Foliar damage by leafcutter bees (Megachile sp.) is associated with increased leaf damage by phytopathogens in an understory perennial

Victoria Luizzi and Judith Bronstein, Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, A. Elizabeth Arnold, School of Plant Sciences and Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ

Background/Question/Methods
Interactions between species typically occur in a community context. Accordingly, the costs and benefits of pairwise interactions often can be illuminated by considering communities of interacting species. In montane forests of southern Arizona (USA), solitary, cavity-nesting leafcutter bees (Megachile sp., Megachilidae) cut discs from living leaves of the herbaceous perennial Clitoria mariana (Fabacae) to line their nests. How such leaf damage impacts plant fitness, both directly and with regard to plant responses to other natural enemies, is not yet known. We haphazardly selected and tagged C. mariana plants in a natural forest understory, counted leafcutter bee cuts on each individual, and quantified reproduction at the end of the season by counting flowers, fruits, and seeds. For a subset of similarly aged leaves with and without leafcutter damage on each plant, we measured leaf area damaged by necrotrophic pathogens and chewing herbivores. We predicted that 1) plant reproduction would differ between plants that leafcutter bees did not damage, and 2) the degree of damage from pathogens and herbivores would differ between leaves that were cut vs. not cut by leafcutter bees.
Results/Conclusions
Plants that were cut by leafcutter bees produced significantly more flowers than non-cut plants, but did not differ in fruit or seed production. Leaf damage due to chewing herbivores did not differ between leaves that were cut vs. not cut by leafcutter bees. However, damage due to necrotrophic pathogens was significantly greater on leaves that were cut by leafcutters vs. similarly aged, uncut leaves on the same plants. Intriguingly, pathogen damage did not differ as a function of leaf damage by chewing insects, suggesting that this pattern is not driven simply by wounds facilitating pathogen entry. Leafcutter bees may be uncommonly effective pathogen vectors, prefer leaves that are expressing less chemical defense, or prefer leaves that harbor pathogenic microbes, some of which may have a protective effect on larvae when introduced into nests. Under each scenario, plant pathogenic microbes could have important effects on leafcutter bee-plant interactions, a perspective supported by studies in cultivated plants that indicate a potential sensitivity of Megachile to foliar microbiomes. In future studies, we will determine which of these mechanisms is at play and explore how plant pathogens, part of the community context of leafcutter bee-plant associations, may affect the ultimate outcome of this interaction.