Print PageRecent phylogenetic studies strongly suggest that the first bees were specialists, limiting themselves to some subset of available floral resources when foraging for pollen. While some bee lineages have since evolved a more generalized diet, many have remained specialists, with some clades undergoing numerous host-shifts. For example, the ancestor to the solitary bee genus Diadasia, with roughly 30 North American species, likely specialized on genera in Malvaceae. While the majority of Diadasia species still exclusively visit Malvaceae, a clade of five have radiated on to Cactaceae, and one species each specializes on Convolvulaceae, Asteraceae, and Onagraceae. Of all the potential hosts for Diadasia, how and why have they radiated onto these distinct and distantly related plant families? Using a combination of floral scent analysis (GC/MS) and spectrophotometry, hypotheses regarding the role of visual and chemical cues in floral host recognition were tested: 1) Diadasia host plants share in common a suite of visual and chemical cues that are lacking in other co-blooming plant genera and 2) Diadasia are either unable to recognize, or are not attracted to, many of the visual and chemical cues presented by non-host plants.
Results/Conclusions
In terms of visual similarities, measurements of spectral reflectance for corollas reveal no consistent pattern in color presentation among host and/or non-host plants—many host plants showed considerable UV reflectance, but this was not unique, nor was it consistent across all host plants. In terms of chemical cues, floral scent for the nearly comprehensive array of Diadasia host plants that were tested includes compounds that are known to attract bees, including monoterpenes (e.g. beta-ocimene), sesquiterpenes (e.g. beta-caryophyllene), and aromatics (e.g. phenylmethanol). However, none of these compounds were unique to Diadasia host plants. Many were ubiquitous in the floral scent of abundant and attractive co-blooming non-hosts that were also analyzed. On the other hand, non-host scent often included a large suite of both mono and sesquiterpenes that never occurred in host flowers. The absence of many compounds in host plant genera, rather than the presence of any particular compound, makes them much more similar to each other than any of them are to non-hosts (average Sorenson’s similarity for host/host comparisons: 0.79, compared to 0.56 for host/non-host comparisons). Our data implies that specialization in Diadasia may be a result of its limited ability to recognize many floral volatiles. Moreover, visual cues appear to be important only within the context of chemistry.
