A cactus bee returns to its nest in the soil carrying a load of pollen collected from prickly pear cactus. Photo by Jeff Mitton
By Jeff Mitton
On a June afternoon, while hiking on Flagstaff Mountain, I paused at a convergence of trails to consider my path into the future. But I never made a choice, for the buzzing of bees distracted me. At first, I thought I had discovered a hive of honeybees, but a closer look revealed an aggregation of solitary bees.
About two dozen bees were on the ground, moving in and out of burrows that they had excavated in one of the paths. Fresh piles of soft dirt adjacent to the tunnel entrances indicated that some bees were enlarging their tunnels. Others were returning from foraging, their back legs thickly coated with pollen and a light dusting of pollen everywhere else except for their wings.
The cactus bee, Diadasia australis, is a species of solitary bee native to the western U.S. and northern Mexico. It takes its common name for its reliance on one or a few species of prickly pear cactus. It is larger and broader than a honeybee, and its long, grey hair gives it a furry appearance. Perhaps most striking is the delicate pattern of light and dark grey in its eyes.
People are most familiar with the honeybee, a social insect living in large colonies. Honeybee hives have a single queen and thousands of sterile female workers that assume different tasks. Some attend the queen, some guard the hive, some ventilate the hive and others forage for nectar and pollen.
In contrast, solitary bees live solitary lives. A female mates shortly after emerging from her natal nest and then chooses a site for her nest. Cactus bees typically dig a tunnel into the soil and excavate a compartment or cell for each egg. They provision the cell with pollen, often mixed with a small amount of nectar to form a paste. All of the work, including excavation of the tunnel and cells and the provisioning of the cells, is accomplished by a female working alone. When the cells are excavated, provisioned and filled with eggs, the female may start another nest so that she can lay more eggs. But many die after finishing a single nest.
Honeybees are generalists and are able to harvest pollen or nectar from a great diversity of species. In contrast, cactus bees and other solitary bees in the genusDiadasiacollect pollen from cacti. Foraging studies reveal that each bee species collects pollen from one or several species of prickly pear cactus.
A study of pollination biology conducted near Pueblo examined the guild of solitary bees visiting the plains prickly pear, Opuntia polyacantha, and the desert prickly pear, Opuntia phaecantha. These cacti occur in the same localities and have flowers of similar size that bloom at about the same time. Colors of plains prickly pear flowers vary from yellow to magenta and overlap the shades of red in the desert prickly pear. Despite all these similarities, cactus bees were found regularly on plains prickly pear but were never found on desert prickly pear. Examination of the pollen carried by cactus bees revealed that many had been foraging solely on the plains prickly pear.
Chemical studies of prickly pear pollen suggest that fragrant oils on the surface of the pollen may provide the cues that allow cactus bees to discriminate among species of prickly pear cactus.
Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.
April 1, 2011