliage is much greater than the biomass of old foliage removed due to decreased production of current-year foliage, and (2) old foliage removal stimulates net photosynthesis in current-year shoots, probably by improving foliar nitrogen relations. Removing old foliage reduced the total photosynthesizing biomass more, and stimulated net photosynthesis of remaining foliage less, than the removal of current-year foliage measured in earlier studies, which helps explain why tree recovery is slower following defoliation by the balsam fir sawfly than after spruce budworm (Choristoneura fumiferana) attack.

Sex biased intra‐tree oviposition site selection and larval foraging behavior of a specialist herbivore

Heterogeneity in the quality of oviposition and feeding sites within plants can significantly influence the distribution and abundance of herbivorous insects, but remains poorly understood. Field surveys and a manipulative study were conducted to evaluate the influence of variation within the crown of black spruce, Picea mariana (Mill.) B.S.P. (Pinaceae), on adult oviposition and larval feeding behavior of yellowheaded spruce sawfly, Pikonema alaskensis Rohwer (Hymenoptera: Tenthredinidae). Most eggs were laid in the mid to lower crown of 1.5–2 m tall trees. However, most of the few eggs that were laid in the upper crown (i.e., whorl 2) were female. Fourth and fifth instars dispersed acropetally, from the mid and lower to the upper crown, causing high defoliation in the upper crown. Late‐instar females were generally more abundant than males on the leader, the most apical shoot on a tree where eggs and early instars rarely occurred, strongly suggesting that more females than males disperse acropetally. This hypothesis was supported in a manipulative experiment, where only 15–20% of larvae in all‐male broods, but almost three‐quarters of larvae in mixed broods, dispersed to the upper crown. To our knowledge, this is only the second study to explicitly demonstrate preferential allocation of progeny sex through oviposition site selection by a herbivorous insect, and the first study to unambiguously demonstrate sex‐biased dispersal by the juveniles of an insect whose adult females can fly. This study emphasizes the important role of intra‐plant variation in shaping both oviposition site selection and the dispersal behavior of juvenile phytophagous insects within their hosts, and suggests that sex‐biased foraging behaviors may be necessary for some insects to accommodate the respective needs of immature females and males within heterogeneous host plants.