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.

Spider Sunday: Arizona Blonde Tarantula

Without a doubt, one of the iconic animals of the Sonoran Desert is the tarantula. What would a Western, or desert horror movie be without a shot of one of these giant spiders moving leisurely across the landscape? Maybe a tumbleweed, too, though a Russian Thistle moves faster than a strolling tarantula.

Tarantulas belong to the family Theraphosidae, and only the genus Aphonopelma is native to North America. There are 54 currently recognized species north of Mexico, and determination to species level is complicated by the fact that these are primitive animals in the evolutionary sense, with simple genitalia that don’t allow for easy comparison. That said, the most common species here in southern Arizona is the “Arizona Blonde,” Aphonopelma chalcodes.

It is mature male spiders that are most often seen crossing roads, especially at night, during the summer monsoon season. The male is easily recognized by his relatively lanky appearance, with a smaller abdomen and longer legs than the female. His front legs also have a spur on the underside of the tibial segment. The spurs are used to hold the female’s fangs at bay during mating. His coloration also differs from that of the female. His legs are entirely black, while the female has black femur segments with the remaining leg segments brown.

Both sexes live in burrows in the ground. Females rarely venture out, lest they become even more vulnerable to tarantula hawk wasps, and other predators such as coatimundi. An occupied burrow will have a thin curtain of silk over the entrance during the daytime. The spider will sit at the lip of its tunnel during the night, the better to ambush any unsuspecting insect or other small animal that comes within striking range. During the winter months the spiders plug their burrows with soil. Fat reserves sustain the spider during those lean months.

Tarantulas begin life as surprisingly small spiders, like the one below. Margarethe Brummermann found this specimen under a cow patty near Sunsites in the Dragoon Mountains. It takes a minimum of seven years for a tarantula to reach sexual maturity, and that is for captive specimens that receive regular meals and are otherwise provided with optimal conditions.

The black patch on the abdomen of the spiderling indicates special hairs that tarantulas use in self-defense. Rather than striking at an adversary with their fangs, our North American species literally kick themselves in the rear. Be careful if you see a tarantula doing this. It is dislodging short, microscopically barbed hairs that easily become airborne. These hairs, if they contact mucous membranes of the nose or eyes, cause extreme irritation. Some people may also have an allergic reaction, including a rash or other inflammation of the skin. Even a molt (shed “skin”) of a tarantula can bring about an allergic reaction in those people who are susceptible.

Tarantulas are among the very few spiders that continue molting after they reach adulthood. This process helps replenish those defensive hairs, and shed mites, other parasites, and dirt. Captive female spiders like the one shown above have been known to live as long as thirty years, but “wild” specimens likely don’t survive nearly as long. Males, after reaching adulthood, abandon their burrows in search of mates. After successfully breeding they usually die.

Members of the genus Aphonopelma are collectively distributed west of the Mississippi River and as far north as Missouri, Colorado, Utah, Nevada, and the southern two-thirds of California.

Tarantula Hawks

I consider wasps my favorite insects. The honest truth is that I first took interest in them because nobody could call me a sissy for collecting an insect that could fight back! The more I learned, though, the more fascinated I became with their behaviors and sheer diversity. The solitary “spider wasps” of the family Pompilidae are one such example.

Here in Arizona, spider wasps reach the gargantuan proportions of the so-called “tarantula hawks” in the genera Pepsis and Hemipepsis. The largest females exceed two inches in body length. The standard appearance of these colorful insects is an iridescent blue-black body with bright orange wings, though some species have black wings with bluish reflections.

Thankfully, despite their size and high voltage sting, the females are generally placid, not easily aroused as they seek nectar at flowers. Milkweed is decidedly their favorite source of nectar, but I have also seen them on blooming creosote bush (as this female Pepsis chrysothemis demonstrates), blue palo verde, eucalyptus, and various mesquites and acacias. Females will also drink water at the edges of puddles and ponds.

Males are often more numerous at nectar sources than the females, and are recognized by slimmer bodies, longer antennae, and long, flattened hind legs. The males can also gather in multi-species “bachelor parties” during the hottest hours of the day, and also overnight and during inclement weather. They gather on certain trees in large, loose clusters. The odd female may join them, though mating does not seem to take place at such congregations. The males of at least some species perform a behavior called “hill-topping,” whereby they perch on trees at the highest point in the surrounding landscape, the better to spot and intercept passing females as they fly below.

Once mated, the females go about the business of earning their name. They search feverishly on foot, with flickering wings and quivering antennae, for the burrow of a tarantula spider. This behavior frequently takes place in the cooler morning, evening, or overnight hours, and is not often observed.

An occupied burrow discovered by the wasp usually results in the spider chasing the wasp back aboveground, and the battle is on. Initially it is a series of strategic maneuvers, each animal well aware of the agility and weapons the opponent possesses. Most of the time the female wasp manages to eventually sting the spider on its underside, striking a nerve center that renders the hairy arachnid paralyzed. She then sets about dragging the helpless victim to the nearest underground cavity, often choosing the spider’s own burrow. The tarantula hawk then lays a single egg on the still-living but comatose spider. The larva that hatches will then consume the spider as it grows ever larger during the long feast. The grub eventually pupates, and emerges as an adult wasp months later. Given the abundance of the wasps, there must be a very large tarantula population in the Sonoran Desert.

Tarantula hawks are diverse, too. I have personally collected seven species in Tucson alone: Hemipepsis ustulata, Pepsis grossa, P. chrysothemis, P. mexicana, P. thisbe, P. cerberus, and P. mildei.

My good friend Justin Schmidt, an outstanding research scientist, ranks tarantula hawks near the top of his “sting scale,” but has learned something very interesting. He concludes, both from personal experience and chemical analysis, that the venom of these wasps causes excruciating, short-term pain in humans (and likely other mammals), but does virtually no damage to tissues, nerves, or any bodily functions. You are in utter agony for about three minutes, and then you generally can resume normal activity.

Personally, I think it pays to heed the bright aposematic (“warning”) colors of these wasps, and to let them be. I’ve also learned that, once in my net, the wasps adopt a threatening posture with wings flared and abdomen curled, while liberating a distinctive fragrance at the same time.

My own fascination with these wasps only continues to grow, and I encourage everyone who encounters tarantula hawks to react calmly and enjoy watching them live out their lives. After all, few insects are so large and colorful.