Comparison of two populations of Cardinal flower (Lobelia cardinalis) for alkaloid production and the phytochemicals' effects on a specialist weevil seed predator, Cleopmiarus hispidulus

Background/Question/Methods

Plant–animal interactions can be complex and multi-faceted.  Secondary metabolites can impact these interactions positively or negatively depending on the particulars of the animal’s interaction with the plant part.   Alkaloids are most often associated with herbivore deterrence, but their interaction with seed predators is not as clear.  We investigated the role of alkaloids in seed predation in Cardinal flower (Lobelia cardinalis), which produces a suite of alkaloids dominated by lobeline.   L. cardinalisplants in two populations were chosen for investigation:  Schenley Park in Pittsburgh, PA and Powdermill Nature Reserve in Rector, PA.   Soil samples from each site were analyzed for percent carbon, percent nitrogen, and pH.  Population parameters such as floral display traits, weevil signs, and weevil presence were recorded throughout the growing season.  After refining a lab-based alkaloid extraction protocol for field-collected specimens, we collected leaves for alkaloid analysis from the elongated stems of plants.   Leaves were stained for alkaloids, digitally processed to a grayscale image, and digitally sampled for average alkaloid concentration.

Results/Conclusions

Powdermill and Schenley differed in plant density, weevil infestation, and carbon and nitrogen content, but not C:N.  Powdermill had significantly more flowers per stem (F_2,232 = 120.61, p<.001) and significantly more flowers per plant (F_2,43 = 30.19, p<.001) leading to larger floral displays in Powdermill’s more distributed population.  Contrary to previous reports, weevils arrived prior to flowering, but weevil establishment in these populations was truncated due to a mid-summer flooding event that submerged the inflorescences.   Powdermill plants were more robust and recovered better from the submergence.  Prior to flooding, weevil sightings at Schenley were few, but post-flood investigation of the mature capsules revealed no weevils at Schenley.  Consequently, evidence of weevil visits was greater on capsules from Powdermill (F_2,455 = 53.21, p<.001) as was the number of weevils recovered from mature capsules (F_2,455 = 32.35, p<.001).  Weevil infestation varied from zero weevils in an entire plant to single capsules holding three adult weevils.  Alkaloid analyses indicate that Powdermill plants produced more alkaloids (F_2,37 = 1631.5, p<.001) , challenging the concept that the alkaloid cocktail in L. cardinalis serves as a deterrent to its specialist seed predator.