I vividly remember the first time I encountered a tarantula hawk wasp (genus: Pepsis). I  was tromping around Saguaro National Park searching for Gila monsters, showing my friend my undergraduate research field site, when we came across a large, black flying insect perched on a rock. A burgeoning entomologist, my friend told me that these wasps had “one of the worst stings in the world”. I’m not one to be afraid of most insects – though I do prefer them not to land on me – but I noted that this two-inch long black wasp with red wings looked particularly menacing. Little did I know just what a menace tarantula hawks can be, to humans and fellow invertebrates alike.

When stung by a tarantula wasp, “screaming is satisfying and helps reduce attention to the pain of the sting” which is described as “instantaneous, electrifying, excruciating, and totally debilitating”. These colorful descriptions come from Dr. Justin O. Schmidt, an entomologist from the University of Arizona (my alma mater). Besides being a researcher at the Carl Hayden Bee Research Center there, Schmidt is the man behind the Schmidt pain index, a scale of painfulness for insect stings.  The scale, first introduced in a paper in 1986, ranges from 0 (practically imperceptible, like a mosquito) to 4 (tarantula hawk) [1].  Before long, Schmidt was getting stung by many insects in his entomological career, and qualitatively logging their stings on his scale.  As he describes it, “all stings experienced occurred during a collector’s enthusiasm in obtaining specimens” (Schmidt).  Schmidt, an Arizona native just like the tarantula hawk wasp, has been stung by them more than 8 times.

A quick search down the internet rabbit hole suggests that stings by tarantula hawks, do, indeed, seem horrific. Videos show grown men willingly getting stung, and then crying on the desert sand as their skin swells. Interestingly, and reassuringly for these YouTubers, the tarantula hawk’s venom has relatively low toxicity. The lethal dose required to kill 50% of mice exposed to this dose (LD50; a common, if not morbid, metric used in toxicology) for the tarantula hawk venom is 160 mg/kg. For perspective, the common honeybee, a stinger that we don’t consider particularly dangerous if one isn’t allergic, produces a venom with an LD50 of 7.2 mg/kg [4]. So, the common honeybee is over 20 times more deadly than the tarantula hawk – not what one would expect based on its sting. It appears that the main purpose of the venom, at least for larger vertebrates, is to cause pain.

So what is the purpose of this excruciating venom for the wasps? Justin Schmidt, once again, has the answer. Because of this high level of pain without fatality, and their aposematic (meaning, warning) coloration of black with reddish-orange wings, the tarantula hawk’s predators quickly learn not to mess with this insect. Since the venom is not at all fatal, the tarantula hawk ensures the predator will live another day, having learned to avoid its distinct color combination in the process. Over time, this led to the wasp having “no meaningful predators” in the wild [3]. This allows the wasps to to grow large, in fact large enough to hunt their namesake prey: tarantula spiders.

This is the part, dear reader, where the tarantula hawk transforms from a mean stinger to an invertebrate villain of science fiction proportions. These wasps not only prey on tarantulas, the big, fuzzy spiders of the Sonoran desert, but also “zombify” them, turning the spiders into helpless slaves for their young. Tarantula hawks belong to the parasitoid wasps, a group of wasps which rely on other invertebrates to reproduce. A female tarantula hawk hunts for tarantulas by looking within their burrows, which are small holes in the desert sand, or ambush male tarantulas as the spiders look for mates during the summer breeding season. She stings the tarantula in rapid succession, paralyzing it without killing it. Then, she drags the tarantula – nearly eight times her size – back to its burrow (or digs it a new one herself, if that is more convenient). Once inside the burrow, she will lay a single egg into the still-paralyzed tarantula’s body and seal the burrow shut on her way out. Over the next few days, a wasp larva will hatch from the egg and begin to eat the tarantula alive from the inside. Importantly, the larva is careful to make its source of sustenance last as long as possible, and saves the most vital organs, like the spider’s heart, for last before finally killing the host [5], Alien chest-burster style.  

Surprisingly, there are few studies on how the venom of the tarantula hawk allows it to terrorize the tarantulas. There are other, more extensive examinations of venoms of other parasitoid wasps, such as the jewel wasp that preys on cockroaches [6]. In this distantly-related parasitoid wasp, the venom contains chemicals that inhibit the dopamine-signaling in the roaches, blocking their ability to move their legs [2]. (N.B.: Dopamine is also an important neurotransmitter in human motion, and it is disrupted in Parkinson’s patients.) Further, the venom of the jewel wasp contains a compound that mimics inhibitory neurotransmitters, “turning down” neural activity in its host’s brain and transforming it into a placid servant for their larvae [2]. Of course, the exact compounds in the tarantula hawk’s venom and its genus remain unknown, an area of research full of open questions.

One answer is sure: Despite our shared neurotransmitters, humans will not be “zombified” by the tarantula hawk for their larvae, and there are currently no threats of chest-bursters on Earth. For humans, the stinger really is the most pressing “danger”, even if not a lethal one. I for one, though, will keep my distance if I happen across one again in my native Arizona desert, and will silently thank the universe for not making me a tarantula and the wasp the last sight I’ll ever see.

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For more information:

Listen to a Science Friday podcast episode on Dr. Schmidt: https://www.sciencefriday.com/segments/from-nettles-to-volcano-a-pain-scale-for-insect-stings/

Read Dr. Schmidt’s book, The Sting of the Wild.

References:

1. Adams, C. (2012) “Did the creator of the Schmidt Sting Pain Index volunteer to get stung by everything on earth?” The Straight Dope.
2. Moreau, S. J. M. & Asgari, S. (2015). Venom proteins from parasitoid wasps and their biological functions. Toxins, 7(7):2385-2412.
3. Schmidt, J. O. (2004) Venom and the Good Life in Tarantula Hawks (Hymenoptera: Pompilidae): How to Eat, Not be Eaten, and Live Long. Journal of the Kansas Entomological Society, 77(4):402-413.
4. Van Vaerenbergh, M. (2013) Honeybee (Apis mellifera) and bumblebee (Bombus terrestris) venom: analysis and immunological importance of the proteome. Dissertation thesis, Ghent University. Accessed from: https://biblio.ugent.be/publication/4194060/file/4336838
5. Williams, D.S. (1996). “Tarantula Hawk”. DesertUSA. Accessed from: https://www.desertusa.com/insects/tarantula-hawks.html
6. Robertson, H. M., Gadau, J., & Wanner, K. W. (2010). The insect chemoreceptor superfamily of the parasitoid jewel wasp Nasonia vitripennis. Insect molecular biology, 19, 121-136.