Arbuscular mycorrhizal fungi influence visitation rates of pollinating insects

1. Arbuscular mycorrhizal (AM) fungi can increase a number of plant traits to which pollinating insects are known to respond. These include total plant size, flower number, flower size, and amount of pollen produced. 2. It was hypothesised that these effects would lead to a different visitation rate of pollinating insects on mycorrhizal and non-mycorrhizal plants. To test this idea, three species of annual plants (Centaurea cyanus, Tagetes erecta, and Tagetes patula) were grown with and without AM fungi and the visits by pollinating insects were recorded over a 2-month period. 3. In all three species, mycorrhizal plants experienced a greater number of pollinator visits per flower per unit time. Diptera and Hymenoptera were the predominant insects and the latter order showed the strongest response. 4. Here, it is suggested that mycorrhizal fungi increase floral visitation rates by insects, but that the mechanism varies from one plant species to another. In C. cyanus, it appears to be due to flower number per plant, in T. patula it is individual inflorescence size, and in T. patula it is nectar standing crop per inflorescence.