Impact of microbial communities on floral nectar chemistry: Potential implications for biological control of pest insects

Abstract

Nectar-producing plants are increasingly used in agroecosystems to provide biological control agents (BCAs) such as predatory insects and parasitoids with necessary sugars to fulfil their nutritional requirements. However, it has recently been shown that nectar is commonly infested by microorganisms which may change the chemistry of the nectar and therefore its attractiveness and nutritional suitability for insects. The aim of this study was to investigate the impact of nectar-inhabiting microorganisms on nectar chemistry of three co-flowering plant species that are commonly used to provide BCAs with sugar resources (Borago officinalis L., Centaurea cyanus L. and Symphytum officinale L.). First, we assessed the abundance and visitation rate of flower-visiting insects on these plant species. Further, we identified the culturable microbes inhabiting the floral nectar and assessed changes in pH, total sugar concentration, main nectar sugars and amino acids. Flowers of all three plant species were found to be visited by a wide range of insects, with B. officinalis being the most attractive plant species for BCAs. Nectar microbial community structure differed significantly between plant species, irrespective of experimental field site. Microbial contamination affected both sugar and amino acid concentration and composition. Especially, sucrose and the fructan sugars 1-kestose and neokestose were found at significantly lower concentrations in contaminated nectar. Further, microbial contamination resulted in a significant increase of alanine and glycine, and a decrease in threonine and valine. Further research is needed to investigate the precise impact of these changes on the efficacy of biological control agents to control pest insects.