Beetle Visitations, and Associations with Quantitative Variation of Attractants in Floral Odors of Homalomena Propinqua (Araceae)

This study investigated floral visitations of two beetles, Parastasia bimaculata (Scarabaeidae) and Chaloenus schawalleri (Chrysomelidae), and examined associations between beetle visitations and variation in attractant traits, such as quantitative variations of attractants in floral odors and heat generation of spadices in Homalomena propinqua (Araceae). Observations showed P. bimaculata visited pistillate-phase inflorescences most frequently during heat generation, whereas C. schawalleri visited regardless of floral stages and heat generation. Chemical analyses of five dominant components of floral odors showed quantities of 2-butanol, veratrole, and alpha-pinene during the pistillate phase were the most abundant during all floral stages, and increased during heat generation. When testing combinations of these five authentic chemicals, some mixtures including 2-butanol or veratrole or both attracted both beetles, and veratrole attracted C. schawalleri. These results strongly suggested that the increased emission of floral odor attractants which accompanied heat generation influences floral visitations by P. bimaculata, but not by C. schawalleri. We therefore hypothesize that P. bimaculataaculata is a reliable pollinator, and that variation in attractant traits is a honest signal for P. bimaculata to seek rewards. In contrast, C. schawalleri can detect the signal even at low levels, and so visits inflorescences steadily during all floral stages.