Sunflower volatiles involved in honeybee discrimination among genotypes and flowering stages

In order to define the part of olfactory cues in the selective behavior of honeybees, observation on their foraging behavior was carried out on various sunflower genotypes in parallel with chemical analysis of aromatic extracts of the genotypes. Foragers show a preference for the early stages of flowering and, when they are given a choice between couples of parental lines of two commercial hybrids, Marianne and Mirasol, they are randomly distributed on Mirasol parents, but they prefer the female line of Marianne. The comparison of relative proportions of compounds among aromagrams obtained from head space trapping from the two couples of genotypes, reveals (1) a phenological stage effect for 17 compounds among 144 indexed compounds for Marianne lines and for 18 among 136 indexed compounds of Mirasol lines; most of these compounds exhibit higher relative proportions in the early flowering stages, which is related to plant attractiveness towards honeybees; (2) a sex effect for 33 compounds among 144 for Marianne lines and for 14 compounds among 136 for Mirasol lines; further semiquantitative analyses reveal a sex effect for only eight compounds of 134 for Marianne lines and 20 compounds of 250 for Mirasol lines, which represents less than 10% of the indexed compounds. These discriminatory compounds were partly identified by coupled GC-MS. Possible relations between such phenological and genotypical volatile fluctuations and forager attraction are discussed.