Mycorrhizal fungi associate with almost all land plants. Recently, it has been recognized that mycorrhizae increase host-plant resistance to insect herbivores by various mechanisms. One hypothesis we tested was that mycorrhizal plants would have lower insect herbivore populations than non-mycorrhizal plants. We observed naturally colonizing mirid (Tupiocoris notatus Distant) populations on two species of Nicotiana, a wild species (N. rustica) and cultivated species (N. tabacum). These mirids are zoophytophagous but mainly feed on stem and leaf-cell contents. Mirid populations were significantly lower (adult p=.024; nymph p=.029) on mycorrhizal N. rustica. But neither population was significantly different on N. tabacum (adult p=.199; nymph p=.109). Plant "biomass" was also significantly affected by mycorrhizal treatment (leaves p < .0001; height p=.039), but when biomass (plant leaf number or height) was controlled, insect populations were still significantly lower on mycorrhizal plants due to the mycorrhizal treatment. Non-mycorrhizal plants were also more heavily damaged than mycorrhizal plants, by visual estimation. Additionally, as the plants became more mycorrhizal (percent root colonization increased), insect herbivore populations became significantly different between mycorrhizal and control treatments. It appears that mycorrhizal colonization decreases host-plant suitability for insect herbivores over non-mycorrhizal plants, most likely a result of changes in host-plant quality. These data also suggest that mycorrhizal fungi may play a bottom-up role suppressing insect herbivores. A bottom-up affect on insect herbivores by soil fungi may play a role that is complementary to the top-down affect inflicted by herbivore natural enemies.
Species 1: Heteroptera Miridae Tupiocoris notatus
Species 2: Lepidoptera Sphingidae Manduca sexta
Keywords: population regulation, top-down
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