HMC Bee Lab: Dream Homes for Chimney Bees

While searching online through available apartments using filters like "pet-friendly" and "washer/dryer", I started wondering what options a house-hunting website would have if it catered to bees instead of humans. Many factors contribute to declines in wild bee populations. Among these, habitat destruction can cause both food deserts and housing crises for bees. Contrary to popular belief, bees are an extremely diverse set of over 20,000 species, with 1,600 of those species living right here in California. These diverse bee species provide essential pollination services to a wide variety of wild plants and crops, helping them to produce seeds and fruits. Some bees, such as the familiar European honey bee, live in large cavities. Others, such as leafcutter bees, live in plant stems. However, most bee species build their nests underground, so the availability of high-quality soil or sand real estate is very important to them.

[1] Chimney bee (Diadasia sp.) digging a nest in a bare patch of gravel road. Only her abdomen and hind legs are visible. Video is at 0.25x speed

The vast majority of bee species are solitary, which means that each adult female builds her own nest, stocks it with food, and then lays an egg and seals the nest. Hopefully, each little larva will hatch and develop with plenty to eat in a safe environment, protected from predators and parasites. Although these bees don't cooperate to collect food or build nests in the way that social bees (e.g., honey bees and bumble bees) do, they often nest very close to many neighbor bees of the same species, forming large aggregations.

[2] A “neighborhood” of chimney bee nests

Unfortunately, people sometimes mistake these bee neighborhoods for the much more aggressive nests of ground-nesting social wasps such as yellowjackets. Although bees and social wasps look different, it can be hard to see them well when they fly quickly in and out of the ground. One way to tell the difference between a colony of social insects and a neighborhood of solitary bees is that solitary bees usually all have their own holes in the ground so you'll see insects coming and going from many different nest entrances.

[3] In contrast to solitary bee nesting aggregations, underground yellowjacket wasp nests tend to have only one entrance. Here are wasps entering/exiting a nest under a water pump access panel

I was wondering about bee house hunting for two reasons:

First, I had recently heard about the new Ground Nesting Bees project, lead by Cornell researchers Jordan Kueneman and Bryan Danforth, which uses the citizen science app iNaturalist to crowd-source information about ground-nesting bee aggregations. Basically, anyone who finds such a bee neighborhood can take photos of bees entering/exiting their nests and post them to the iNaturalist project. On the project site, they recommend temporarily covering a nest with a clear container such as a plastic cup to slow a bee down and get better photos (just remember to remove the cup afterwards so that she can get in and out of her nest again!). Along with the photos, you provide the date/time you saw the bees/nests, the rough GPS coordinates (by clicking on a map), and an estimate of how many nest entrances were visible in the ground at that site.

Second, this is the third year that we in the HMC Bee Lab have noticed a neighborhood of Diadasia bees under construction during late May. Each year, they nest in the gravel road next to our observation hive shed. (Thank you to WSU Entomological Collections Manager Joel Gardner for IDing the genus of bees and for comments on this blog post!) Their common names are chimney bees, mallow bees, or cactus bees. The name chimney bee comes from the funny chimney-like nest entrances they form as they dig tunnels in the soil to create their nests (although many of their nest entrances just look like perfectly-round holes in the ground).

[4] Left- Chimney bees get that common name from the fact that sometimes they build odd, chimney or turret-shaped nest entrances. [5] Right- Female bees in this genus have very long hairs on their hind legs such as this Diadasia rinconis photographed by a volunteer at the USGS

Like all bees, they get their protein and lipids from pollen, and the species in this genus tend to be picky about which flowers they collect pollen from. We often see the bees burrow their way through the tangle of anthers in nearby bright yellow prickly pear cactus flowers, covering themselves in yellow pollen that they then pack onto the long hairs on their back legs. These fluffy leg hairs make the bees look a bit like they're wearing cowboy chaps. Not surprisingly, we see the bees actively digging and provisioning nests during the same time of year that the cacti are in bloom. The adults are only active for a few weeks to mate, build/provision nests, and lay eggs. Bees in this genus spend most of their lives in a hibernation-like state either as prepupae (kind of like bee tweens) or as adults, waiting until their preferred sources bloom again.

[6] Bee collecting pollen from a prickly-pear cactus

[7] Female chimney bees entering their nests to add pollen for their offspring. Video is 0.25x speed

Apparently, ground nesting bees, much like humans, often like to settle down in the same place that they grew up, so it's not too surprising that they show up in the same spot year after year. That could be because they are often picky about the soil that they nest in. For example, some bees prefer looser soil or moist soil while others prefer dry, compacted soil like the gravel road that we found these bees nesting in.

During the first year we saw them (2021), we were a little concerned about a bunch of bee flies (bombyliids) we saw hovering around the nests. These fuzzy flies mimic bees, but their adorable adult appearance provides a sharp contrast with their destructive behavior as larvae. The adult female flies try to lay eggs around nest entrances so that their larvae can crawl into the nest and eat the bee larvae! Fortunately, whatever negative effect they had didn't noticeably decrease the numbers of bees there in 2022 and 2023.

[8] Video of bee fly (bombyliid) hovering near a bee nest entrance

The new iNaturalist project lets us contribute these observations to help the researchers at Cornell determine what sorts of habitats attract and support different species of ground nesting bees. They plan to combine the location/timing of bee observations with other data sets about soil characteristics, land cover, topography, and more and use mathematical modeling to better understand bees' housing needs. The hope is that this understanding will help land managers find ways to better conserve diverse species of bees and in turn improve pollination services to a wide variety of crops.

So next time you see buzzing insects flying out of many holes in the ground, consider snapping a few photos and sharing your discovery. It may help future bees to find their dream home!

HMC Bee Lab

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Friday, June 2, 2023

Dream Homes for Chimney Bees

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