Floral nectar is a primary factor mediating plant-pollinator interactions, yet the composition of this key trait varies both across species and within populations. In addition to sugars, nectar frequently contains secondary metabolites associated with plant defense. The function for these noxious compounds may include defending plants from floral antagonists such as nectar robbers, however, studies suggest that nectar secondary metabolites can have negative impacts on pollinator visitation. Given that these compounds may incur such an ecological cost, and may also have allocation costs for plants, inducible nectar defenses employed when robbing actively occurs may be a more adaptive strategy than constitutive nectar defenses. Linaria vulgaris is a non-native weed that experiences heavy robbing by the bumble bee Bombus occidentalis in high alpine meadows of Colorado. We conducted a three-year field study at the Rocky Mountain Biological Laboratory to examine whether nectar robbing induces iridoid glycosides in the floral nectar of L. vulgaris. We exposed flowering stems to either natural robbing by B. occidentalis or artificial robbing via mechanical damage or jasmonic acid. We quantified iridoid gylcoside concentrations in leaves, stems, floral tissue, anthers and floral nectar using gas chromatography and compared these concentrations across plant parts and treatments.
Preliminary analyses indicate that nectar iridoid glycosides were significantly higher in the natural robbing treatment in the first year of study, with an overall trend towards increased nectar iridoid glycosides after natural and artificial robbing treatments across years. Interestingly, we did not find any evidence of induction in floral tissue after robbing treatments, indicating that floral tissue gains no additional protection after damage. Within years, iridoid glycoside concentrations differ significantly between plant parts and while these concentrations are correlated between some plant parts, the correlations differ between years, suggesting no general pattern of chemical induction or allocation within plants. Our results suggest that nectar robbing can induce nectar secondary metabolites, which may be a strategy to deter further robbing, protecting remaining or subsequently produced nectar for pollinators. Whether these increases in nectar secondary metabolites actually reduce robbing and the effects of induction on pollinators and pollination will be evaluated in future studies. Understanding the role that nectar chemistry plays in plant-pollinator interactions may be particularly important in alpine systems where the growing season is short and effects of climate change are magnified.