Many bees in the warm deserts of North America are pollen specialist species, which suggests pollen host specialization and high species richness of bees are related. The bees that visit creosote bush (Larrea tridentata) flowers offer an insight into an advantage that pollen specialization may provide to desert bees. Creosote bush is one of the most drought-tolerant desert shrubs in North America and hosts 20 species of pollen specialist bees, along with many pollen generalist species. Samples made across its range in the United States unexpectedly found that specialist bees dominate where drought has been most common over the last century. The pollen specialist bees of creosote bush were most abundant and species-rich where their host blooms least frequently. Bees, like many insects, can skip years when drought interrupts flowering that would occur in wetter years. Unpredictable flowering due to drought may favor the origin of pollen specialization if specialists are better able than generalists to predict when flowering occurs (Minckley et al 2000). A drought that occurred during our San Bernardino Valley bee survey provided additional support for this hypothesis. During a severe drought, those pollen specialist bees reliant on plants that did not bloom remained in diapause whereas bees were active that visited the few plants that bloomed regardless of rain (Minckley et al., 2013).

Unpredictable drought is an ecological constraint that may favor the evolution of pollen specialization in desert bees. Broad-scale community studies also suggest that ecological opportunity underpins why pollen specialization evolves. Oligolectic bee species often use plants that have open, easily accessible flowers with superabundant resources. Plants with these characteristics include willows (Salixspp.) sunflower (Helianthus annuus), Prickly pear cactus (Opuntiaspp.), and creosote bush (Minckley & Roulston 2006). Not only do they attract large specialist bee faunas, generalist bee species are also abundant and surely equally suitable as pollinators as pollen specialist bees. The data for bees and their hosts from the San Bernardino Valley can be examined here.

One bee that specializes on a superabundant resource is Dieunomia triangulifera,a specialist bee that visits only sunflower (Helianthus annuus) for pollen. Females collected more pollen per trip, and took less time per trip if they are collecting pollen when most pollen is available over the day and during the season (Minckley et al., 1994). The synchrony in life history and behavior contribute importantly to how this bee interacts with its sole floral host.

Pollen specialist bees use a tremendous array of plant hosts and there is a vast, and growing, literature that their pollens differ in chemistry, morphology, nutritional value and how accessible pollen is in the flower. Plant attributes, phylogeny, and the ecological context contribute to the origin and maintenance of pollen host breadth in bees (Danforth, Minckley & Neff 2019; Minckley & Roulston 2006)