Nectar antimicrobial compounds and their potential effects on pollinators

•Nectars are nutrient-rich solutions that can easily harbor microbial growth.•Nectars often contain antimicrobials to limit microbial growth.•The interplay between microbes and antimicrobials can change nectar chemistry.•Nectar chemistry, controlled by plants and microbes, influences pollinator visitation. Nectar is a sugary, aqueous solution that plants offer as a reward to animal mutualists for visitation. Since nectars are so nutrient-rich, they often harbor significant microbial communities, which can be pathogenic, benign, or even sometimes beneficial to plant fitness. Through recent advances, it is now clear that these microbes alter nectar chemistry, which in turn influences mutualist behavior (e.g. pollinator visitation). To counteract unwanted microbial growth, nectars often contain antimicrobial compounds, especially in the form of proteins, specialized (secondary) metabolites, and metals. This review covers our current understanding of nectar antimicrobials, as well as their interplay with both microbes and insect visitors.