The Biogeography of a Solitary Bee

Tucked away in the human mind, stirring in the imagination, alighting into myth and metaphor, is the bee. Upon mention of the bee a picture forms almost immediately: honey, the queen, flowers, the bumblebee, ah… the drone. But the honeybee, it turns out, is an introduced species from Europe (Powell & Hogue 1979). In California there are over 1000 species of native bees, most of which are considered solitary; there are no queens, workers, or drones, and the female bee is the only caretaker of her offspring (Powell & Hogue 1979). Within the genus Andrena over 150 species are found in California (Powell & Hogue 1979). One of these, Andrena (Hesperandrena) limnanthis, is a specialized species that relies on flora found in the margins of vernal pools (Thorp 1990).

The solitary bee, A. (H.) limnanthis, is smaller than your average honeybee. Females average 9mm in length and males tend to be 1mm shorter (Thorp 2001). A. (H.) limnanthis is not flashy in its coloration; it sports a dull black body and its thorax, abdomen, and legs are dusted with light brown hairs. The hairs are adaptations that aid in pollen collection (Powell & Hogue 1979). Delicate, transparent wings extend gracefully from its body. Taxonomists named the order Hymenoptera, under which all bees are classified, after the membranous wings found on bees, wasps, and some ants. Like all bees, A. (H.) limnanthis has two sets of wings with a larger pair in front and a smaller set behind. In flight, the bee links its wings together with hooklets found on the front of the hind wing, and when landed, it disengages and folds them against its body (O’Toole and Raw 1991).

As mentioned above, A. (H.) limnanthis is dependent on flora found at the edges of vernal pools. Vernal pools in California are related to our Mediterranean climate (Stone 1990). They are seasonal pools that form during the rainy winter and are generally gone by late spring or early summer. They form as winter rains build up underground, trapped by a hardpan layer, and surface in depressions of shallow soil (Thorp & Leong 1995: 4). They are famous for their unique flora (Stone 1990). Grassland plants and marsh plants cannot colonize this specialized habitat because of inundation throughout the growing season and very dry conditions in the summer and fall (Stone 1990). Thus, gorgeous annuals often endemic to this unique Californian climate, such as Limnanthes (meadow foam), can dominate the vernal pool habitat (Stone 1990). It is to this annual, Limnanthes, that the solitary bee A. (H.) limnanthis is true. A. (H.) limnanthis is oligolectic on (pollinates and derives its sustenance only from) certain species of Limnanthes, which is found in vernal pools in California. Females of this species of solitary bee mainly collect pollen from Limnanthes douglasii, but they also visit L. alba, L. montana, L. striata, and an endangered species L. vinculans (Thorp & Leong 1998). Because they collect pollen for food they inadvertently play an important role in pollinating these specialized vernal pool Limnanthes plants. These bees depend on the vernal pool habitat and they contribute to the richness of species in California.

This vernal pool bee, while restricted to the area inhabited by its host for food, nests in adjacent upland areas in the spring (Thorp & Leong 1998). The females need to nest in a drier area. They burrow small tunnels into the ground that extend vertically for 10-30 cm and finish with a lateral “single brood cell” (Thorp 1990, 112). The female bee lines this terminal cell with a secreted waterproof substance and then deposits collected pollen from the meadow foam plant into the prepared space (Thorp 1990). This substance, secreted from Dufour’s gland, and consisting of chemicals called terpenes, is critical because it helps to keep water out and it fights fungal attacks (O’Toole and Raw 1991).

It is worth mentioning that solitary bees are different than most insects in their reproductive strategies (O’Toole and Raw 1991). They lay fewer than thirty eggs in their lifetime (O’Toole and Raw 1991). Most insects lay hundreds of eggs, even in one day, in order to increase the chances of survival. The solitary bee assures the survival of some of her offspring by putting her time and energy into provisions for them, a strategy known as ‘maternal investment’ (O’Toole and Raw 1991).

The males live a shorter life than do the females, but they are protandrous: they emerge from their cells a few days earlier (O’Toole and Raw 1991, Thorp 1990). The males then wait for the females to emerge so that they can mate with them. Male bees from the genus Andrena mark non-flowering plants to create a patrol territory inside the host plant area (Ramel 2001). Inside their patrol they fly along trails or circuits that they establish by marking plants with mandibular gland scents, and if a female enters the circuit they will pursue and attempt to mate with her (O’Toole and Raw 1991, 160).

The order Hymenoptera contains insects known as bees, wasps, ants, sawflies, and horntails. Insects are thought to have appeared sometime during the Devonian, but the fossil record does not really begin in until the Upper Carboniferous (Boudreaux 1979). Given the vast number of insect species that exist today (they far outnumber all other species combined), and the sparse insect fossil record, it is difficult to assign ancestry to a fossil specimen (Boudreaux 1979). Such is the case for bees. Bees evolved from solitary wasps sometime during the Cretaceous period (76 million to 146 million years ago (O’Neill 2001; O’Toole 1991). Wasps all belong to the suborder Apocrita, as do bees, but it was from an ancestor of one distinct family of wasps, Sphecidae, under the superfamily Apoidea, that over 20,000 species of bees descended (O’Neill 2001). This evolution, from wasp to bee, was characterized by a change in feeding behavior. Bees feed on pollen and nectar, and wasps, for the most part, feed on insect prey. O’Neill, a wasp researcher, sums up the situation by asserting that bees are just “exceedingly hairy wasps” (2001: 135). He explains that specialized pollen feeding has evolved independently three times (O’Neill 2001). For example, under a completely different superfamily of wasps, Vespoidea, some species have evolved bee-like behavior in that they collect pollen and nectar, instead of insect prey, to feed themselves and their larvae (O’Neill 2001; O’Toole 1991; Malyshev 1968). A. (H.) Limnanthis is considered to be a lower sphecid bee, which means that it derives from the line of ‘true bees’ under the superfamily Apoidea (Malyshev 1968).

Phylogenetic theory seeks to explain these evolutionary relationships within groups of organisms. Phylogeneticists often use cladograms (similar to a family tree) to show these relationships by delineating likely points of divergence in the evolution of a group (Boudreaux 1979). It is assumed that a split has occurred within a group when different and derived traits of corresponding features are found in one subgroup and not the other (Boudreaux 1979). Through this line of study, evolutionary taxonomists have come to argue about the evolutionary sequence, and thus the classification, of bees. Most traditions of classification have placed the bees into nine and sometimes eleven families (Brothers 1999, O’Toole 1991). However, some cladisticians insist that there are no easily distinguishable splits that necessitate separate bee families (Brothers 1999). Brothers also argues that a single bee family would lend consistency to the Aculeate cladogram based on the classification of other organisms in the division (ants and wasps) (1999).

Bee-like behavior only became possible during the Cretaceous period when the first true flowering plants appeared (O’Toole 1991). Malyshev hypothesized in 1968 that true bees evolved from ancestors of the sphecid genus Psenulus (O’Toole 1991). This genus of wasp feeds on honeydew, a sweet substance excreted by aphids, and they feed the aphids themselves to their larvae (Malyshev 1968: 284-285; O’Toole 1991: 20). The idea is that Psenuloid wasp ancestors must have begun to eat these sweet substances as well, and this was a precursor to the eventual development of wasps that began to feed on pollen (Malyshev 1968: 286). It follows that as some wasps began to feed on pollen and left behind their carnivorous ways, thus becoming bees, they began to have an effect on flowering plants (O’Toole 1991). Plants with flowers that were attractive to bees were more likely to proliferate and diverge due to reproduction through cross-pollination (O’Toole 1991). Thus, a coevolutionary process was begun and bees and flowers began to adapt and change in relation to each other. It is clear that the coevolutionary process has taken place between certain Limnanthes varieties and A. (H.) limnanthis. Like all vernal pool flowering plants and their specialist bees, Limnanthes and A. (H.) limnanthis have developed evolutionarily in close association with each other in vernal pool habitats in California (Thorp & Leong 1995).

Figure 6: Composite cladogram based on Brothers 1999. Shows the phylogeny of the three Aculeate superfamilies that are part of the 14 superfamily evolutionary lineage forming the Hymenoptera (O’Neil 2001). Shows relationships of taxa at the family level and at the subfamily level for the Andrenidae family, under which A. (H.) limnanthis is classified.

The distribution of bees on earth is continuous where vegetation can be found on the ground (O’Toole and Raw 1991). This cosmopolitan distribution is not surprising given that the continents were just beginning to diverge when bees evolved during the Cretaceous (O’Toole 1991). The Andrenidae family of bees is found on all continents except Australia (O’Toole and Raw 1991). Within the genus Andrena, bees live in Eurasia and North America (O’Toole and Raw 1991). However, the species A. (H.) Limnanthis is endemic to California. Furthermore, this species is endemic to the geographically scattered vernal pool habitats that contain the plant species Limnanthes douglasii (Thorp & Leong 1998).

The vernal pool habitats were not always so scattered. A shallow sea covered the Great Valley of California during the Tertiary and into the earliest Pleistocene (Stone 1990). When it began to dry in the late Pleistocene, the ancestors of most contemporary vernal pool endemics probably invaded the newly available land (Stone 1990). It was at this time that the climate was beginning to produce wet winters and dry summers. As inundated areas disappeared, annuals would have given way to perennials except where poorly drained depressions resulted in seasonal pools (Stone 1990). Native bees forming close associations with these annuals would have become more disjunct in their distribution as well. Thus, an isolated situation emerged that allowed for highly derived species of oligolectic bees to evolve (Thorp 1990).

An oligolege of Limnanthes, A. (H.) Limnanthis is found within the ranges of the outcrossing Limnanthes plants (Limnanthes plants that reproduce with the aid of pollinators instead of self-pollinating) (Thorp & Leong 1998). These plants are found primarily in vernal pool settings as far south as San Luis Obispo, north to Humboldt County, and as far east as Fresno County (Thorp & Leong 1998). Thorp and Leong explain that it is difficult to know for certain the distribution of A. (H.) limnanthis in its vernal pool habitat (1998). While this species is relatively easy for bee researchers to distinguish and recognize, field research yielding evidence of geographic distribution is incomplete (Thorp & Leong 1998). However, Thorp and Leong were able to add to their own field collections with museum collections, and they show the data graphically in a map of specific site locations (Thorp & Leong 1998). Figure 9 is an adaptation of their map.

Several other endemic plants and animals have evolved in vernal pool habitats to form unique ecosystems in California. Fairy shrimp, spadefoot toads, salamanders, migrant waterfowl, shorebirds, grasses, goldfields, and blue downingia are just a few of the organisms that succeed in healthy vernal pool settings (Eaton 2001). However, vernal pool habitat has been adversely affected and lost to development. Agriculture and urbanization have claimed a quarter to half of this habitat in California (Eaton 2001). Much of the land that is characterized by these special ecosystems is privately owned and therefore more difficult to protect (Eaton 2001). Some habitat is protected and designated as refuge areas or preserves (Eaton 2001). Because endemism is so high, these ecosystems add to California’s biodiversity and should be protected. Conservation is critical because of the number of endangered, threatened, and candidate species that depend on vernal pools (Eaton 2001). It is important to remember that lands adjacent to actual pools are often part of the ecosystems as well (like the drier, upland, nesting area of solitary vernal pool bees) and must be included in conservation efforts.

One approach to conservation has been restoration and creation of vernal pools. Ferren and Gevirtz insist that such practices must be rooted in sound scientific and technologic processes (1990). However, they stress that restoration or creation of vernal pools is still in an experimental stage and cannot replace natural pools in mitigation of impact processes (Ferren & Gevirtz 1990).

With regards to A. (H.) limnanthis and other native vernal pool bees, Thorp and Leong have determined that these specialist solitary invertebrates should be included in conservation efforts (1995). They are important pollinators of vernal pool plants that in turn provide sustenance to small rodents and seed-eating birds (Thorp and Leong 1995). Moreover, these bees are limited in their flight and dispersal capabilities (Thorp and Leong 1995). Thus, they may not be able to travel the distances to find new habitats when their habitats are destroyed. Their endemism alone is reason enough to preserve the ephemeral pool habitat to which they are inextricably linked.

Brothers, D. J. 1999. “Phylogeny and evolution of wasps, ants and bees (Hymenoptera, Chrysidoidea, Vespoidea and Apoidea).” Zoologica Scripta 28(1-2): 233-249.

Ferren, W. R. Jr. and E. M. Gevirtz. “Restoration and creation of vernal pools: cookbook-recipes or complex science?” pp. 147-178. In H.I. Ikeda, Robert A. Schlising, F.J. Fuller, L.P. Fuller, L.P. Janeway, and P. Woods. eds. Vernal Pool Plants: Their Habitat and Biology. Chico, Ca.: The University Foundation.

Malyshev, S. I. 1968. Genesis of the Hymenoptera and the Phases of their Evolution. London: Methuen & Co. Ltd.

Neff, J. L. and B. B. Simpson. 1993. “Bees, Pollination Systems and Plant Diversity.” pp. 143-167. In J. LaSalle and I.D. Gauld, eds. Hymenoptera and Biodiversity. Wallingford, UK: C.A.B. International.

O’Neill, K. M. 2001. Solitary wasps: Behavior and natural history. Ithaca, New York: Comstock Publishing Associates.

O’Toole, C. and A. Raw. 1991. Bees of the World. London: Blandford Publishing.

Powell, J.A. and C.L. Hogue. 1979. California Insects. Berkeley, Ca.: University of California Press.

Ramel, G. (2001, September, 8) “Gordon’s solitary bee page: mating and nesting habits.” Earth-Life Web Productions. [Online] Available: http://www.earthlife.net/insects/solbees.html#2 [Accessed 4 October, 2001].

Schlising, Robert A. 1990. Photograph p. ii In H.I. Ikeda, Robert A. Schlising, F.J. Fuller, L.P. Fuller, L.P. Janeway, and P. Woods. eds. Vernal Pool Plants: Their Habitat and Biology. Chico: The University Foundation.

Stone, R.D. 1990. “California’s endemic vernal pool plants: Some factors influencing their rarity and endangerment.” pp. 89-108. In H.I. Ikeda, Robert A. Schlising, F.J. Fuller, L.P. Fuller, L.P. Janeway, and P. Woods. eds. Vernal Pool Plants: Their Habitat and Biology. Chico: The University Foundation.

Thorp, R. W. 1990. “Vernal pool flowers and host-specific bees.” pp. 109-122. In H.I. Ikeda, Robert A. Schlising, F.J. Fuller, L.P. Fuller, L.P. Janeway, and P. Woods. eds. Vernal Pool Plants: Their Habitat and Biology. Chico, Ca: The University Foundation.

Thorp, R. W. and J.M. Leong. 1995. “Native bee pollinators of vernal pool plants.” Fremontia 23(2): 3-7.

____________. 1998. “Specialist bee pollinators of showy vernal pool flowers.” pp. 169-179. In C.W. Witham, E.T. Bauder, D. Belk, W.R. Ferren Jr., and R. Ornduff, eds. Ecology, Conservation, and Management of Vernal Pool Ecosystems. Sacramento: California Native Plant Society. Based on proceedings from a 1996 Conference. [Online] Available: http://www.cnps.org/vernal pools/thorp.pdf/ [9 October, 2001].


Figure 2: Anatomy of a generic bee. Source: O'Toole and Raw 1991.	Figure 3: Andrena (Hesperandrena) limnanthis collecting pollen from Limnanthes douglasii. Source: Thorp 1990.