Pollen-host specificity and evolutionary patterns of host switching in a clade of specialist bees (Apoidea: Diadasia)

Abstract

We examined the levels of pollen-host specificity in North American Diadasia (Hymenoptera: Apoidea), a clade of specialist bees. We analysed the scopal pollen loads of 409 individuals representing 25 of the 30 species of Diadasia that occur in North America. Each Diadasia species showed a preference for one of five plant families. However, the 25 species varied in their level of host specificity: the average percentage by volume of the preferred host in pollen loads ranged from > 99% to < 75%. In 17 of the 25 species, all or most individuals examined contained pure loads of one host taxon, while in eight species individuals were less specialized and carried mixtures of several unrelated host taxa. Mapping these host preferences onto a phylogenetic tree indicated that Malvaceae is the most likely ancestral host for the genus, and use of other hosts can be explained by a single switch to each of the other four host-plant families. Thus, most speciation events were not associated with a host switch; this pattern does not support host switching as a niche partitioning strategy to avoid competition. Diadasia species are more likely to use host-plant families that are used by other Diadasia and Emphorine bees; however, there was no evidence of residual adaptation to ancestral hosts. Diet breadth appears to be a labile trait: transitions from narrower to broader host use, as well as vice versa, were observed. The observed patterns of host-use evolution may be driven, in part, by host morphology and/or chemistry.

Phylogeny and host-plant evolution in Melittidaes.l.(Hymenoptera: Apoidea)

“…Most cases of host-plant variation involve shifts in the host-plant visited with little change in diet breadth. This pattern is consistent with previous studies on the evolution of host-plant associations in bees (Müller, 1996a;Sipes and Tepedino, 2005;Patiny et al, 2007;Larkin et al, 2008).…”

“…However, several studies showed that field records are not always reliable enough to confidently determine the host-plant range because field records rarely distinguish between pollen and nectar foraging (e.g. Westrich and Schmidt, 1986;Westrich, 1990;Müller, 1996a;Sipes and Tepedino, 2005;Timmermann and Kuhlmann, unpubl. data.).…”

“…Host switches based on morphological similarity among host-plants has been demonstrated in some previous studies. For example, several genera within the subfamily Rophitinae (Halictidae) (Penapis, Protodufourea and Xeralictus; reviewed by Patiny et al, 2007), Diadasia (Apidae; Sipes and Wolf, 2001;Sipes and Tepedino, 2005) and Macrotera (Andrenidae; Danforth, 1996) present equivalent shifts to unrelated, but morphologically similar, host-plants.…”

“…Only a few previous studies have examined the pattern of host-plant evolution in specialist bees using phylogenies at the species level (Müller, 1996a;Sipes and Wolf, 2001;Michez et al, 2004b;Sipes and Tepedino, 2005;Larkin et al, 2008). Such studies have generally indicated that host-plant associations are maintained over multiple speciation events (suggesting that host-switching is not always associated with speciation), but that when host switches occur, they occur to distantly related host-plant families.…”

Analysis of pollen loads in a wild bee community (Hymenoptera: Apidae) – a method for elucidating habitat use and foraging distances

“…Firstly, pollen analysis revealed the actual use of pollen resources and excluded nectar-only collection and other bee activities that were included in observation of bee visits. Sipes and Tepedino (2005) pointed out that bees visit a relatively large number of plant species because of their nectar rather than pollen production (e.g., in our case, probably Dianthus carthusianorum). Secondly, results of pollen analysis demonstrate that the bee visits were not limited to typical plant species of sand ecosystems but that they also utilise resources in the surrounding agricultural landscape.…”