Fungal endophytes in oak trees: experimental analyses of interactions with leafminers

Endophytic fungi are diverse and abundant in woody plants and are thought to increase resistance of host trees to invertebrate and vertebrate herbivores. We performed two experimental manipulations of endophytic fungi in the field to test the long-term and short-term effects of individual endophyte species on growth, development, and survival of leafmining larvae of Cameraria sp. nov. (Lepidoptera: Gracillariidae). In the first experiment, we compared long-term survival and mass of larvae surviving to pupation on branches with elevated infections of the endophytic fungus Asteromella sp. to survival and mass of larvae inhabiting control branches. In the second experiment, we compared short-term survival, developmental time, and mass of pupae in mines injected with one of three common endophytic fungal species Asteromella sp., Plectophomella sp., or a filamentous yeast, relative to controls. Long-term survival and size of surviving larvae did not differ between leafminers on control branches and leafminers on branches with elevated endophyte infections. Likewise, larvae within mines injected with known quantities of spores and hyphae from three endophytic fungal species did not differ in survival or pupal mass from larvae in control mines. However, larvae in leaf mines injected with two of the endophytes developed more slowly than did larvae in control branches and thus may suffer more mortality from leaf abcission. Our results call into question the generality of mutualisms between endophytic fungi and their host plants predicated on increasing resistance to macroherbivores. Interactions between micro- and macroherbivores in foliage of woody plants are likely to be as highly variable and complex as interactions in other well-studied communities.