Selection for late summer phenology?
Caveat: the following includes free-association speculation not based on careful reading of literature. This is a journal; I get to do this.
If you are from the east, the WIllamette Valley climate is an adjustment. In Michigan or Connecticut it actually rains in the summer, so you can find many flower-visiting insects on any decent day from April to November. Late summer is particularly rich, as goldenrod and aster flower in dense stands. By contrast, western Oregon is dry from mid-June until mid-fall, and nectar and pollen resources fall to near-zero in most habitats. Any non-honeybee attempting to forage in August is wasting its time.
Unless it lives in the city. In lots of urban areas, floral resources abound. Where I live (Corvallis), most yards are dead grass, but on any block there will be one or two properties jammed with late summer flowers and an abundance of bees and flies. I've seen Halictus, Megachile, Triepeolus, Agapostemon, Melissodes, Bombus, Ceratina, and honeybees. Plus syrphids including Eristalis, Eupeodes, Myathropa, and Syritta. And cabbage, alfalfa, skipper, and gray hairstreak butterflies. There has to be a powerful selective advantage for insects that are active for two or three months beyond the natural wildflower season. For bees this would include larger or better-provisioned broods. For some syrphids, it could mean additional generations. But of course it is also complicated. Consider possible cases.
Halictus ligatus as a species exhibits some plasticity in the colony cycle, which can be multivoltine, with pauses during dry conditions (in southern Florida). Could either feature appear in bees in Oregon's Mediterranean climate? If so, that might amplify reproduction. But what would happen to the sex ratio during the colony cycle? I note that where I am, males are predominant (but not exclusive) in September; these might have minimal effect on populations.
Osmia lignaria of the Willamette Valley is the subject of a just-out publication examining the (impressive) rate of brood cell production over the month or so that nests are active. The life cycle is probably of fixed length; O. lignaria will enter diapause regardless of an extended availability of floral resources. If so, effects of an extended floral season might be indirect, favoring competitors or cuckoo bee enemies.
Eristalis tenax is observable in the Willamette Valley in any month of the year--a longer activity window than other common Eristalis. Scanning iNaturalist observations, it is pretty well represented on wildflowers--esp yarrow and Queen Anne's lace--that occur in dry conditions late in the season. But in a city, these resources are dwarfed by garden landscape plants that are also exploited by E. tenax, which enjoys multiple generations. There is at least the potential that their populations would be significantly augmented by late summer landscape plants, favoring this species over others.
There is some literature on this question - E.g., Temporal dynamics influenced by global change: Bee community phenology in urban, agricultural, and natural landscapes, which reviews fairly scant literature. But urban landscapes deserve consideration, as mini-experiments in adaptation, and as habitats that are expanding with increasing interest in native plant landscaping.
Along these lines, I have the pet idea that it would be really interesting to study pollinators in botanical gardens, which have diverse kinds of flowers in a large area, well defined and generally isolated from other irrigated landscapes.
Below: some August/September flower visitors in Corvallis.
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I also wonder about the trophic effects of these late-season "abundant" floral resources. But, first ,can we assume nectar and pollen are constant? Does the tap slow down when things get really dry? This year, we have had a lot of late season yellow jackets - more than I recall - and i've wondered if it is related to our increasing efforts to provide floral resources for our honeybees (from which the other taxa benefit as well). Each year, we plant more late- and long-flowering plants (lately borage and sunflower). I can see that our bees are still pulling in regular load of pollen. Our yellow jacket traps are also filling up fast. Another neat observation is that while mowing our prairie I disturbed multiple yellow jacket nests. The swallows were thick around the tractor as they dive-bombed them.
Another anecdote: Melissodes, males and females, minor players in natural areas, are super-abundant in Corvallis this year, absolutely the dominant visitor to sunflowers, which become available after flowers in unmanaged areas are senesced. One of the more surprising consequences of the war in Ukraine.
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