Parasites are some of the most bizarre, fascinating, and skin-crawlingly creepy organisms on this earth. While some species worry themselves with boring, energy-costly things like building social networks or foraging, parasites have a different approach to survival. Nature’s con-men, parasites are experts at infiltration and exploitation. Some parasites are in it for the long con, like Sacculina carcini, a barnacle that turns its host crab into a zombified mobile home. While some parasites take what they need and go, viciously tearing through their host, like Euderus set, a parasitoid wasp that infests another wasp species’ mind, only to gorily burst through its head cavity. Our focus today is on a far less dramatic free-loader: a parasitic bee by the name Inquilina schwarzi.
First, we’ll need to meet its host, Exonera robusta, also known as a reed bee, a type of bee native to Australia.
E. robusta belongs to a subset of bees called allodapine bees, which are quite different than your typical honeybee or bumblebee. They have a far more primitive form of eusociality than we associate with bees. They typically don’t have very defined queen and worker castes, and all females work together to communally rear young (“Exonera”). Their ‘hives’ are typically much smaller than you’d expect too. The average size for an allodapine colony is about 15 females. This is where it gets interesting. Typically, the bigger the colony the more successful the offspring rearing: many hands make short work, after all. If this is true, you might expect allodapine bee colonies to be much bigger than they are, all things considered. So why are they so small? The answer may be parasites.
I. schwarzi is a social parasite. Instead of infiltrating its host’s body, it just invades its home. I. schwarzi, which belong to the same genera as its host, finds and integrates itself into the unsuspecting colony, sometimes taking over as the ‘queen.’ Gruesomely, it has a habit of eating the host species’ eggs, presumably to make space for its own.
Jaclyn A. Smith and Michael P. Schwartz wanted to know how I. schwarzi chooses its mark.
Smith and Schwartz gathered various E. robusta colonies of different sizes. Colonies with 1-3 females were classified as small, colonies with 4-8 were medium, and any colony with 13 or more females were considered large. In a controlled experiment, I. schwarzi was placed in a chamber and given access to three different colonies, one from each size group. The researchers were then able to record which colony the I. schwarzi chose to infiltrate. Of twelve individuals, ten were able to successfully infiltrate a colony. Amongst these successful parasites, they found that I. schwarzi chose the large nests 70% of time. More so, in the 20% of I. schwarzi that chose medium nests, this was only in nests that were 8 females strong. This indicates that I. schwarzi has a preference for colonies that have at least 8 females. Which makes sense, there’s more resources that way, and its easy to disguise yourself among a crowd. The specific mechanisms for this ‘choice’ are unknown. Are the I. schwarzi able to consciously discriminate between nest sizes? Perhaps, but evidence also suggests that they might just be following their nose. I. schwarzi is able to locate host colonies based on odor, so it may just be following the strongest odor, which would be from the larger colonies.
Still, the fact remains that they bigger they are—the more likely I. schwarzi is going to be able to slip on in. If larger colonies are more at risk for parasites, then it makes sense that there would be a selection pressure against them. This could be one of the reasons why we still see very small colonies of E. robusta and other allodapines. Sure it’s harder work on the individual, but at least you don’t run as high a risk of infanticide!
References
Exonuera robutsa. 23 May 2017. In Wikipedia. Retrieved 8 Oct 2017, from https://en.wikipedia.org/wiki/Exoneura_robusta
Smith, J. A., & Schwarz, M. P. (2009). Decisions, decisions, decisions: the host colony choices of a social parasite. Journal Of Ethology, 27(3), 385-389. doi:10.1007/s10164-008-0131-y
Nataly Ulloa
October 9, 2017 — 10:00 pm
This is super interesting because it brings into the play the whole idea that parasites can drive the evolution of their hosts i.e. in this case small colonies were selected for and thriving more so than larger ones. I also like the disease element.
Megan Mickel
October 10, 2017 — 5:19 am
Wow, there’s something morbidly cool about this type of lifestyle. Do the other females ever realize that they were infiltrated? And what happens after that? Likewise, if the hives keep progressively getting smaller (a brilliant response to the problem), will the parasite adapt its sense of smell to locate them?
Alicia Bukowski
October 10, 2017 — 6:39 pm
I’m not sure. I’d be interested in learning what other kind of parasitism prevention measures the bees have! As for the size. Theres actually conflicting pressures between a) increasing brood strength/size and b) keeping nests small bc of parasites, so I assume there’s always going to be variability in size.
Alaina Bandanza
October 12, 2017 — 5:14 pm
This is a really cool article! I’ve never heard of social parasitism before–I normally think of the stereotypical examples of ticks, tapeworms, or those wasps we read about parasitizing the Hawaiian crickets (so creepy).