Creature Feature: Crotch’s Bumblebee

I met my first Crotch’s bumblebee (Bombus crotchii) during the spring of 2021. I had just started fieldwork for my PhD at the University of McLaughlin Reserve in central California. I stood in a field of purple hairy vetch flowers, recording data on plant-pollinator interactions. A very large, fuzzy bumblebee landed on a vetch flower. Her yellow and black stripes glinted with sunlight. She began drinking nectar. I noticed a fiery orange band on her abdomen and pumped my fist with excitement. A Crotch’s bumblebee! Although the bee’s flashy orange butt is a key feature for identification, she is named after the entomologist George Robert Crotch [1]. According to the reserve directors Cathy Koehler and Paul Aigner, that spring was the first time they had found Crotch’s bumblebees at the McLaughlin Reserve.

A Crotch’s bumblebee visits invasive hairy vetch flowers. The striking orange band on the abdomen is a good way to tell Crotch’s bumblebee from other bumblebees [Source: Rebecca Nelson].

A Crotch’s bumblebee lands on a serpentine sunflower, a plant endemic to the California coast range [Source: Rebecca Nelson].

The Crotch’s bumblebee lives primarily throughout the state of California but has disappeared from at least 70% of its historic range [2]. The population of Crotch’s bumblebees has decreased so drastically that the species is listed as endangered by the International Union for the Conservation of Nature and is in the process of being listed as endangered by the state of California [2-3]. While Crotch’s bumblebees were once abundant in California’s Central Valley, habitat loss due to rapid urbanization and conversion of land to agriculture led to its decline [1-3]. The Crotch’s bumblebee has a relatively narrow climate preference, so climate change may further threaten this species [2]. Other potential causes for concern include pesticide use and competition with non-native honeybees [2-3].

Crotch’s bumblebees are typically found in grasslands and shrublands such as this grassland at the University of California McLaughlin Reserve [Source: Rebecca Nelson].

I followed the Crotch’s bumblebee worker from vetch flower to vetch flower before she zoomed off into the hills. I was intrigued. With the help of an intrepid undergraduate, I began surveying different areas of the reserve on the search for Crotch’s bumblebees. We found that the Crotch’s bumblebees visited both non-native flowers such as the hairy vetch and yellow starthistle as well as native flower species such as the foothill larkspur and serpentine sunflower. We saw mostly worker bees but found at least two queens. Watching a queen bumblebee buzz from larkspur to larkspur, I marveled at how such a large, floofy bee could be so majestic and graceful in flight. We found Crotch’s bumblebees in areas of the reserve that had burned during the 2020 wildfires as well as in unburned areas of the reserve.

A Crotch’s bumblebee visits an invasive yellow starthistle plant [Source: Rebecca Nelson].

I followed the bumblebees from flower to flower, hoping they would eventually lead me to their nest. Crotch’s bumblebees live in social colonies with non-reproducing workers, a queen bumblebee, and reproductive males [2]. They nest in abandoned rodent burrows, piles of rocks, cavities in dead wood, and old bird nests [2]. The worker bees emerge in spring, foraging on flowers, and are active through the end of the summer. The bumblebee queens overwinter in the nest. Although queen overwintering success is critical to the survival of Crotch’s bumblebee colonies, very little is known about what types of habitats and conditions they prefer to overwinter in [2]. Learning more about their overwintering preferences is important for Crotch’s bumblebee conservation. Although I even followed one of the bees as she visited over 100 flowers, I never did find a nest. Each time I followed one of them, the bumblebee would rocket off at a pace I couldn’t keep up with as a mere human.

A Crotch’s bumblebee pollinates a foothill larkspur. While most workers have a bright orange band on the abdomen, the color can sometimes vary to burnt orange or gold in the case of this individual [Source: Rebecca Nelson].

One day, I stood in a larkspur patch. Several Crotch’s bumblebees whirred between flowers. One bumblebee flew right up to my face and stared at me with large, black eyes. Did she think my bright pink shirt was a flower? Was she curious about my presence? She flew within a few inches of my nose and hovered there for a few seconds before landing on a larkspur blossom. Bumblebees are often particularly good at pollinating larkspurs and other species of flowers. They, along with other wild bees, provide $3251 per hectare in pollination services to crops [4].

A Crotch’s bumblebee prepares to drink nectar from a foothill larkspur. Her long tongue is sticking out [Source: Rebecca Nelson].

The sudden appearance of Crotch’s bumblebees at the McLaughlin Reserve could be a sign of successful habitat restoration. Some ways we can help to save the Crotch’s bumblebee and other pollinators include restoring native grassland habitats in California, limiting the use of pesticides near bumblebee habitats, and encouraging pollinator-friendly farming practices such as creating native plant hedgerows [2]. Both the Crotch’s bumblebee and the imperiled Western Monarch butterfly really like milkweed, so bumblebee conservation efforts can have the added benefit of helping other pollinator species [2]. If you live in California, you can plant larkspurs, lupines, and other flowers to attract bumblebees to your area. You can document your bumblebee observations on community-science platforms such as the California Bumblebee Atlas [5] and iNaturalist [6].

Rebecca Nelson is a second year PhD student in the Graduate Group in Ecology at UC Davis in Dr. Susan Harrison’s lab. She studies how invasive species and restoration strategies affect plant-pollinator interactions. She also writes nature poetry.

References:

[1] Los Padres ForestWatch. (2020, June 27). Crotch’s Bumblebee. Retrieved January 15, 2022, from https://lpfw.org/our-region/wildlife/crotchs-bumblebee/

[2] Crotch’s Bumble Bee | Xerces Society. The Xerces Society. Retrieved 22–01-15, from https://xerces.org/endangered-species/species-profiles/bumble-bees/crotchs-bumble-bee

[3] Hatfield, R., Jepsen, S., Thorp, R., Richardson, L. & Colla, S. 2015. Bombus crotchii. The IUCN Red List of Threatened Species 2015: e.T44937582A46440211. https://dx.doi.org/10.2305/IUCN.UK.2015-2.RLTS.T44937582A46440211.en. Accessed on 15 January 2022.

[4] Kleijn, D., Winfree, R., Bartomeus, I., Carvalheiro, L. G., Henry, M., Isaacs, R., … & Potts, S. G. (2015). Delivery of crop pollination services is an insufficient argument for wild pollinator conservation. Nature communications, 6(1), 1-9. https://doi.org/10.1038/ncomms8414

[5] The California Bumblebee Atlas. The California Bumblebee Atlas. Retrieved January 15, 2022, from https://www.cabumblebeeatlas.org/

[6] Observations. (2022). INaturalist. Retrieved January 15, 2022, from https://www.inaturalist.org/observations?taxon_id=271451

Main image source: Rebecca Nelson.

[Edited by Alexandra Dwulit]