|Foothills Palo Verdes bloom along Picture Rocks Road in the Tucson Mountains, Arizona|
Legume Trees are blooming all over the Sonoran Desert. Those hunger specialists with nitrogen-binding symbionts attached to their root system dominate habitats with poor sandy soils from Texas to California.
|Ironwood Tree in Saguaro National Park West|
|Ironwood flowers are built like bean or pea flowers|
While Acacias and Mesquites expose their pollen in very reduced flowers to any willing pollinator and even the wind, PaloVerde and even more so Ironwood and the western Smoke Tree have retained most of the typical leguminose flower architecture that requires insects with a short proboscis but a sturdy body to to push their way inside towards the nectar. This triggers an upward movement of the flower's stylus which then rubs along the furry underside of the bee to ensure pollen transfer to the stigma.
|Centris pallida at Ironwood flower|
The main pollinator of Palo Verde and Ironwood Trees is the Digger Bee Centris pallida. As soon as the chilly desert night gives way to the warmth of an early May morning, the droning sound of the big bees fills the air around every blooming tree. Females zap from flower to flower, visiting each only for seconds but quickly filling the pronounced pollen baskets on their hind legs.
|A white-faced Centris pallida male|
Both strategies, searching for and digging up emerging females, and releasing pheromones and defending territories, have been shown to be energy intensive (Vinson et al. 1982). During my attempts to photograph them, I found that males often take brakes from their vigil, usually on the inner branches of the tree. There they fall for several minutes into a torpor like state. I actually coaxed one onto my finger where he kept sleeping peacefully. This was the chance to shoot close-up portraits showing clearly the male's white clypeus (the female has a dark face).
|Female collecting nectar and Pollen|
|Female digging a brood chamber and distributing debris|
Literature: Chemical Ecology of Bees of the Genus Centris SB Radleigh, V Inson, I Gordon, WF Rankie, HJ Williams 1996