PARENT SESSION
Contributed Oral Session 80: Mutualism / Parasitism: Pollination, Dispersal, Ants, Aphids, and Fungi
Wednesday, August 10, 8:00 AM - 11:30 AM, Meeting Room 518 C, Level 5, Palais des congrès de Montréal

Fungi→plants→herbivores: Mycorrhizal fungi indirectly benefit a specialist insect herbivore.

Laird, Robert^*,1, Addicott, John¹, ¹ University of Calgary, Calgary, Alberta, Canada

ABSTRACT- Plants bridge the soil-air interface and therefore provide a conduit through which below- and above-ground ecological processes can interact. For example, by aiding in the acquisition of soil nutrients, arbuscular mycorrhizal fungi have the potential to alter their host plant's nutrient content, which in turn can affect the feeding performance of above-ground insect herbivores. We examined this type of trait-mediated indirect interaction by studying the effects of mycorrhizal fungi on the performance of larvae of the specialist sunflower beetle (Zygogramma exclamationis Fabricius) when feeding on leaves of the common annual sunflower (Helianthus annuus L.). Plants were collected as four-leaved seedlings and reared in pots on a greenhouse roof. Fifty plants were treated with the fungicide benomyl to create a reduced mycorrhizal fungi treatment; an equal number served as control plants. Larvae were collected from Dinosaur Provincial Park, Alberta, and were weighed at the beginning and end of eight days of feeding (one larva per plant). After the feeding bout, plants were harvested, leaf area eaten was determined, and the plants were analysed for nutrient content. We documented three types of benefits of mycorrhizal fungi: First, larvae fed on mycorrhizal plants suffered lower mortality compared to larvae fed on non-mycorrhizal plants. Second, larvae had a greater relative growth rate when fed on mycorrhizal plants due to a greater food conversion efficiency. Third, larvae had a lower relative consumption rate when fed on mycorrhizal plants, which is a benefit insofar as time spent feeding trades off with other activities, such as hiding from predators. These results do not appear to be a consequence of differences in the nutrient content of mycorrhizal versus non-mycorrhizal plants. We discuss the alternative explanation that mycorrhizal fungi might alter their host plant's secondary compounds to the benefit of specialist herbivores. Specifically, mycorrhizal fungi can enhance a plant's production of secondary compounds, and specialist herbivores can frequently exploit these putatively 'defensive' substances. Finally, we discuss the feedback effects of mycorrhizal fungi on plant performance via changes in herbivore feeding behaviour.

Key words: arbuscular mycorrhizal fungi, fungus-plant-insect interactions, herbivory, mutualism