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Mycorrhizal fungi and plant resistance to herbivory: Does mode of herbivore feeding matter?
Borowicz, Victoria*,1, 1 Illinois State University, Normal, IL, USA
ABSTRACT- In addition to facilitating phosphorus uptake, mycorrhizal fungi have been hypothesized to alter relations between plants and their enemies, including herbivores. Early experiments with caterpillars suggested that mycorrhizae reduce herbivore performance but subsequent experiments with aphids found the opposite trend. Based on these studies investigators speculated that mycorrhizae increase resistance to chewing herbivores but reduce resistance to insects that feed by sucking. Using data from published studies I conducted a meta-analysis to test the hypothesis that effects of mycorrhizal fungi depend on herbivore mode of feeding. A literature search produced only nine papers that presented data in a form amenable to meta-analysis, yielding a total of 27 separate experiments. Two experiments involved ectomycorrhizal fungi and oaks, and the remaining involved arbuscular mycorrhizal fungi and herbaceous plants. Herbivores included aphids ("suckers" - 8 experiments), and root or leaf chewers (13 experiments), leaf scrapers (4 experiments), and stem gallers (2 experiments). Due to small sample sizes of two categories and low within group variance when pooled, all non-sucker studies were combined in one group and compared to suckers. The effect of mycorrhizal fungi on insect mass or growth rate was large and positive for suckers; aphids grew larger or faster when they fed on mycorrhizal hosts. Mycorrhizal fungi did not significantly affect non-sucker growth. Nine experiments with non-suckers also included data on survival. Mycorrhizal fungi had a moderately large and significant negative effect on herbivore survival; insects exhibited lower survival on well-colonized plants. Studies thus far provide equivocal support for the hypothesis that mycorrhizal fungi alter plant resistance and that the direction of the effect is associated with mode of feeding. The trends were not robust and are likely to be challenged as more data become available.
Key words: herbivory, mycorrhizal fungi, aphids