Hymenoptera (ants, bees, wasps) are second only to beetles for insect diversity in North America. Most people recognize honey bees and bumble bees, the two common social bee families, but that is only the beginning. In fact, most bees have neither hives nor queens. The majority of bees live solitary lives. Instead of hives, they excavate nests in dirt or dead wood. These self-sufficient little creatures have unique habits which, when combined with their big eyes and fuzzy bodies (the 'cute' factor), make them intriguing objects of study.
Megachile spp. (brevis?) says: "What're you looking at?"
Their small size and high diversity can make them difficult to identify to species without magnification in a lab setting. However, I have convinced myself that I got at least two bees to genus on a prickly pear (Opuntia spp) blossom last Saturday.
When getting a rough ID of a bee on the fly, there are a number of traits to consider. Things like behavior (posture, vocalizations, choice of food), body shape, placement of fuzzy bands, placement of scopa (pollen collecting hairs), and coloration are all good characteristics to remember.
Megachile spp. and crab spider on Opuntia (prickly pear) blossom
Keeping this in mind, I decided that two of the bees I observed fussing over the cactus flowers were in the Diadasia and Megachile genus, respectively. Megachilids (commonly referred to as 'leaf-cutter' bees for their habit of excising semicircular patches from leaves to furnish their nests) are easy to spot. They have relatively large bobble-heads for their size, make a high-pitched humming noise, and, most distinctively, collect pollen on their bellies instead of their legs. They forage with a 'bottoms-up' posture that looks a little goofy on the flower.
Megachile "bottoms up!" pose
Diadasia can be a little more challenging. To the relatively untrained eye (i.e., mine), Diadasia resemble Melissodes. Both are medium-sized fuzzy bees with large scopa on their hind legs. The differences are in the details. Diadasia are overall a bit fuzzier, and they have round foreheads (vertices) instead of a flattened vertex like the Melissodes bees. The final factor that hinted "Diadasia" was that this bee was observed on a cactus.
Confirmed as Diadasia australis on BugGuide.com by Dr. John S. Ascher
Diadasia are one of the many bees referred to as 'cactus bees'. All sorts of bees love cactus, but Diadasia species are often classified as 'oligoleges' (think oligo- few), that is, they specialize on a few sources of pollen. A number of Diadasia have a special affection for cactus flowers. While many Diadasia love cacti, they do feed on other flowers. Oligoleges tend to be more particular about the pollen they collect than the nectar they sip. Another common name for Diadasia is "chimney bee". They build turrets around the entrance to their nests in the ground. I didn't find this little lady's nest, but if you see a tunnel with a turret, you'll know a Diadasia might be calling that nest home.
One last thing to know about the vast body of bees: despite the fact that you can see I totally got up in their faces, these native bees didn't so much as buzz me. Most bees rarely sting. This is doubly true for solitary bees without a hive to defend.