The influence of host plant phenotype and interspecific competition on the abundance of four willow sawfly species was tested on potted cuttings of the arroyo willow, Salix lasiolepis, in northern Arizona. Interspecific competition by a stem—galling sawfly, Euura lasiolepis, was demonstrated on a leaf galler, Pontania sp., a leaf folder, Phyllocolpa sp., and a petiole galler, Euura sp. The competitive influence of the stem galler on these species is likely to be absent at low stem gall densities. Density of the stem galler is spatially patchy, so competition should be variable among willow clones in the field. The three other sawfly species form galls that are distal to stem galls and/or do not affect the shoot in a way detrimental to the stem galler, therefore this competition appears to be asymmetrical, an "amensalism." All sawfly species exhibited significant differences in gall densities among potted, replicate willow clones under uniform environmental conditions. This suggests underlying genetic variation among clones in susceptibility to sawfly oviposition. Each sawfly species seemed to respond to variation among clone phenotypes idiosyncratically. Under experimental conditions, host plant phenotype accounted for 1.5 to 3.2 times as much variation in the density of subordinate species as did interspecific competition; under field conditions host plant phenotype is likely to play a more important role than competition in the community patterns of herbivorous insects.

The frequency of coevolution as a process of strong mutual interaction between a single plant and herbivore species has been questioned in light of more commonly observed, complex relationships between a plant and a suite of herbivore species. Despite recognition of the possibility of diffuse coevolution, relatively few studies have examined ecological responses of plants to herbivores in complex associations. We studied the impact of two specialist herbivores, the horse nettle beetle, Leptinotarsa juncta, and the eggplant flea beetle, Epitrix fuscula, on reproduction of their host, Solanum carolinense. Our study involved field and controlled-environment experimental tests of the impact on sexual and potential asexual reproduction of attack by individuals of the two herbivore species, individually and in combination. Field tests demonstrated that under normal levels of phytophagous insect attack, horse nettle plants experienced a reduction in fruit production of more than 75% compared with plants from which insects were excluded. In controlled-environment experiments using enclosure-exclosure cages, the horse nettle's two principal herbivores, the flea beetle and the horse nettle beetle, caused decreases in sexual reproduction similar to those observed in the field, and a reduction in potential asexual reproduction, represented by root biomass. Attack by each herbivore reduced the numbers of fruits produced, and root growth, when feeding in isolation. When both species were feeding together, fruit production, but not root growth, was lower than when either beetle species fed alone. Ecological interactions between horse nettle and its two primary herbivores necessary for diffuse coevolution to occur were evident from an overall analysis of the statistical interactions between the two herbivores for combined assessment of fruit and vegetative traits. For either of these traits alone, the interactions necessary to promote diffuse coevolution apparently were lacking.

No 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.

The densities of four species of gall-forming sawflies were found to vary significantly among willow host plant clones. Two of the speices varied among host plants at four sites in each of three years. The other two species varied in density among host plants at most of the sites in two of the three years. Total sawfly density also varied significantly among clones. Individual species densities on willow clones were significantly positively correlated between years when all sites were combined and frequently when sites were considered separately. Most pairwise species combinations were independent in density between years, but some negative correlations existed between the stem galler and the leaf galler. Gall-former densities also were largely independent among clones within years with all sites combined and with sites considered separately. The significant correlations were nearly all positive. At all four sites the combination of significant variation in sawfly densities among willow clones in the field and independence of species densities among clones resulted in significantly different communities (relative abundance of species) among willow clones in three years. Although sawfly abundances differed substantially among the four sites, this remained true. It is argued that the pattern of community structure among clones is the result of variation in host plant quality of clones. We propose an hypothesis to account for patterns of herbivore species associations based on intrapopulation host plant variation.