Numerical relationships of the Solidago altissima stem gall insect-parasitoid guild food chain

Abstract

The field site conditions (soil pH, soil moisture, soil nutrient availability, etc.) and abundances of Solidago altissima (often included in S. canadensis sensu lato), three S. altissima specific stem gall formers, and the parasitepredator guilds for two of the three gall insects were investigated. It was found that S. altissima is tolerant of a wide range of site conditions. Herbivore (stem gall insects) occurrences were positively correlated with plant occurrence, in a linear fashion. However, there was no disproportionate increase in stem gall insect densities with plant density as might be predicted by the resource concentration hypothesis. Parasitoid guilds were exploiting stem gall insect populations over a wide range of occurrence, but were under-utilizing fields of higher herbivore occurrences. Path analysis showed a high degree of predictability in the causal models, with all but 14% of the ball gall parasitoid guild and all but 43% of the elliptical gall parasitoid guild occurrences explained by the direct influences of stem gall insect occurrence and the indirect influences of goldenrod occurrence and site conditions. The numerical relations of this three trophic level system suggest a well-integrated and well-controlled food chain.