Impact of Shoot Galler Attack on Sexual Reproduction in the Arroyo Willow

Abstract

The assumption that herbivores cause reduction of plant fitness is central to plant herbivore theory, but empirical support from field studies is scant. We tested the hypothesis that the shoot—galling sawfly Euura lasiolepis significantly influences the fitness of its obligate host, the arroyo willow (Salix lasiolepis). Surveys of reproductive attributes on field willow clones at different elevations indicated that the presence of shoot galls led to a significant reduction in reproductive buds, and thereby inflorescences, on individual shoots on both male and female plants. Shoots with galls suffered a loss of 43% of their reproductive buds relative to shoots without galls. Inflorescence size, measured as flowers per inflorescence, seed mass, germination success, and pollen germination success were not significantly affected by gall presence. Two sawfly exclosure—enclosure experiments on plants that had been established from cuttings from clones showed reduced reproductive bud production on plants with galls at both the whole—plant and individual—shoot levels; whole—plant losses of 34—56% and per—shoot losses of 17—55% of reproductive buds on different clones were coupled with densities that varied from 0.56 to 2.70 galls per shoot. Most plants suffered relatively small reproductive losses because of low sawfly densities. However, 20% of all plants suffered losses of 10%, while 7.5% experienced losses of 20% or more of their reproductive potential. A small willow clone attacked over the range of 0.5—2.0 galls per shoot would lose 381 000—1 500 000 seeds of a potential 3 760 000 seeds in the year of attack. Genetic variation among plants in susceptibility to sawfly attack and subsequent herbivore—related reproductive loss was suggested by a common garden study; some clones had 12 times as many galls as others.