Use of novel pollen species by specialist and generalist solitary bees (Hymenoptera: Megachilidae)

Abstract

If trade-offs between flexibility to use a range of host species and efficiency on a limited set underlie the evolution of diet breadth, one resulting prediction is that specialists ought to be more restricted than generalists in their ability to use novel resource species. I used foraging tests and feeding trials to compare the ability of a generalist and a specialist solitary mason bee species to collect and develop on two pollen species that are not normally used in natural populations (novel pollens). Osmia lignaria (Hymenoptera: Megachilidae) is a generalist pollen feeder; O. californica, is more specialized. Adults of the specialist were more limited in use of novel hosts, but only in some contexts. Both bee species refused to collect one novel pollen. The specialist accepted a second novel pollen only when it was presented along with its normal pollen, whereas the generalist collected novel pollen whether presented alone or with normal pollen. Surprisingly, larvae of the specialist were more flexible than were generalists. The specialist grew well on mixtures of normal and novel pollen species, in some cases better than on its normal host alone. Larvae of the generalist grew more poorly on all diets containing novel pollens than on their normal host. Data on these two species of bees suggest that specialization by itself need not reduce flexibility on novel hosts. The findings also provide information about mechanisms of specialization in bees. Similar to some folivores, specific cues of the pollen host and the bee's interpretation of these contribute, along with foraging economics, to pollen choice by adults. The ability of the larvae to cope with specific components of one pollen species need not interfere with its ability to use others.

Diet characterisation of solitary bees on farmland: dietary specialisation predicts rarity

“…One of the suggested mechanisms by which oligolecty is maintained is that plants may chemically protect their pollen to prevent overexploitation, thus necessitating specialisation to process difficult metabolites (Praz et al 2008). Asteraceae pollen is known to have a low protein content (Roulston et al 2000;Hanley et al 2008), is lacking in essential amino acids (Wille et al 1985) and may possess a toxic pollenkitt, the oily liquid found on the surface of the pollen grain (Williams 2003). Consequently Asteraceae pollen is difficult to utilize by non-specialised bees, even in widely polylectic species such as the honey bee (Herbert et al 1970) and solitary bees such as Osmia lignaria (Williams 2003).…”

“…Asteraceae pollen is known to have a low protein content (Roulston et al 2000;Hanley et al 2008), is lacking in essential amino acids (Wille et al 1985) and may possess a toxic pollenkitt, the oily liquid found on the surface of the pollen grain (Williams 2003). Consequently Asteraceae pollen is difficult to utilize by non-specialised bees, even in widely polylectic species such as the honey bee (Herbert et al 1970) and solitary bees such as Osmia lignaria (Williams 2003). Even bees specialised on Asteraceae pollen may incur other costs such as extended development time, as in a comparison of specialised bees the Asteraceae oligolec Heriades truncorum had the longest development time on its preferred pollen despite being the smallest bee in the comparison (Praz et al 2008).…”

An examination of synchrony between insect emergence and flowering in Rocky Mountain meadows

“…For example, although Hoplitis fulgida missed Lathyrus flowering at several sites in 2009, it maintained fairly good synchrony with Potentilla, a commonly used pollen source for this bee. However, even in generalist bees, larval growth and survival can vary significantly according to the pollen species in the diet, even if these are pollens that adult females are willing to collect for their offspring (Williams 2003). Studies monitoring the consequences for bee populations of changes in resource use will therefore be an important, but challenging, future step.…”

Specialized Bees Fail to Develop on Non-Host Pollen: Do Plants Chemically Protect Their Pollen

“…Only a few observations on the performance of bee larvae on non-host pollen have been reported. Two larvae of the oligolectic bee Nomadopsis zonalis grew normally on non-host pollen (Rozen 1963); Lasioglossum galpinsiae, a bee species strictly specialized on Oenothera (Onagraceae), developed on pollen of Medicago sativa (Fabaceae) (Bohart and Youssef 1976); and the larvae of the Asteraceae specialist Osmia californica developed on pollen of Hydrophyllaceae and Brassicaceae (Williams 2003). Such sparse evidence led to the tentative suggestion that floral specializations in bees are not linked to the chemical composition of the pollen (Wcislo andCane 1996, Minckley andRoulston 2006).…”