Pollination of Scutellaria mexicana (Lamiaceae) in Southern Nevada by Anthophora Bees

Scutellaria mexicana (Lamiaceae) is a common shrub in the Mojave and northern Sonoran Deserts in North America. The plant produces small tubular and bilabiate flowers that are nototribic by enclosing 2 pairs of stamens and a style within a hooded upper corolla lip. I investigated the pollination of the species in southern Nevada during April 2018 by aspirating insects from flowers. I estimated amounts of S. mexicana pollen on the front of the head and dorsum of the thorax, determined proportions of conspecific pollen on the head, thorax, and middle and hind legs, and examined correspondence between measurements of insect and flower structure. All insects collected on flowers were similarly sized bees in Anthophora (Apidae), mostly comprising A. coptognatha females, followed by A. centriformis males and females. Bees landed on flowers and fed on nectar with the front of the head against lobes that extended laterally from the base of the upper corolla lip. Large and varying amounts of conspecific pollen were carried on the head and thorax of bees, consistent with S. mexicana's nototribic flowers. Bees appeared specific to the plant because they carried high proportions of S. mexicana pollen. Proportions of conspecific pollen on bees differed among collection sites, likely due to different floral compositions. Lower proportions of conspecific pollen on the legs compared with the head or thorax indicated that pollen from plants other than S. mexicana was more likely to be transferred to the legs by bees of both sexes and transported to larvae by females. The length of Anthophora's long glossa corresponded to the depth of the corolla and the availability of nectar. The vertex of the head and anterior dorsum of the thorax of bees feeding on nectar would align with the anthers and stigma on the flowers. Scutellaria mexicana in southern Nevada appears to be dependent on Anthophora bees for pollination.