Opposing hypotheses predict that mycorrhizae may increase or decrease herbivore performance, depending on the underlying mechanisms. The Nutritional Quality Hypothesis predicts that mycorrhizae will promote herbivore performance due to higher quality foliar tissue, whereas as the Modification of Defenses Hypothesis predicts that mycorrhizal plants will improve their defenses against herbivores, thereby suppressing herbivore performance. Furthermore, it is unclear how inducible defenses are influenced by mycorrhizae, though it is hypothesized that mycorrhizae will exacerbate herbivore suppression on primed plants. In order to resolve how herbivore performance responds to mycorrhizal mutualisms, we established an experiment that tested the independent and interactive effects of mycorrhizal inoculants (none, single species, multispecies) and past herbivory (primed, not primed) on herbivore (Spodoptera frugiperda) performance on Solidago altissima. To help determine underlying mechanisms, we measured several plant traits, including biomass proxies (leaf counts and total heights), leaf surface area, foliar nutritional content, root water content, and root colonization.
Results/Conclusions
The presence of mycorrhizae increased herbivore growth rates (P=0.002). Herbivores grew 9% faster on plants with a diversity of mycorrhizal mutualists than plants inoculated with a single mycorrhizal species (P=0.002). Most interestingly, however, mycorrhizal status and herbivore history interacted to alter subsequent herbivore growth rates (P = 0.01). In the absence of mycorrhizae, herbivores gained equally as much weight on primed and unprimed plants (P=0.26). However, herbivores gained more weight on primed plants than unprimed plants when inoculated with a multispecies mycorrhizal mix (P = 0.06), a surprising result that indicates that mycorrhizae may influence plant-insect interactions in unexpected ways. Herbivores also gained more weight on primed, multispecies mycorrhizal plants than primed, single species mycorrhizal plants (P=0.002). Mycorrhizal status and herbivory history may interactively influence herbivore performance not only in magnitude, but also directionally, promoting herbivore performance under some circumstances and suppressing herbivore performance in others. Nutritional analysis and plant trait analysis will also be presented to possibly elucidate the plant traits underlying mycorrhizal mediation of herbivore performance.