Q: Why Do The Carpenter/bumblebees Attack Each... - RCOW - Tumblr

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Q: Why do the carpenter/bumblebees attack each other when they’re flying around in my front yard?

A:

The short answer is that bees are engaged in some combination of flirting and fighting. In most species of carpenter bee (genus xylocopa) and some species of bumblebee (genus bombini), male bees—also known as drone bees—emerge from their nests when they are mature and the weather is warm and seek to stake out territory that will be attractive to lady bees, while keeping other drones of their species out of their territory. Male carpenter bees famously do this by “dive-bombing” other carpenter bros, in which they aggressively fly towards one another and try to shove the other bee out of their claimed territory. While these battles can be startling to watch, they are generally harmless to humans, since male carpenter bees do no have stingers and have no interest in attacking people.

Similar behaviors (or bee-haviors, if you like portmanteaus) have been observed in some species of bumblebee, though in some species it has also been noticed that the drones pick the same locations together, often without regard to whether there is food or suitable nesting sites for bumblebee queens, and often leave pheromone trails on the exact same objects—one speculative theory on why this might be advantageous is that bumblequeens may seek out males by scent, and that more drones leaving pheromone trails in the same place makes it easier for the queens to detect the smell.

Drones of some species of bumblebees have been observed chasing queen bees—or what they think are queen bees based on visual cues, which can be any small flying object (including birds)—and then tackling and mating with the queens if they guess correctly. Though it is not a scientific term, I like to think of this guess-and-check mating strategy as “bumbling.”

Now, you may be wondering: do female xylocopa and bombini ever attack each other, or other bees? The answer is yes, though the manner in which is occurs is more complicated, and closely related to the ways in which carpenter bees and bumblebees have different lifecycles.

Carpenter bees are considered semi-solitary bees, meaning that they don’t forage or raise young together in colonies, though they do live together in nests with their siblings for a time. The carpenter bee mating season begins when mature male and female bees emerge from their nests in late spring or early summer, and begin their frenzied search for mates. Among some species, individuals of both sexes mate with several other individuals before the fertilized females leave to build their nests and forage.

To build a nest, carpenter bee moms chew out tunnels through any suitable piece of wood using their mandibles, which are slightly wider than their own bodies and branch out into smaller cells, where she lays each egg. She then goes out to forage plant nectar and pollen to feed herself and her young until they reach maturity in the fall, at which point she plugs the opening of the nest with a wad of wood pulp and hibernates with her brood through the winter. In the springtime, the new adult bees and their bee mom all chew their way out of the nest and begin the cycle again. (While carpmoms do this several times in a lifetime, carpdads do not hibernate after their first summer, and instead die as soon as the weather gets cold. This may explain some of their live-fast-die-young antics in your front yard, since this one season is their only chance at reproductive success.)

Ideally, carpenter moms like to live in soft wood, at least two inches thick, with a straight grain, which makes it easier to chew. They also like to expand upon earlier nests, rather than build entirely new homes each year. However, if there are not enough suitable nesting sites in an area, female bees will take over the nests of other carpenter bees by simply pushing all the eggs and larvae out of an existing nest and replacing them with their own. This is less overt than dive-bombing other female bees for territory, but effective, nonetheless.

Bumblebees, on the other hand, are social bees, which do build nests together. In fact, most species of bumblebees are eusocial species, in which a class of infertile or sterile individuals in a colony works to raise the young of the reproductive class. Among eusocial bumblebees, most female bees are “worker bees,” who do not mate, and are rendered functionally infertile for most of their lives by pheromones emitted by the “queen bee” of the colony, who is the only fully reproductive female (though she does not actually have monarchical powers of the colony, as far as any humans can tell).

To grow new queen bees, worker bumbles bees feed some female larvae more than others, and feed them a special food called “royal jelly,” which allows them to grow larger and more sexually developed than their sisters. When the new queens and new drone bumblebees mature in late summer, they leave the nests where they were born to forage and mate, after which the queens hibernate and through the winter, and the males die once the weather gets too cold. Unlike some carpenter bees, all bumblebee queens mate with only one male bee in their lives, leading to greater genetic similarity among their offspring than those of carpenter bees.

When the bumblequeens emerge in the spring, they find somewhere nice to raise their larvae—ideally an abandoned rodent’s nest, or a pile of thatch or dead grass—and build a small “honeypot” of beeswax and pollen, where she lays her first eggs. The queen then sits on this first brood to keep it warm, and forages pollen and nectar to feed the larvae until they mature into worker bees. The worker bees then build the rest of the nest tor the spring and summer, forage food for the colony, and raise the rest of the larvae.

At this point, you may be wondering if there is another way to tell carpenter bees apart from bumblebees bee-sides watching their bee-havior closely. The easiest way to do this is to look at the end of the bee’s abdomen—carpenter bees have a shiny, black abdomen:

while bumblebees have fuzzy abdomens:

Towards the end of the bumblebee colony’s one-year lifecycle, the queen stops emitting the pheromones that make the workers infertile, and then lays a set of eggs that will grow into male, “drone” bumblebees. At this point, the bumblefights begin between some worker bees and their bumblemoms. Since the queen is no longer emitting hormone-suppressing pheromones, some of the largest, most developed worker bees start laying their own eggs, and may become aggressive towards the queen bee and try to eat her eggs. Due to the complexity of bee chromosomes, all the eggs laid by these worker bees will grow up to be male bees, since they are from unfertilized eggs, but they will be fully functional male bumblebees. More importantly, the sons of the worker bees are genetically closer to their mother than the sons of the queen bee (the worker bee’s brother), giving the worker bees some incentive to try to increase their likelihood of reproductive success over that of the queen bee’s sons. Similarly, queen bees may try to eat the eggs laid by worker bees as they come out of their bodies.

As another exception to the rule about bumblebee eusociality, about 45% of all bombini species are cuckoo bees. Rather than build their own nests, fertilized female cuckoo bees sneak into the nests of eusocial bees, suppress or kill the queen of the colony, and then lay their own eggs to be raised to adulthood by the workers. So, bumblefight among female bumblebees do certainly happen as well, but not usually out in the open.

Sources:

Bumblebee Conservation Trust. Lifecycle. Retrieved from http://bumblebeeconservation.org/about-bees/lifecycle/
Goulson, D. (2009). Bumblebees: Behaviour, Ecology, and Conservation (2nd Edition). Oxford, GBR: Oxford University Press. Available from UNC.
Jones, S. C. Carpenter Bees. Retrieved from http://ohioline.osu.edu/hyg-fact/2000/2074.html
Penn State College of Agricultural Sciences. (2015). Carpenter Bees. Retrieved from http://ento.psu.edu/extension/factsheets/carpenter-bees.

Image Credits:

Cemx. (2015). Carpenter Bee Galleries. Wikimedia Commons. Retrieved from https://en.wikipedia.org/wiki/File:Carpenter_Bee_Galleries.jpeg. Licensed under CC BY 3.0.
Cfpresley. (2006). Eastern Carpenter Bee. Wikimedia Commons. Retrieved from https://en.wikipedia.org/wiki/File:Carpenterbee.jpg. Licensed under CC-BY-SA 2.5.
Evans, S. (2006). Appalachian Bumblebee. Wikimedia Commons. Retrieved from https://commons.wikimedia.org/wiki/File:Appalachian_Bumble_Bee_%28180992746%29.jpg. Licensed under CC BY 2.0,
Sanjoin, P. (2008). Bumblebee Nest. Flickr. Retrieved https://www.flickr.com/photos/30098304@N00/2634700990. Licensed under CC BY 2.0.

By Nora Weston