3-WAY INTERACTIONS AMONG ECTOMYCORRHIZAL MUTUALISTS, SCALE INSECTS, AND RESISTANT AND SUSCEPTIBLE PINYON PINES

Herbivores and mycorrhizal fungi are important associates of most plan ts, but little is known about how these organisms interact. In a 9-yr experiment, we examined how the pinyon needle scale (Matsucoccus acaly ptus) affects and is affected by the ectomycorrhizal mutualists found on the roots of scale-resistant and -susceptible pinyon pines (Pinus e dulis). Three major results emerged. First, removal experiments demons trated that scales negatively affected ectomycorrhiza. Second, althoug h ectomycorrhiza could either positively or negatively influence scale performance by improving plant vigor or increasing plant investment i n antiherbivore defenses, we found no ectomycorrhizal effect on scale mortality when we experimentally enhanced levels of ectomycorrhiza. Th is represented the first test of whether ectomycorrhiza promote plant resistance and contrasted with studies showing that arbuscular mycorrh iza negatively affected herbivores. Third, pinyon resistance to scales mediated the asymmetrical interaction between fungal mutualists and s cale herbivores. High scale densities suppressed ectomycorrhizal colon ization, but only on trees susceptible to scales. Similarities between mycorrhiza-herbivore interactions and competitive interactions among herbivores suggest broader generalities in the way aboveground herbivo res interact with belowground plant associates. However, because mycor rhiza are mutualists, mycorrhiza-herbivore interactions do not fit wit hin traditional competition paradigms. The widespread occurrence and i mportance of both herbivores and mycorrhiza argue for incorporating th eir interactions into ecological theory.