The last few years I regularly come across reports and specimens of spring bees that have been found in the fall. I have been looking for records from before 2000, but couldn't find any very reliable records so far.
In collection material from a large survey of the Badlands National Park in South Dakota I found a female of Andrena cressonii. The bee was captured by us in pan traps in the third week of October 2010. In 2011 around half October I found a female Andrena spiraeana on frost asters.
In both cases the female bees were clearly freshly emerged individuals, without any signs of worn hairs or wings.
In recent years I came across several records of vernal bees found
during fall. I could make the following provisional list of species that
emerged in fall in Western Europe: Andrena vaga, Andrena
carantonica, Andrena cineraria, Andrena nitida, Andrena praecox,
Andrena chrysosceles, Andrena haemorrhoa, Andrena flavipes, Andrena ventralis, Osmia
aurulenta, Nomada alboguttata and Lasioglossum pallens.
In older literature I could find records of spring anthophorid bees, like Anthophora plumipes,
Anthophora canescens, Anthophora crassipes, Anthophora dispar, Melecta italica, Melecta
leucorhyncha and Melecta albifrons. These anthophorids have been found to emerge in the fall mostly in the Mediterranean.
There are certainly more species to add. The emergence of vernal
bees in fall is likely nothing new, but it seems to occur more
regularly over the past few years. The effect of global warming on weather patterns might be an important factor.
The increased interest in solitary
bees in many parts of Europe also contributes to the increasing number of records. So, it might be an overlooked phenomenon that occurs every year. The emergence occurs from the end of September to November.
There are several causes that have been discussed:
- disturbance ex. erosion of nest sites causing the exposure of nest cells with adult bees in diapause.
- weather patterns ex. unusual warm periods late in fall after a cold, rainy summer
- parasites ex. Stylops melittae
The absence of any records
from common vernal mason bees like Osmia rufa or Osmia cornuta in Europe is
remarkable. It seems the bees found so far make underground nests and
one constructs nest cells in snail shells.
The list also includes both bivoltine and univoltine species.
One would also
think that more male Andrena would be recorded, as they emerge usually earlier and occur in larger number at the beginning of the flight period. This is maybe true, but there are
also many records of female Andrena emerging in fall.
phenomenon is not restricted to bees. There are also records of vernal
beetles and moths in fall over the last few years in W-Europe.
I would be very grateful for any additional information on this phenomenon.