For several years running, the Oak Titmice nested in a nest box on the front of our little house in a canyon near UCLA, fledging clutches of young each year. They had started building a new nest this year, which I knew because we clean out the box each year to reduce the potential build-up of parasites. By May, when we should have already heard the nestlings, there was no activity. I took a closer look at the nest box late one afternoon and saw a bumblebee coming out. That’s odd, I thought.
Then I saw a bumblebee going into the nest box, and one hanging out at the edge of the hole. It was then I noticed the yellow spots on the front of the nest box, which I had attributed earlier to messy titmice, but clearly was the color of pollen, which got me thinking.
Notwithstanding a bit of prior experience with angry stinging insects, I gave the side of the nest box a couple of firm taps.
The resulting buzz sounded like the box itself was alive and awakened that visceral awareness of one’s surroundings and potential dangers that anyone who has ever heard a rattlesnake’s rattle would recognize.
We most certainly had a bumblebee nest, and I had a bit of an adrenaline buzz.
They were, according to a more expert friend, Black-tailed Bumblebees (Bombus melanopygus; their proper scientific name). Many bumblebees nest in the ground, such as in an old rodent burrow, but I learned that this species also has a penchant for abandoned bird nests. I did wonder whether the “abandoning” in this instance took place before or after the bees decided they wanted to set up shop.
The species is one of the moderately urban tolerant native bumblebees in California (McFrederick and LeBuhn 2006). In 1911, local naturalist Anstruther Davidson, transplanted to Los Angeles from his native Scotland, reported that he had found a colony of Black-tailed Bumblebees “warmly housed in the nest of a cactus wren” in San Fernando (Davidson 1911).
Bumblebee nests are temporary affairs. The queen overwinters alone, presumably snugly burrowed in the soil. In the spring, she emerges and starts looking for a place to nest. She finds a spot and collects a ball of pollen in which to lay her eggs. She then incubates the clump of pollen where she deposited the eggs, keeping it warm by shivering her flight muscles. She can find the pollen lump in the dark, through chemical cues she detects with her antennae. These cues are chemicals (pheromones) that she has deposited, leaving herself a marker where her eggs are found (Heinrich 1974).
The eggs hatch to larvae, which eat the pollen. The queen must keep them warm and feed them by fetching more pollen and nectar. After a few weeks, the larvae turn into pupae, just like butterflies do, and undergo metamorphosis into adult bees. The first ones in a season are always females that set about helping the queen to raise more larvae and adults (their sisters). The new bees have jobs — some shiver their wings to keep the temperature up, or fan to cool the nest, some forage for nectar and pollen, and some stand guard against predators and remove dead larvae or adults. As the season progresses, the queen finally lays eggs that develop into new queens and males, which leave the nest, find mates, and then the mated queens find spots to hunker down (hibernate) and wait until the next spring to start the process over.
A nest might hold 30 bees, which explains the loud buzz in response to my taps. After I discovered the bees had taken over, I would go out periodically and give the box a tap to hear the buzz. By June 18th they were gone, just as we headed into the dry, hot summer, and flowers in the hillsides were becoming scarce.
Black-tailed Bumblebees are found throughout the west and north into Canada and to Alaska. They all look the same, except those north of California have a red patch on their abdomen (and are known familiarly as “red butts”). The ones in California used to be classified as a different species, but it turned out there was just one gene that affected the red patch, and they were otherwise no different (Owen et al. 2010).
There are several other species of bumblebee that you might encounter in Los Angeles, with names like Bombus californicus (named after the state), Bombus sonorus (named after Sonora, Mexico), Bombus vosnesenskii (named after naturalist Ilya Voznesensky who was part of the Russian commercial expedition that was colonizing the part of California that would become Sonoma County in the early 1800s). Our Bombus melanopygus, named after its black abdomen (Greek: melan– = black, dark; pygus = rump), was previously known as Bombus edwardsii (Cresson 1878), named for William H. Edwards (who made his fortune mining West Virginia coal but is remembered for his contributions to entomology, in particular the epic three volume Butterflies of North America, 1868–1897) (Calhoun 2013).
Black-tailed Bumblebees are not aggressive in the least, and my periodic taps only elicited buzzing and maybe a few guards coming to the entrance hole. If you are so lucky as to encounter a nest, let it be, and rest assured that its presence will be seasonal. Then you can celebrate being in the middle of a patch of nature capable of supporting a nest-commandeering flower-powered social insect colony!
After the bees had gone, I cleaned up the box, moved it to a second location and added a new box for the titmice in their old spot. The titmice did bring around fledglings from somewhere close, and we’ll eagerly wait to see where they nest in the spring.
Citations
Calhoun, J. V. 2013. The extraordinary story of an artistic and scientific masterpiece: The Butterflies of North America by William Henry Edwards, 1868–1897. The Journal of the Lepidopterists’ Society 67:73–110.
Cresson, E. T. 1878. Descriptions of new species of North American bees. Proceedings of the Academy of Natural Sciences of Philadelphia 30:181–221.
Davidson, A. 1911. The bumble bees of Los Angeles. Bulletin, Southern California Academy of Sciences 10:66.
Heinrich, B. 1974. Pheromone induced brooding behavior in Bombus vosnesenskii and B. edwardsii (Hymenoptera: Bombidae). Journal of the Kansas Entomological Society 47:396–404.
McFrederick, Q. S., and G. LeBuhn. 2006. Are urban parks refuges for bumble bees Bombus spp. (Hymenoptera: Apidae)? Biological Conservation 129:372–382.
Owen, R. E., T. L. Whidden, and R. Plowright. 2010. Genetic and morphometric evidence for the conspecific status of the bumble bees, Bombus melanopygus and Bombus edwardsii. Journal of Insect Science 10:109.
Travis Longcore 