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Does size matter for bees?

Landscape simplification is bad for bees. As agriculture becomes more widespread, less natural and semi-natural areas remain intact, and fewer valuable foraging and nesting resources are available to wild bees. Most wild bees are solitary, meaning that unlike their social honey bee and bumble bee cousins, a single female is responsible for provisioning food for her offspring which emerge the following spring or summer. These offspring are laid in cells in a small cavity, such as a hollow stem, an excavated tunnel in wood, or most commonly, a tunnel in the ground.

Illustrated life cycle of a stem nesting bee

The life cycle of a stem nesting or cavity nesting bee. Illustrated by Sarah Scott.

When food (pollen and nectar from flowers) is hard to find, bees are forced to expend more energy and time searching for food. This usually also means fewer total foraging trips, which can result in lower fecundity (fewer offspring). Research also suggests that over the past century, bees have gotten smaller on average (Oliveira et al., 2016). But not much is understood about what is driving this pattern. One possible explanation is that when less natural habitat is left in the landscape, it's harder for bees to find resources. Bees will then have fewer floral resources available to provision their nests with pollen and nectar, leading to smaller offspring.

Predicted response of wild bees to landscape simplification. The circular images show changes in landscape, moving from a diverse landscape of land cover and use (left), to a simplified landscape with less diverse land cover and use, as is seen in agricultural settings (right). The graphs illustrate how bee body size is predicted to respond to landscape change, showing a wider distribution of bee sizes, including large bees (left), to a narrower distribution of mostly small bees (right). Graphic from "Landscape constrains body size of native bees," Heather Grab. Presentation at Entomological Society of America, 2016 Conference.

Dr. Heather Grab, a postdoctoral researcher in the Poveda Lab at Cornell, is testing this prediction - how body size in wild bees changes with the surrounding landscape. Heather reached out to us last year to ask if the Isaacs Lab would like to collaborate by sharing data collected from various projects in Michigan. All bees were caught on blueberry and cherry farms. Some of these farm sites had wildflower enhancements planted in field margins. These plantings were specifically designed to create attractive forage and act as a buffer from management disturbances. Other sites had no wildflower enhancement plantings, and instead bees were caught in grassy field margins.

Since most of the Poveda Lab’s collections take place on fruit farms in New York, our collaboration would provide a better sense as to whether or not the patterns they were seeing in New York would be found in other states. We agreed to measure seven wild bee species: Andrena vicina (a species of mining bee), Lasioglossum leucozonium (a common sweat bee), Halictus ligatus (another sweat bee), Ceratina calcarata (a small carpenter bee), Agapostemon virescens (a green sweet bee), and Augochlorella aurata (another green sweet bee). To act as a control we also measured Andrena carolina (another mining bee), an early spring, blueberry specialist, as it flies before peak bloom of wildflowers and would have sufficient forage regardless of the presence of wildflowers.

Overhead view of bee with head capsule width and ITD noted

The most effective measurements for analyzing bee size are: head capsule (the distance between the outermost edges of the eyes), and intertegular distance (or ITD, the distance between the innermost edges of the tegula, or the bee’s equivalent to shoulders). To take these measurements, pinned specimens were placed under a microscope and measured using a graticule (a tiny ruler etched into a lens piece).

Photo from Sam Droege, USGS

In total, more than 1500 additional bee specimens from four collection years were measured, and these data were sent to Heather. So what has Heather found so far? In her preliminary data analysis of Michigan bees, Heather is finding that bees at sites with wildflower plantings are generally larger than bees at sites without plantings. Her analysis indicates that wildflower plantings with greater floral area and flowering species richness have larger bees, and the overall size of the planting has little effect on bee size. She is also finding that some species decrease in size as the landscape becomes more agricultural, while others are not greatly affected.

What are the implications of bees decreasing in size? Research conducted by Renauld et al. between 2014-2016 found that smaller females of a mining bee species in New York carried 40% less pollen by weight, compared to their larger counterparts (females that were 20% larger on average). The inability to carry large pollen loads could result in fewer offspring produced in a season, and potentially smaller offspring produced the following season, though little is known about this effect. Decreased body size and smaller pollen loads could also lead to reduction in pollination services of agricultural crop and wildflowers, with further ramifications in fruit and seed set.

If the changing sizes of bees has got you down, don’t get too discouraged yet! Heather’s work is showing that bee size may be recoverable when native wildflowers are added back into the landscape. So how can you help? If you are a homeowner, you can consider including more native plants in your landscaping choices. Not only are native plants necessary for bees and other pollinators, they are often much easier to care for than ornamentals, and many are deer resistant! If you are a grower, converting unused, grassy margins into wildflower enhancements provides great habitat for bees and non-bee pollinators. These wildflower enhancements encourage bees to make homes close to your crop, provide forage for bees when your crop is not in flower, and may even improve pollination services to your crop. There are federal programs, such as the USDA NRCS Conservation Reserve Program , that help cover some of the overhead costs of installing wildflower enhancements to growers and landowners. If you rent your home, or do not have yard space but you still feel passionate about improving bee habitat, consider volunteering with Pollinator Partnership, or become a citizen scientist for the Xerces Society.

For more information about bees and attracting these pollinating powerhouses to your property, visit Michigan Pollinator Initiative.

For more information about bee size and landscape change, visit Dr. Grab's personal page here .