Response of Galling Insects to Natural Browsing by Mammals in Alaska

Abstract

Along the Tanana River, near Fairbanks, Alaska, natural browsing by the snowshoe hare, Lepus americanus, and moose, Alces alces, pruned back growth and resulted in resprouting of more juvenile growth in host plants. This resulted in longer leading shoots of stems in young trees of Populus balsamifera and ramets of the shrub, Salix novae-angliae, both species in the family Salicaceae. Total numbers of shoots were the same on uncaged and caged plots, the latter excluding moose and snowshoe hare, but the ramets of Salix novae-angliae were slightly younger on uncaged plots, indicating that dieback of older ramets accelerated under natural browsing. The most common herbivores in August 1993 were two leaf-folding sawfly species in the genus Phyllocolpa (Hymenoptera: Tenthredinidae). Densities of the two Phyllocolpa species, one on Populus balsamfera and one on Salix novae-angliae, were 83 and 85% higher on ramets or stems in uncaged plots. Also, the combined number of other rarer galling species were higher in browsed, uncaged plots. Mean length of leading shoots in ramets and stems accounted for 35 and 30% of variation in densities of Phyllocolpa species on Populus balsamifera and Salix novae-angliae in plots, respectively. Thus, the increased galling insect densities are accounted for as far as our studies permit by increased vigor of host plants indicated by shoot length. On Salix novae-angliae also faster dieback of older senescent ramets may have an important effect on the resource quality for gallers.