Red Mason bee male tactics

I posted about the Red Mason Bees (Osmia rufa) nesting in our bee post last year. The males  have now been active for a few weeks, and they are quite interesting to watch, as they are very inquisitive and interactive with other insects. The different behaviours of male bees depend to some extent on how the resources used by females - such as nesting sites and food plants - are distributed. These resources indicate where females are to be found, and to maximise their reproduction males are to encounter and mate with as many recently emerged females as possible. In Osmia rufa, females are thought to mate only once shortly after emerging from their nest, as males guard females after mating and impregnate them with chemicals that make the females unattractive to other males, effectively ensuring their paternity. As many other bee species, Red Mason Bees are polylectic - they are quite generalistic as to what flower species they feed on - and their nests are quite scattered (at least in natural habitats!), and therefore it does not pay to be territorial. In a 1988 paper I read today, Karsten Seidelmann graphically described Red Mason Bee behaviour based on 364 individually marked males:

In order to find mates, each male roamed about his home range. These home ranges were located around flowering groups of foods plants, but also by nesting places. Occasionally sunny places (exposed leaves, stones, etc.) or bushes of dwarf pine in the surrounding of large nest aggregations were inspected and patrolled as well. Home ranges sometimes also consisted of several small patchy encounter sites. Males then searched within the patches and flew in a straight line between them. Every home range contained an exposed sunny place where the resident male basked between patrol flights at irregular intervals. When home ranges become shadowed, males turned to other areas, returning when the area was in the sun again. Areas abandoned because of shadows were not occupied by other males.


The photo heading this post shows a male next to a potential nesting hole in our bee post. The males often enter and inspect the holes and look out.
These little bees often chase and make contact with other bees - not only of their own species but also other insects such as butterflies in their home range - and I had misinterpreted this behaviour as agressive, an indication of territorial behaviour. Seidelman's detailed observations, on the contrary, shows that the Red Mason Bee is not territorial. It does have a 'home range' but these are not exclusive to one individual and home ranges of different males partially or totally overlap. Males also occasionally went on 'excursions' away of their home range. Their chasing and making contact with other visually detected insects is an 'inspection' behaviour: they check if they are conspecific females or not, but they do not attempt to drive them away:

Home ranges ranged from 3 to 30 m2 in size. Males continuously altered their home ranges in response to changes in possible encounter sites. Plants that started to flower were integrated in adjacent home ranges, or new home ranges were established and old ones were abandoned. If females emerged from a large aggregation of nests, the searching activities of several males were concentrated entirely on these nesting places, with males inspecting nest entrances frequently.
Males inspected all resting and flying insects with an O. rufa-like shape during their patrol flights. They approached other solitary bees (e.g., Osmia, Megachile, Anthophora, Andrena), as well as honeybees (Apis), bumblebees (Bombus), or even flies (e.g., Calliphora). However, only receptive females of their own species were mounted, whereas males of O. rufa and other insects were abandoned immediately after a short contact.


Seidelman also analysed the reproductive success of males, that is, how many times they mated throughout their lives and concluded that this is independent of the male body size.

More information at:
Karsten Seidelmann (1999) The Race for Females: The Mating System of the Red Mason Bee, Osmia rufa (L.) (Hymenoptera: Megachilidae). Journal of Insect Behavior, Vol. 12:13-25. here.

Robert John Paxton (2005) Male mating behaviour and mating systems of bees: an overview. Apidologie 36:145–156. here.