Mycorrhizal composition influences plant anatomical defense and impacts herbivore growth and survival in a life-stage dependent manner

While arbuscular mycorrhizal (AM) fungi may have a prominent role in trophic ecology, mycorrhizal improvement or reduction on herbivore growth and survival may also be dependent on herbivore's stage of development. Solanum lycopersicon (tomato) was grown on sterile background soil treated with either mycorrhizal inoculant (AM+) or non-mycorrhizal control (AM−). Mycorrhizal treatments included four single species of AM-fungi (Entrophospora infrequens, Funneliformis mosseae, Claroideoglomus claroideum, and Racocetra fulgida) and a mixture of all four species (fungal community). To determine if mycorrhizal treatment indirectly alters the ability of beetle larvae (Leptinotarsa decemlineata) to access plant resources, plant damage and trichome density were quantified as plants were infested with a single neonate (early-stage) for 96 hours (h). In a second experiment, beetle growth rate was assessed as plants were infested with a single third-instar (late-stage). After 72 h of late-stage beetle infestation, beetle mass was measured. It was found that early-stage beetles inflicted more damage on AM+ tomatoes. Interestingly, this corresponds with fewer trichomes on AM+ tomatoes, as well as higher early-stage beetle survivorship. Specifically, AM taxon, C. claroideum increases herbivory and thereby reduces beetle mortality. Among late-stage beetles, C. claroideum does not improve beetle growth nor rate of survival. This suggests that AM taxa that are beneficial to early-stage beetles may not necessarily provide an advantage to late-stage beetles. Taken together, these findings highlight potential dependencies of AM-fungal effects on herbivory and herbivore life history, including growth and life-stage specific survival.