Effects of Temperature on Growth, Sporulation, and Competition of Mountain Pine Beetle Fungal Symbionts

The mountain pine beetle, Dendroctonus ponderosae, depends on two fungi, Grosmannia clavigera and Ophiostoma montium, to augment a nutrient-poor woody food resource. Because the two fungi exert differential effects on the host beetle, temperature-driven differences in fungal growth and competition outcomes have a strong potential to influence host population dynamics. Weisolated fungi from beetles and wood from three locations in Montana and Utah, USA, and measured their growth rates and sporulation between 5 and 35 °C on artificial media. We also measured growth rates and percent resource capture for each fungus at 10, 15, 21, and 25 °C during inter- and intra-specific competition. G. clavigera excelled at resource capture at most temperatures. Its optimal growth temperature occurs around 20 °C while that of O. montium occurs near 30 °C. There was no effect of collection site on growth or sporulation; however, O. montium exhibited greater variability in response to temperature than did G. clavigera. Sporulation of G. clavigera was greatest at 30 °C while O. montium sporulated at low levels across all temperatures. During competition experiments, G. clavigera captured more resources than O. montium at most temperatures and captured a greater percentage of resources at a greater rate during inter-specific competition than during intra-specific competition. In contrast, O. montium captured a greater percentage of resources during intra-specific competition. These results demonstrate that temperature can differentially affect growth, sporulation, and resource capture of the two symbionts, indicating that it may be an important factor influencing the composition and dynamics of the symbiosis.