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Bumblebees!: Nesting Habitat

In North America we have 46 species of fuzzy, little bumblebees.  They generally follow similar life history patterns: queens emerge, find a nesting site, seek pollen and nectar to establish a colony, workers emerge, they grow the colony, generate new queens and male bees, those fly off and mate, new queens find a place to over winter, a cozy little hibernacula of their own, and all the other bumblebees die.  (Sorry to end their story on such a morbid note, but that’s where everything ends in the end, right? ) No!  Of course that’s not the end!: the new queens emerge the next spring and the whole thing starts over again.  That is the general, well-worn, well-loved pattern, but we see differences in species’ phenology, or timing.  

Some bumblebees emerge early and some emerge a bit later.  And in general, it’s thought that each species is following the blooming phenology of their preferred habitat.  Early emerging bumblebees have a stronger woodland association, using woodlands for early blooming spring nectar and pollen and nesting, and late emerging bumblebees have a grassland association, using grasslands for finding flowers and nest sites.

In late April, you can hear early bumblebees buzzing around the redbuds.

According to the Xerces Society of Invertebrate Conservation, over 1/4 of North American bumblebees are threatened with extinction, and when looking over these bumblebee declines, researchers in Great Britain and North America, have seen a trend that bumblebee species with late emerging queens (who are seeking flowers and nest sites in grasslands) are more likely to be showing population declines.  And really, this makes sense.  In Great Britain, North America, and other places, too, grasslands have been, and continue to be, declining.  For example, a recent study out of Michigan notes that their agricultural grasslands/hay fields, full of timothy and clover (a good resource for bumblebees), have decreased by over 92% since 1925.  

A big brown-belted bumblebee (I think) visits July blooming swamp milkweed.

As for grassland loss in Iowa, well, first we have the astonishing history of the destruction of the prairie, with over 99.9% of the prairie lost to agriculture and development after European settlement. In more recent history, though, Iowa and the Midwest have continued to lose grasslands, especially over the last 15 years.  A land-use study out of the University of Wisconsin, demonstrated that, grasslands accounted for 77% of the land used for agricultural expansion between 2008 and 2012 -a loss of about 5.7 million acres of grassland, due to the 2005 Renewable Fuel Standard (a.k.a. ethanol mandates).

But what has all this meant for bumblebees in Iowa?  The fact is, research is scarce, but the Xerces Society for Invertebrate Conservation considers 5 of Iowa’s 16 bumblebee species vulnerable to extinction, and one species endangered.  

A bumblebee visiting a purple coneflower in July.

An answer to habitat loss is to create or restore high quality habitat, but when it comes to what really makes high quality bumblebee habitat, there are still many unknowns. What exactly are the most preferred flowers, nesting sites, and overwintering sites for each species? We just don’t have super researched answers yet. But that said, we definitely have enough information get started making bumblebee friendly gardens at home.

Add Bunch Grasses to Your Flowers

“Creating and conserving nesting habitat in natural areas and habitat fragments can potentially be a significant contributor to the reproductive success of bumble bees.”

Xerces Society, Conserving Bumble Bees

A number of studies have suggested that a lack of nest sites may be a limiting factor for bumblebee population growth. In a natural setting, bumblebee queens will usually nest under a clump of grass, or an abandoned mouse or chipmunk burrow, or cavities in dead trees, or in rock piles. But special attention should probably be paid to the bunch grasses. A 2005 study in Iowa showed that bumblebee abundance in prairie remnants was significantly predicted by the percent of grasslands in the surrounding landscape (suggesting additional nest sites and flowers were likely beneficial), and a couple studies out of Europe showed bumblebee queens actually prefer withered grasses and tussocks as nest sites. Considering their loss from the landscape, it seems that planting grasses, specifically bunch grasses, is a great step forward. And gardens are a great place to start.

It’s been shown that gardens, parks, and urban areas can provide significant nesting habitat for bumblebees. In 2004, a citizen science effort in England, found about 3 bumblebee nests per acre, with the majority of nests found in people’s gardens. And lucky for our gardens, bunch grasses are beautiful and versatile, adding wonderful textures and colors, weed suppression, and habitat.  

Little bluestem, in the foreground, is an excellent addition to pollinator gardens.

A bunch of choices:

Bunch grasses come in many heights, colors, and with a beautiful diversity of seed heads. Here’s just a few good options. There are many more! Photos courtesy of the Lady Bird Johnson Wildflower Center.

Little bluestem
Schizachyrium scoparium
A blue-green grass that turns a beautiful red in the fall.

Prairie dropseed
Sporobolus heterolepis

Indian grass
Sorghastrum nutans

Can be aggressive

Big bluestem
Andropogon gerardii

Bumblebee Garden Maintenance: Less is More

To help with your bumblebee habitat, it’s recommended to not cut back bunch grasses, but to leave them be.  The fallen grass is used for overwintering hibernacula for queen bees, as well as nest sites the following spring.  Similarly, areas with leaf litter, brush, and fallen logs also provide beneficial nesting and overwintering sites.  So, I guess what I’m saying is, we can plant grass and flowers, and then forget about it.  Don’t mow as much.  Don’t rake up your leaves.  Don’t pick up all your sticks and fallen logs, in fact, just leave some piles around (in secret locations if you don’t like the “naturalistic” appearance of brush piles). And as always try to eliminate pesticides and herbicides from your yard.

Have you found any bumblebee nests in your yard? If so, where was it?!

Resources:

Conserving Bumble Bees, The Xerces Society for Invertebrate Conservation

Narrow pollen diets are associated with declining Midwestern bumble bee species. Wood, T. J., Gibbs, J., Graham, K. K., and Isaacs, R.. 2019. Ecology 100( 6):e02697

Decline of bumble bees (Bombus) in the North American Midwest. Jennifer C. Grixti, Lisa T. Wong, Sydney A. Cameron, Colin Favret. Biological Conservation. 2009. 142:1.

Diversity and Abundance in Tallgrass Prairie Patches: Effects of Local and Landscape Floral Resources, Heather M. Hines, Stephen D. Hendrix, Bumble Bee (Hymenoptera: Apidae), Environmental Entomology, Volume 34, Issue 6, 1 December 2005, Pages 1477–1484

Cropland expansion outpaces agricultural and biofuel policies in the United States. Tyler J Lark et al 2015 Environ. Res. Lett.10

Quantifying and comparing bumblebee nest densities in gardens and countryside habitats. Osborne, J.L., Martin, A.P., Shortall, C.R., Todd, A.D., Goulson, D., Knight, M.E., Hale, R.J. and Sanderson, R.A. (2008), Journal of Applied Ecology, 45: 784-792.

Habitat preferences of nest-seeking bumble bees (Hymenoptera: Apidae) in an agricultural landscape, Birgitta Svensson, Jan Lagerlöf, Bo G. Svensson, Agriculture, Ecosystems & Environment, Elsevier, February 2000

How to Garden for Bumblebees, by Tom Oder

A Quest for Bumble Bee Nests: The Missing Link, by Amanda Liczner

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