Earlier this week, I came across a very unusual Cicada Killer Wasp (Sphecius speciosus, family Bembicidae) soil burrow site in southwest Ohio. Cicada killer wasps are so named because females target annual cicadas. However, the “killer” appellation is not entirely accurate.
The females grab annual cicadas from trees. Although cicada killer wasps are the largest native wasp found in Ohio, their prey are commonly equal or larger in size. The females tip the scales in their favor by immediately stinging the cicadas to paralyze them. The paralyzed cicadas can’t struggle or presumably plead for mercy during their final flight.
Paralyzing rather than killing their cicada prey provides another benefit. Once the killers drag the live, but fully aware, paralyzed cicadas underground, they will remain fresh to serve as larval nourishment. Dead cicadas would spoil. It’s an effective workaround for the lack of refrigeration.
Cicada killer (paralyzer?) wasps are not social insects like paper wasps, yellowjackets, and baldfaced hornets (family Vespidae), with “queens” at the top of the organizational chart. Instead, individual cicada killer females excavate separate burrows in the soil with no apparent communication or coordination of efforts among the females.
However, cicada killer females commonly burrow in aggregations, sometimes comprising hundreds of females. Males cruise above the aggregations competing for territory by aggressively driving off competitors. Things can get dicey for anyone who ventures too close. The males react to trespassers with the same belligerent buzz-fests normally reserved for their killer-competitors. Of course, they are males, so they lack ovipositors (a.k.a. stingers).
Cicada killer females do not engage in the boy-sterous roughhousing. Although they have impressively long stingers, they are non-aggressive, reserving their paralyzing stings for hapless annual prey.
Research has shown that the driving forces behind the cicada killer wasp’s collective soil burrowing behavior are twofold. The high concentration of females increases their chances of getting acquainted with males. Females also gather together because they share the same environmental requirements for their soil burrows.
One of the most frequently cited scientific papers in the cicada killer wasp literature is Dambach and Good (1943). This is what the authors wrote about the location of cicada killer wasp burrows: “With few exceptions cicada killer colonies are located in well-drained, light-textured soil, in full sunlight where vegetation is scant or absent.”
The environmental requirements for cicada killer wasps are repeated in other references, both online and published. Indeed, all of the cicada killer sites that I’ve ever observed, as well as the reports that I’ve received, have conformed to the description published by Dambach and Good. That is, until this past Monday.
An Unexpected Observation
After our OGIA/OSU Greater Cincinnati Diagnostic Walkabout at the Cincinnati Zoo and Botanical Garden (CZBG), I was shown a sizable cicada killer aggregation under a dense planting of Grey Owl Juniper (Juniperus virginiana 'Grey Owl').
The first image below shows Steve Foltz, Director of Horticulture (CZBG), and one of the CZBG horticulture interns, viewing the planting. The second image shows the planting from a different angle. The third image shows a cicada burrow.
The planting was established 3 years ago in a light-textured, well-drained soil mix. The wasp aggregation did not predate the planting, and no noticeable numbers were observed in the location last year. However, mating was observed, which signaled that new cicada females had emerged to support the expansion of the aggregation.
The number of cicada killer males buzzing the junipers was impressive. The images below show a male cicada preening, perhaps to make himself more presentable to ladies.
The nest aggregation appeared to extend almost the full length of the planting, with the killers rising through the foliage. Males perched on the juniper tips to survey their “territory.” Following a “rinse and repeat” pattern, the males launched themselves in pursuit of their rivals; then they returned to their perch, time and again. The images below show the same male returning to the same spot through three sorties. I just kept my camera focused on the same place.
The literature notes that new females are mated shortly after they emerge from their burrows. The females only mate once, and mating occurs on the ground near the abandoned burrow. The image below shows a mating cluster with a group of males trying to … gain an advantage.
The second image shows the same activity in the juniper site. If you look closely (not too closely; that would be voyeurism), you’ll see a female wasp surrounded by males. Her compound eyes are much larger (small wonder). The picture illustrates that the dense juniper foliage covering the burrows had altered the mating behavior reported in the literature. The female had moved up and through the foliage rather than being mated on the ground.
The site provided two of the three key ingredients that support site selection by cicada killer wasp females. Good drainage is needed so the larvae won’t drown. Light soil texture aids digging by the females. Both are why we often see aggregations in bare soil in playgrounds, or in or around sand volleyball courts and golf course sand traps. What is glaringly absent in the site at CZBG is direct exposure to sunlight.
Amended Management Options
Cicada killers are considered beneficial insects by helping to suppress annual cicada populations. The females are not aggressive, with stinging encounters extremely rare. I do not encourage the application of insecticides.
The best way to manage cicada killers, if they appear where they're not wanted, is to modify their habitat. Raking mulch to disturb and redistribute possible burrowing sites will convince females to nest elsewhere. The same is true for golf course sand traps and sand volleyball courts. Periodic raking will prevent the wasps from becoming established.
However, we should modify a common recommendation that focuses on modifying cicada killer habitat to increase shading. Based on site requirements published in the scientific literature as well as my own observations, the approach is effective. But based on my observation at CZBG, it’s more accurate to say mostly effective. Obviously, cicada killers don’t read what we write about them.
The cicada killer wasp aggregation at CZBG is in a high-profile location readily accessible to the public. However, CZBG’s response is in keeping with their dedication to conservation and outreach education (visit “The World of Insects” at CZBG!). Instead of taking drastic measures to eliminate the cicada killers, they addressed fears by using signage to educate the public.
While taking pictures of the cicada killers at CZBG, I watched a mother read the sign out loud to her young son. Neither appeared bothered by the male cicada wasp practicing their faux aggression. I observed the same indifference from a group of five adults and two children as they read the sign. Education is a powerful ally. Although cicada killer wasps don’t read what we write about them, people will read what we write about them.
Selected References
Dambach, C.A., & Good, E. (1943). Life history and habits of the cicada killer in Ohio. The Ohio Journal of Science 43: 32–41.
https://kb.osu.edu/server/api/core/bitstreams/c7eb3ddb-1871-5f88-b612-63f29eb820f9/content
Coelho, J. R., & Holliday, C. W. (2008). The effect of hind‐tibial spurs on digging rate in female eastern cicada killers. Ecological Entomology, 33(3), 403-407.
https://doi.org/10.1111/j.1365-2311.2007.00981.x
Coelho, J. R., Holliday, C. W., & Hastings, J. M. (2011). The geographic distributions of cicada killers (Sphecius; Hymenoptera, Crabronidae) in the Americas. Open Entomol. J, 5, 31-38.
Everhart, M. J. (2022). Photo Essay: Observations of an Eastern Cicada Killer Wasp (Sphecius speciosus) at High Park, Derby, Kansas. Transactions of the Kansas Academy of Science, 125(3-4), 233-235.
https://doi.org/10.1660/062.125.0311
Hastings, J. M., Holliday, C. W., Long, A., Jones, K., & Rodriguez, G. (2010). Size-specific provisioning by cicada killers, Sphecius speciosus, (Hymenoptera: Crabronidae) in North Florida. Florida Entomologist, 93(3), 412-421.
https://doi.org/10.1653/024.093.0315
Lin, N. (1978). Contributions to the ecology of the cicada killer, Sphecius speciosus (Hymenoptera: Sphecidae). Journal of the Washington Academy of Sciences, 75-82.
https://www.jstor.org/stable/24536854
