Masons, cutters, and carders – oh my!

The leaf-cutter bee expeditions are almost completed. Here is a quick overview.

Megachilidae is a family of bees with more than 600 species in the United States and over 4000 species worldwide. Their common names come from the materials they use to build nests. Leaf-cutter bees cut disks from leaves and petals (and sadly, plastic too) to line the walls of their nurseries. Mason bees use mud to build their nests. And carder bees use plant fibers or animal hairs (carding is the process of combing cotton or wool fibers into parallel strands to be woven together). Still others collect plant resins and are called resin bees. One thing they all have in common is that they provision their larvae with pollen and nectar. All this flower visiting means lots of pollen getting spread from one plant to another.

Most bees, including megachilids, are solitary, meaning they do not live in colonies with a caste structure like honey bees and bumble bees. Because honey bees maintain large colonies of overlapping generations, and seem okay living in wooden boxes that are moved from farm to farm, they became the dominant species for commercial pollination. Similarly, greenhouse tomatoes are often pollinated by bumble bee colonies, which will fly at cooler temperatures. But it is much more difficult to wrangle large numbers of solitary bees for commercial pollination. Enter the alfalfa leafcutter bee, Megachile rotundata. These bees will use stacks of hollow reeds or straws to make their nests, and so can be collected, stored, and deployed in alfalfa fields to increase yield.

Because of their importance as pollinators in natural, agricultural, and urban landscapes, the CalBug team chose megachilid bees as important ecological indicators and set about photographing their labels from our collections. It was over three years ago that we launched the first leaf-cutter bee expedition. Now, after nearly 50,000 specimens we are in the final stretch with “CalBug leaf-cutter bees 18”. So, let us look at some stats.

The first thing I noticed is that the Essig Museum has megachilid bees collected every year since 1892, (except for 1893, 1894, 1898, 1899, and 1903). The peak in the late 1930’s marks the beginning of the California Insect Survey, which led to the formalization of the Essig Museum at UC Berkeley. The 1950’s, 60’s, and 70’s, following World War II, are often considered the “heydays” of Entomology when researchers sought out all species, not just the groups they studied, and collections across the country grew exponentially. Since then researchers have become much more focused in their field efforts. A current peak, beginning around 2010 with renewed interest in native pollinators, is not on this chart because these specimens were databased in-house and not part of the Notes from Nature expedition.

Although the Essig Museum has a major focus on California arthropods, we documented megachilid bees from 53 countries and 45 US states. These specimens represent 493 species in 37 genera. The greatest number of specimens was in the genus Megachile (19,469), followed by Osmia (7208), Ashmeadiella (3781), Hoplitis (3526), Trachusa (3326), and Anthidium (3320).

For this last set of leaf-cutter bee specimens, we would love to know if we fill in some of gaps, like the eastern states not yet represented in the data, or specimens from the 1890’s. The next steps will be to tabulate the host plants these bees visit and look for any populations shifts such as seasonality due to climate change, or the contraction or expansion of particular species over the last 130 years. Thanks to all who have contributed to the monumental task of capturing data on some of our most important native bee species!

— Peter Oboyski, Essig Museum of Entomology