Floral visitor diversity and pollinium acquisition in three pollinator taxa of Asclepias syriaca, Common Milkweed (Apocynaceae)

Background/Question/Methods

Different factors can affect zoophilous-angiosperm pollination including the number of flowers that a pollinator visits per inflorescence, time that a pollinator spends per inflorescence, pollinator taxon, and sampling year.  To learn more about these factors, we studied pollinators in two Asclepias syriaca populations in 2008 and 2009 in Eastern U.S.  This plant produces macroscopic pollen packages termed pollinia which attach to pollinators’ bristles and hairs and to each other.  Pollinators fertilize A. syriaca flowers by depositing pollinia into stigmatic slits.  We tested the null hypothesis that in A. syriaca, pollinator taxon, number of flowers visited per cyme, time spent on a cyme, sampling year, and interactions among these factors do not statistically predict the number of pollinator-transported pollinia.  Before a cyme started blooming, we covered it with a pollinator-excluding, mesh bag.  When a cyme reached full anthesis, we uncovered it and recorded the number of flowers that the first-arriving insect visited and the time that it spent foraging on the cyme.  As it was leaving the cyme, we captured it and put it into a labeled plastic bag that we placed on ice.  In our laboratory, we taxonomically identified the insect and recorded the number of pollinia that it carried. 

Results/Conclusions

In our 258 observations of cyme visitors, non-native Apis mellifera (Western Honey Bees), four native Bombus spp. (bumble bees), and 19 native lepidopteran spp. (butterflies and moths in six families) were the principal pollinium-carrying groups.  Taxon, number of flowers visited per cyme, and study year were significant predictors of the number of pollinia carried by pollinators (quasi-Poisson general linear model).  The three taxa did not differ significantly in the mean time that they spent on cymes.  Apis mellifera visited significantly fewer flowers per cyme than Bombus spp., and A. mellifera carried significantly more pollinia than the other two taxa.  Further, A. mellifera’s behavior suggests that it is effecting more cross-pollination than the other two taxa, and this could increase fruit production in this primarily self-incompatible plant. Therefore, this introduced bee, which has been in North America for about 400 years, might be affecting A. syriaca reproductive success and cyme-size evolution.  To our knowledge, our study is the first one to examine pollinium number carried by pollinators with regard to the factors taxon, time spent per cyme, number of flowers per cyme, study year, and interactions among them in Asclepiadoideae which comprises about 2,900 spp.