Of Pollen Pigs and Faithfull Bees in Saguaro National Park West

There are a few nice Poppy fields in Saguaro National Park West right off Picture Rocks Road, just where I cross the Tucson Mountains when I drive to the University. Randy and I consider this commute through the park a very good reason to live on a dirt road  NW of Picture Rocks.

 Otherwise poppies are scarce in SNMW, but the variety of wild flowers is great, from the earliest Fairy Dusters, Brittle Bushes and Lupines along the roads, Penstemons in sandy washes and many different members of the Mallow family, some of them spectacular fountains of orange, pink or white.

Desert Globemallow, Sphaeralcea ambigua

Tiny 'belly flowers' (so named by photographers with viewfinder cameras) sprout among warm red eroding rocks in sandy washes.

All of them attract insects, either offering nectar and pollen to more or less specialized pollinators, or not so voluntarily (I assume) offering to nurture larvae and adults with everything from buds to leaves, petals, developing fruit and final seeds. Many insects are so specialized that they can only be found in certain years at the exact time when there are enough  flowers of a certain species ...

Others are great generalists, like the ubiquitous honey bee. This, by the way, makes her a much less reliable pollinator than some of our native bees. Local entomologists call Apis mellifera the Pollen Pig: Abundant and able to shoulder the smaller native bees away from their food sources, ripping the base of deep-throated flowers to steal their nectar....they are the boars of the insect world.

Caliche Globemallow, Sphaeralcea laxa

Yesterday in Kings Canyon, I found Caliche Globe Mallow (Sphaeralcea laxa) bushes buzzing with small bees. Male and female bees visited flower after flower, even forced their way into some half-open buds.

Adrena sphaeralceae visiting its Globemallow. Above female, lower right male

 Reaching for nectar at the bottom of the flower the bees curled their furry bodies around the center column that is formed by distinctive dark purple anthers and the pistil. Powdered richly with pollen in the process, they immediately visited another mallow flower. They never seemed to stray away from that particular species of mallows.

Adrena sphaeralceae

John Asher identified the bees from my photos as Adrena sphaeralceae and called them oligoleges of  Globe Mallows, Sphaeralcea. In pollination ecology, Oligolecty refers to the narrow, specialized preference of certain bees species for pollen sources, typically for a single genus of flowering plants.
Interestingly, there are other bee species from different genera tribes that are also specialized on Malvaceae, for example certain Melitoma  and  Diadesia species. 

Although Darwin already described some of the extremest cases of interdependency between certain bee and orchid species, many questions about  the evolution of oligolecty are still unanswered. While the benefit for the flowering plant seems obvious, cost and advantage for the bee are more difficult to analyze. It is not quite clear whether oligolecty is based on the inability of bee larvae to digest any but the preferred pollen, or if the adults are just not able to recognize other flowers as food sources. Speculations have been discounted that smaller species with a shorter flight radius are most likely to be oligoleges.
I was surprised to learn that about 30% of European and Asian bees are oligoleges. I couldn't find any data for the Americas, but they are probably similar.Definitely a topic to learn more about.