Vernal pool flowers and their 
specialist bee pollinators

Robbin W. Thorp
Professor of Entomology Emeritus, University of California, Davis

with most photos by Dennis L. Briggs,
taken at Jepson Prairie Reserve, 11 miles south of Dixon, Solano Co., CA

Visitors to Vernal Pool Flowers

Showy masses of early spring wildflowers characterize vernal pool ecosystems. Flowering plants specialized for life in vernal pools include: Blennosperma (yellow carpet), Limnanthes (meadowfoam), Lasthenia (goldfields) and Downingia (downingias or sky blues). Although pleasing to our eyes, their colorful displays are designed to attract insect visitors, especially native solitary bees that specialize in collecting pollen from them. 

Each of the four plant genera has one or more native, solitary, specialist bees that collect pollen only from them. In turn, these bees provide effective pollination services for their floral hosts (Figs. 1, 3, 8, 12, 14). Female bees that specialize in collecting pollen from one or a few closely related species of flowering plants are called oligoleges, the process is oligolecty (oligo = few; legere = to gather). These specialist (oligolectic) bees require pollen from their floral hosts to provide protein for development of their offspring. Life cycles of these bees are closely synchronized with those of their pollen host flowers (see below). 

Generalist bees and other flower visiting insects also visit most of these flowers (Figs. 9, 10, 11, 15) and may potentially pollinate them. Do the flowers require the specialist bees for their reproduction? Certainly in the case of Blennosperma nanum, Dr. Joan Leong demonstrated in her PhD thesis, that seed set was significantly lowered in experimental plant arrays placed at a created vernal pool site where only generalist pollinators were available as compared to those experimental plant arrays placed at Jepson Prairie Reserve where there is a complete guild of flower visitors including the specialist (oligolectic) bee, Andrena (Diandrena) blennospermatis. Her experiments show that the specialist bee is important to the reproduction of the plant.

Figure 1:
A female of the solitary, pollen specialist bee, Andrena (Diandrena) blennospermatis collecting pollen from Blennosperma nanum

Figure 2:
Downingia concolor male phase

Figure 3:
A female of the solitary, pollen specialist bee, Panurginus new species, collecting pollen from Downingia

Figure 4:
 Downingia concolor female phase

Figure 5:
Downingia bacigalupii male phase

Figure 6:
A generalist female solitary bee, Lasioglossum (Dialictus) species, collecting pollen from Downingia bacigalupii

Figure 7:
Downingia bacagalupii female phase

Figure 8:
A female of the solitary, pollen specialist bee, Andrena (Diandrena) submoesta, collecting pollen from Lasthenia

Figure 9:
A female cuckoo bee (Nomada species) nectaring on Lasthenia

Figure 10:
A bee fly (Bombyliidae) nectaring on Lasthenia

Figure 11:
A soft winged beetle (Melyridae (Dasytidae)) feeding on pollen of Lasthenia

Figure 12:
A female of the solitary, pollen specialist bee, Andrena (Hesperandrena) limnanthis, collecting pollen from Limnanthes douglasii var. rosea

Figure 13:
A male of Andrena (Hesperandrena) limnanthis that has spent the night in a flower of Limnanthes douglasii var. rosea

Figure 14:
A female of the solitary, pollen specialist bee, Panurginus occidentalis, collecting pollen from Limnanthes [alba?]

Figure 15:
A generalist worker honey bee, Apis mellifera, collecting pollen from Limnanthes douglasii var. rosea

Life cycle solitary ground nesting bees 
of the genus Andrena [Hymenoptera: Andrenidae]

Female bees of the genus Andrena that specialize on pollen of vernal pool flowers are on the wing only in early spring when their host plants are in flower. The life cycle of the bee is closely tuned to that of its host plant bloom period. The bees emerge at or just before their host flowers initiate bloom. Male bees emerge shortly before the females. Mating occurs as soon as females emerge. Females initiate construction of their brood nests immediately after mating. 

The female bees construct their nest in the soil of the upland areas surrounding the pools. The presence of such bee nests is revealed by ant mound-like clumps of excavated soil (tumuli) at the surface of of the soil. These are best seen in the early morning while the soil excavated and pushed to the surface by the female during the night is still damp. As these circular piles of earth dry, they may be eroded by wind and rain. The tumuli (Fig. 16) may be clustered in areas of suitable soil, or scattered over the area. They may be associated with bare soil, but often become hidden beneath growing vegetation such as the expanding basal leaf rosettes of filaree. There is no indication of social structure in the biology of these bees. A closed tumulus (Fig. 17) indicates the female is in her burrow constructing it. An open tumulus (Fig. 18) indicates the female bee is afield foraging for pollen and/or nectar as provisions for her offspring. 

The basic nest architecture (Fig. 19) consists of a vertical shaft penetrating the soil for a few inches before becoming a lateral tunnel ending in a single chamber (brood cell). A newly constructed brood cell (Fig. 20) has a polished look from the water-proof lining. This waxy lining is secreted by the female and painted over the inner surface of the brood cell. 

When brood cell construction is complete, the female bee forages (Fig. 21) for pollen on her preferred host flowers. She grooms pollen from her body and packs it into specialized brushes of hair (scopae) on her hind legs for transport back to her nest. As the dry pollen food provisions (Fig. 22) accumulate in the brood cell, the female adds nectar, moistening the mass to dough-like consistency. When the pollen ball (Fig. 23) is complete, she lays an egg on top. She then constructs a brood cell cap (Fig. 24) closing off the entrance to chamber with spirally arranged soil particles. 

After closing off the brood cell the mother bee has no further contact with her offspring. Thus, she is a solitary bee in contrast to the social honey bee. She then digs another lateral tunnel, pushing the soil into the tunnel leading to the most recently completed brood cell. In the photo showing the nest architecture (Fig. 19) a second, completed, brood cell with pollen ball and egg is visible to the left. The tunnel that connected this cell to the vertical shaft was filled and no longer visible. If one could look through the soil from above, the nest would look like a series of spokes radiating out from the vertical entrance shaft, each spoke ending in a single brood cell. 

The newly hatched larva (Fig. 25) begins feeding on the provisions. After consuming all the provisions, the post-feeding larva (Fig. 26) defecates and remains quiescent in its brood cell throughout the remainder of the spring and well into summer. In about September, it transforms to a pupa (Fig. 27) which is the transition stage between the grub-like larva and the adult bee. The adult remains quiescent in its brood cell until the following spring when it emerges to continue its life cycle in association with the flowering of its pollen host plant. 

NB: The majority of the life cycle of these bees is spent in the brood cell. Overwintering as an adult rather than a post-feeding larva is an adaptation to synchronize these bees with the early spring bloom of their pollen host plants. In contrast to most female insects that pump out hundreds of eggs, these solitary bees may lay only 20 to 30 eggs in their lifetimes. Most of their effort goes into providing a secure haven with a mass of food provisions adequate for their development. This maternal investment and year long, single generation life cycle means that populations of these bees recover very slowly from any catastrophic losses.

Figure 16:
Nest tumuli of Andrena on the surface of the ground

Figure 17:
A closed nest tumulus with the female bee inside her burrow

Figure 18:
An open nest tumulus indicates that the female bee is outside foraging

Figure 19:
Nest architecture - a vertical shaft and a lateral tunnel ending in a single brood cell . A second, completed brood cell with pollen ball and egg is visible to the left.

Figure 20:
Brood cell with water-proof lining

Figure 21:
Female of Andrena (Hesperandrena) species collecting pollen from Lasthenia

Figure 22:
Food provisions - early stage of formation of pollen ball in a brood cell

Figure 23:
Pollen ball with bee egg on top

Figure 24:
Brood cell cap (closure) inner view

Figure 25:
Newly hatched bee larva on top of the pollen ball provision

Figure 26:
Post-feeding larva in its brood cell

Figure 27:
Pupa in its brood cell

About the Contributors

Robbin Thorp is Professor Emeritus of Entomology. Robbin initiated research on native bees and flowers of the vernal pool ecosystem shortly after joining the faculty at University of California, Davis in 1964. He continues his studies following his retirement in 1994. He also retains appointment as Chair of the Jepson Prairie Reserve Advisory Committee at UC Davis.

Dennis Briggs was a Staff Research Assistant for Professor Thorp's pollination research from 1967 to 1990. Dennis is responsible for most of the close-up photographs of bees and flowers in this contribution.

Contact Information

Robbin W. Thorp
Professor Emeritus
Department of Entomology
University of California
One Shields Avenue
Davis, CA  95616-8584

email: rwthorp@ucdavis.edu