Killer magnolias: assessing the effect of Magnolia macrophylla’s stigmatic secretion on its floral visitors.
Many flowers are attractive to insects and facilitate reproduction; therefore, species with flowers lethal to visitors, such as Magnolia macrophylla (big leaf magnolia; Magnoliaceae), present a paradox. Previous observations suggest that a stigmatic secretion produced by M. macrophylla is the cause of lethargic insect behavior and death within flowers. This study documents the behavior of the floral visitor taxa Xylocopa virginica (eastern carpenter bee), Apis mellifera (European honey bee), Bombus impatiens (common eastern bumble bee), Formicidae (ants), Halictidae (sweat bees), and Coleoptera (beetles) and quantifies the effect of stigmatic secretion on worker B. impatiens. I recorded video observations of floral visitors to five M. macrophylla ssp. macrophylla and three M. macrophylla ssp. ashei trees at the Arnold Arboretum in Boston, MA to ascertain visitor behavior within flowers. To assess if stigmatic secretion was the cause of lethargy, I performed assays testing 66 laboratory-raised worker B. impatiens receiving either distilled water (diH2O) or stigmatic secretion application. Videos were scored blind for behaviors including the total length and longest period of immobile, active behavior (as a measure of lethargy), the number of leg flails, and instances of abdomen and antennae grooming. At the Arnold Arboretum, floral visitors, mostly X. virginica and A. mellifera, were observed within flowers and either tried to escape or displayed lethargic behavior. During lab assays, stigmatic secretion application only increased B. impatiens abdomen grooming. These results do not directly support stigmatic secretion as the cause of lethargic behavior; however, they provide a strong basis for future research on Magnolia macrophylla floral visitor interactions.