Specialists vs. Generalists
Most bees can be separated into two categories of female pollen preferences, either specialists or generalists. Specialist bees have evolved a specific relationship with a few or even just one plant species. Some specialist bees forage for pollen that can only be found on one plant species. These specialists emerge from their nests at the same time their host plant begins to flower. The host flower sometimes depends on pollination from one specific bee species and the bee depends on pollen from their specific flower species. This mutualistic relationship can be found all over the world. On the other side of the spectrum, generalist bees are less picky about the flowers they visit. They often visit a wide range of flower types and species when seeking out pollen.
Male squash bees waiting for a female.
In our Berkeley, CA garden we have both specialist and generalist bees visiting the flowers. Two plants that attract specialist bees are squash (Cucurbita spp.) and sunflower (Helianthus annuus). The squash bee (Peponapis pruinosa) is found all across North America and follows the blossoms of squash plants (Cucurbitaceae). Squash bees are most active very early in the morning when the blossoms are open. While the squash bee is the main pollinator of this plant, we have seen other generalists like honey bees visiting the flowers on occasion.
Diadasia ochracea on Malacothamnus.
The specialist on sunflowers (Helianthus annuus) is the sunflower bee (Diadasia enavata), though many other bee species can be found foraging on these flowers. In our Berkeley, CA garden even a small patch of sunflowers has been able to draw the sunflower bee. Other species of Diadasia also specialize on different flowers including: cactus, mallows, morning-glory, or clarkia.
A female Megachile perihirta on seaside daisy.
Generalist bees are by far the most abundant bees in our Berkeley, CA garden. In summer the two bees that are most common are the long-horned (Melissodes robustior) and leaf cutter bees (Megachile perihirta). These two bees can be found foraging on any number of flowers including Mexican aster (Cosmos bipinnatus), blanketflower (Gaillardia grandiflora), sunflower (Helianthus annuus), sea daisy (Erigeron glaucus), bush sunflower (Encelia californica), and Golden Goddess (Bidens ferulifolia). Their pollen needs are not restricted to one specific flower host, which allows them to forage more freely and for a longer period of time. These bees have a longer “season” than some other bees because they are able to forage on such a wide variety of flowers.