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An unusually (good) year for Colorado bumble bees so far

A Bombus occidentalis (Western Bumble Bee) worker forages on Japanese Spirea on the CSU Campus outside my office. Photo by John Mola

There have been several interesting and notable bumble bee observations along the Front Range this year. I wanted to summarize them to bring attention to these intriguing observations in hopes of gathering more information and energizing folk’s interest in seeking more bumble bee observations. 

It’s possible that this seemingly good year for bumble bees in the region may not continue as we get into hotter and drier conditions. Unfortunately, a good year for bumble bee queens and early colonies does not always result in a good year for mature colonies and, ultimately, their reproduction. So, stay tuned!

Two-Spotted Bumble Bee (Bombus bimaculatus)

This year, there have been multiple sightings of the Two-Spotted Bumble Bee (Bombus bimaculatus) around Fort Collins. This species is far outside its expected range – which we would typically think of its more eastern distribution.

Figure 1. A quick look at Bombus bimaculatus records in GBIF with more recent records in lighter blue. There are just a handful of records this far west, and they are all from recent years. Is this perhaps range expansion? Or just a more thorough recording of bumble bee communities due to more folks interested in observing?

Bombus bimaculatus (Two-Spotted Bumble Bee) was first seen this year in east Fort Collins on June 2nd by iNaturalist user Schendy. An additional iNaturalist sighting was made in southern Fort Collins on the 22nd – and members of our research group located a worker foraging at the CSU trial gardens on the 20th. The distance among these sightings suggests multiple colonies, rather than observations of workers from a single active colony. 

Sightings just north of Denver have also occurred this year. Here is the iNat search to see all of these records. 

It’s intriguing to consider whether this species has long been present at low abundances or if its recent appearance is a new arrival and is here to stay. Bombus bimaculatus is a common species throughout much of its range in the east – whether it could become common here is also a notable question as we observe homogenization of pollinator communities throughout urban areas worldwide.

Western Bumble Bee (Bombus occidentalis)

The Western Bumble Bee is a formerly common species throughout western US and Canada. The species has undergone dramatic declines and is predicted to continue to do so. I co-authored a paper on this topic a few years back (Janousek et al. 2023).

There is a possibility this species will become listed under the Endangered Species Act – as such, knowing where we can reliably find populations of the species is of critical importance should future research or conservation efforts become actionable.

This year, there have been several exciting observations of B. occidentalis.

Notably, queens have been observed in some abundance around Fort Collins and at higher elevations. A rather striking observation was made by Ralph Kettle of the Northern Colorado Beekeeper’s Association, who snagged some photos of a B. occidentalis queen attempting to enter one of his honeybee hives. A daring mission for a queen – exactly why she would attempt this is unknown to me, though I suspect it may have been in search for early season food if not much was blooming yet in early April.

Photo from Ralph Kettle via iNaturalist (user: rkettle) of a Bombus occidentalis queen attempting to enter a honeybee hive in north Fort Collins in early April. Perhaps she had emerged before abundant resources were available elsewhere and wanted to take her chances robbing these non-natives?

Our research group has made several observations of B. occidentalis on campus, around town, and in various natural areas, especially in the mountains. On campus, we’ve spotted them foraging on false spirea, Japanese spirea, and buckeyes. 

As of June 28th, there are a total of 20 records for B. occidentalis on iNaturalist in this region this year. 

Notably, I made several observations of B. occidentalis at mid-elevation (~8,000 feet) along riparian areas foraging on Streptopus amplexifolius (Watermelon Berry) and nectar robbing Lonicera involucrata (Twinberry Honeysuckle). They were in such abundance, that essentially every time I found a patch of these plants, I found B. occidentalis. From checking the Bumble Bees of North America database, interactions with these plant species are new associations for the Western Bumble Bee.  

This past week, members of our research team also found a nest in the foothills. We’ll monitor this colony as it develops. 

A Western Bumble Bee (left) foraging on False Spirea alongside a Brown Belted Bumble Bee (right) on CSU’s campus, June 27th, 2024. We observed dozens of Westerns foraging on this patch this day. Photo by John Mola.

Higher abundances of less common species/species outside their “usual” range

In addition to these striking observations of unexpected species, there has been an overall pretty exciting year for bumble bees in the region. As I am largely at my desk more than the field these days, I can speak most confidently about the Fort Collins region. 

I’ve only been in the area for 4 prior years, but this is the first year I have seen Bombus centralis in the city or at lower elevations. Although common at ~7000 feet and above, I have rarely (if ever?) seen centralis along the Front Range cities. Additionally, I’ve received emails and records of the species by backyard observers.

Similarly, we have several sightings of B. fraternus queens. This species is also thought to be declining. Although this species is expected/occurs in Front Range cities every year, there may be more observations than usual.

Records of B. insularis and B. bifarius, as well as passing sightings of these species by our team, have occurred at low elevations this year as well. Again, we regularly see these species at higher elevations, but less commonly are they reported along Front Range cities. 

Why is this happening?

Of course, part of the reason for these shifts in observation could simply be improvement in record keeping, increase in the use of iNaturalist, the popularity of “bee spotting” as a hobby, or even just the fact that I live here now and people increasingly report these things to my lab group. 

On the other hand, Virginia Scott, Adrian Carper, Lisa Mason, and others along the Front Range have been bee spotting for some time now and also report similar feelings/findings of this being a banner year for bumble bees. And comparisons of our observations alongside records on iNaturalist and databases suggest that at least some (if not most) of these records suggest this is a strong year for the genus. 

Why might this be? One possibility is that the previous year was quite wet throughout the area. A solid production of flowers due to wet conditions, combined with a long floral season (as they don’t get “burnt out” if conditions remain wet all summer) could have resulted in a strong production of queens. 

Combined with a relatively mild winter and few late season snows (which perhaps maybe kill early emerging queens), we may be observing a larger than usual amount of queens successfully establishing young colonies. 

Initial reports from the mountains and around town suggest that things are drying out rather quickly, however. So if this trend of being a good bumble bee year will continue is still to be determined. 

What can you do?

We’re able to put these observations together and begin forming this story because of the interest and data sharing of many individuals. Get involved by reporting sightings of bumble bees to iNaturalist or more directly getting involved with Bumble Bee Atlas or CSU Extension’s Native Bee Watch

If you need help with identification of sightings, feel free to reach out to my group or make use of the excellent BeeMachine. Reporting sightings on iNaturalist also usually yields a reliable identification. 

Please also reach out with any particularly striking observations or thoughts on why conditions seem so favorable for bumble bees this year!

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