Wasp Wednesday: Pepsis grossa

There are many species of “tarantula hawk” wasps in the western U.S., but the largest is Pepsis grossa, formerly known as Pepsis formosa. Interestingly, this insect exhibits both an orange-winged (xanthic) form, and a black-winged (melanic) morph. The two are almost never found together in the same location.

female xanthic form

My experience with P. grossa has been confined to southeast Arizona, but the species ranges from southern California and Nevada to Kansas, Texas, Arkansas, and Missouri. It also occurs throughout the Caribbean islands, Mexico, Central America, and northern South America. The xanthic form is found from Arizona north and east, and also in central Mexico. In southern Ecuador and northwest Peru, there is a “lygarochromic” variation whereby the wings are dark at the base, with a median patch of dark amber, and a pale wingtip.

These are enormous wasps impossible to overlook. Females average a whopping 43 millimeters in body length (30-51 mm). Males are smaller, 24-40 millimeters.

male melanic form

These giants can still be confused with the very similar, but smaller P. mexicana. Males of P. grossa can be distinguished by the fact that they have twelve antennal segments. No other Pepsis species has that number of segments. Females of P. grossa have long, coarse hairs beneath the femur of the front leg, though this feature can be abraded in older, worn specimens.

Females hunt for their tarantula prey mostly in the morning and evening to avoid overheating in the intense summer sun. Flying low over the ground, they may detect the presence of a tarantula burrow by sight or smell. Occupied tarantula burrows have a silk curtain over the entrance by day, and perhaps the wasp is tuned in to chemicals in the silk that indicate a spider is at home. The wasp may also land randomly and scour the soil on foot, flicking her wings and bobbing her antennae feverishly.

Once she does find a burrow with a spider inside, she cuts away the silk curtain and cautiously enters the burrow. Soon, both wasp and spider erupt from the burrow. This eviction behavior is crucial to the wasp’s success in securing her prey. She would have far less room to maneuver inside the spider’s tunnel.

The wasp steps back, grooms herself thoroughly, and then sizes up her adversary. She uses her antennae to entice the spider into raising itself off the ground; or even antagonizes the arachnid into a threat posture whereby the tarantula raises its front legs high, exposing its fangs. The wasp then seizes the second leg and thrusts her stinger between the base of its leg and its sternum. She strikes a nerve center in that location which causes the spider to become paralyzed.

The wasp may feed on fluid from the wound created by her stinger, or groom herself again before commencing the laborious procedure of carrying her prey away.

Tarantula hawks often simply drag their paralyzed burden into back into its old burrow, but on occasion they may dig another burrow and bury the spider there. I am pretty certain I witnessed the very end of this sequence one evening at Tohono Chul Park in northwest Tucson. The female shown below was busy filling a shallow depression by scraping sand into it.

female melanic form

The female lays a single egg on the tarantula and then seals the burrow. The larva that hatches from the egg will then consume the spider. Once finished, the larva will spin a silken cocoon, metamorphose into the pupal stage, and eventually emerge as an adult wasp sometime later.

Look for both male and female tarantula hawks on flowers, especially milkweeds (Asclepias spp.), where they fuel themselves on nectar. During the heat of midday, they will seek shelter amid foliage on trees and shrubs, sometimes gathering by the dozens on one plant. Males, and the odd female, may also spend the night in such aggregations.

Despite their intimidating size, these are rather placid animals unless provoked. An agitated tarantula hawk adopts a threat posture with wings splayed and abdomen curled under. They also secrete a strong, but not unpleasant, odor. Take heed! Stings from Pepsis are not life-threatening unless you are prone to allergies, but the pain is incredible. According to Justin Schmidt, you are in agony for about three minutes, and then it is pretty much over (the pain). Actual results may vary, so I am not volunteering to experiment any time soon. Neither should you.

Sources: Alcock, John. 2000. “The Tarantula Hawk Wasp’s Potent Sting Stuns and Kills the Much Larger Tarantula,” Arizona Highways, 76(10): 40-41.
Hurd, Paul David, Jr. 1952. “Revision of the Nearctic Species of the Pompilid Genus Pepsis (Hymenoptera, Pompilidae),” Bull. Am. Mus. Nat. Hist. 98(4): 261-334.
Punzo, Fred. 2007. “Interspecific Variation in Hunting Behavior of Pepsis grossa (Fabricius) and Pepsis thisbe Lucas (Hymenoptera: Pompilidae): A Field Study,” J. Hym. Res. 16(2): 297-310.
Schmidt, Justin O. 2004. “Venom and the Good Life in Tarantula Hawks (Hymenoptera: Pompilidae): How to Eat, Not be Eaten, and Live Long,” J. Kans. Entomol. Soc. 77(4): 402-413.
Vardy, C.R. 2002. “The New World tarantula-hawk wasp genus Pepsis Fabricius (Hymenoptera: Pompilidae) Part 2. The P. grossa- to P. deaurata-groups,” Zool. Verh. 337: 1-134.

Wasp Wednesday: Wasps in a Bubble

One of the strangest inquiries I have ever received was the image below, from a Facebook friend named James Adams. He took the picture at the Rio Santiago Nature Lodge near Santa Ana, Honduras (northern coast). Not familiar with the tropical fauna there, I forwarded the image to Dr. James Carpenter at the American Museum of Natural History in New York City. Dr. Carpenter is a world authority on social wasps with much experience in the neotropics.

© James Adams

Here is the original post to James Adams’ Facebook page:

”OK so this is one of the weirdest things I've seen in nature. These small, black wasps created a normal-looking nest amidst leaves of a soursop tree. Then either they, or some other creature, encased the entire thing in this "plastic" bubble. Great protection from the rain, but WTH ????? There's a little oval-shaped hole in the bottom so they can get in and out. Anyone like Court Whelan or Deborah Matthews know what these wasps are called? Crazy! Rio Santiago Nature Resort, Honduras —“

Dr. Carpenter wrote the following in response to my query:

"It's a nest made by a species of Protopolybia. There are several species in the Amazon that make envelopes of "wax paper," such as chartergoides - you may encounter descriptions in the literature under the generic name Pseudochartergus. The paper that paper wasps make is most commonly wood fiber bound with saliva - these critters have just left out the wood."

The other comments on the original Facebook post of this image were delightful. Piotr Naskrecki, an extremely experienced explorer in his own right, said “Absolutely amazing, I have never seen anything like this.” Holly Overmyer was jealous: “I wish I could make a little plastic bubble when I get caught in the rain.” John Afdem quipped: “I'm just guessing here, but perhaps its one of the Saranwrapipedes???” Kenneth Barnett, not to be outdone, replied “John, you must talking about Polyrapus opaqous?”

James Adams runs The Lodge at Pico Bonito, La Ceiba, Atlantida, Honduras. I just may have to visit both nature lodges to see this kind of phenomenon for myself.

Wasp Wednesday: A Tropical Social Wasp

Attention: You. Yes, you. I welcome your images of mystery insects and spiders from anywhere in the world, and will blog the answer to share with others if I have your permission to publish the images. The following is one example of how this might appear.

I often receive inquiries through my private e-mail, asking if I could identify this insect or that spider, sometimes from other corners of the world besides the United States. This is always a welcome challenge, but the emphasis is usually on “challenge.” Not long ago, my friend Paul Kaufman sent me three wonderful images of a social wasp taken by Pablo Yoder, a missionary in Nicaragua. At least I recognized it as something in the family Vespidae, but there is a vast number of genera and species in the neotropics.

© Pablo Yoder

Thanks to Google image searches, I eventually pinpointed the wasp as the species Polybia emaciata. It is one of the few social vespid wasps that builds its nest of mud instead of paper. The more durable nature of the mud envelope allows the wasps to “hunker down” or flee when faced with a potential attack by a vertebrate predator. Contrast this behavior with the violent attacks launched by social wasps that build relatively flimsy nests of paper.

Enclosing a nest helps protect the eggs, larvae, and pupae that inhabit each cell in the horizontal layers of combs made by social wasps. Exposed combs are more vulnerable to predators and parasites since the adult wasps cannot be everywhere at once guarding each member of the brood.

© Pablo Yoder

So what is the wasp in two of these images doing with a big drop of water in its jaws? It is essentially bailing out the nest, keeping the mud from becoming saturated. It rains hard and often in the jungle. Even though the nests of wasps are usually ensconced beneath foliage in the rainforest canopy or understory, a good quantity of water still reaches those nests. The workers begin drinking the water, then walk to the edge of the nest and regurgitate droplets. The thick mud walls of the nests of this species take longer to become saturated than do paper nests of other species, but there is still cause for concern on the part of the nest inhabitants.

Nests of Polybia emaciata are typically inhabited by a small number of adult wasps. Colonies usually number between 100 and 500 wasps, while other species would have a much larger number. The nests vary in size from roughly nine centimeters to 22 centimeters in length, as least in one study in Panama (O’Donnell & Jeanne, 2002). The nests may also persist for several years due to their durability.

© Journal of Insect Science

O’Donnell and Jeanne elicited nest defense responses by tapping on the exterior of the nests, or blowing into the entrance hole. Most wasps would respond by rushing out of the nest and either attacking immediately or adopting an agonistic (warning) posture while perched on the exterior of the nest envelope. Not so with Polybia emaciata, which either flew away (though not abandoning the nest), or took refuge inside. Only sustained, escalated provocation resulted in the wasps exiting the nest, then pummeling and attempting to sting the researchers. Natural predators of tropical social wasps include birds, bats, and monkeys.

© Pablo Yoder

The diversity of wasps in the tropics, their survival strategies, and their relationships to other organisms is fascinating. I hope to one day see some of these remarkable insects for myself, but in the meantime I appreciate my readers sharing their own experiences and images.

Source: O’Donnell, Sean and Robert L. Jeanne. 2002. “The nest as fortress: defensive behavior of Polybia emaciata, a mud-nesting eusocial wasp,” J Insect Sci 2.3. 5 pp.