Wasp Wednesay: Hemipepsis ustulata

The term “tarantula hawk” applies to more than one species of large wasp. It even applies to more than one genus. Here in the United States, the genera Pepsis and Hemipepsis are the only tarantula hawks, and the largest of our spider wasps in the family Pompilidae. Hemipepsis ustulata is a common species across much of the western U.S., one I have encountered in both Arizona and Colorado.

It is not easy to separate Hemipepsis from Pepsis, but I find that Hemipepsis are matte black in color, not the iridescent blue-black that their relatives are. Hemipepsis also tends to fly earlier in the season than Pepsis, especially in the southwest, but both genera can be found together in the fall, Hemipepsis being in the second generation in autumn. Wing venation is apparently the only sure way to tell the two apart, but most folks understandably don’t want to get that close to such a large, stinging insect to find out which is which.

Tarantula hawks in general have interesting behaviors, especially the males. Dr. John Alcock has studied in-depth the territorial behavior of male Hemipepsis ustulata in the Superstition Mountains near Phoenix, Arizona. They are among the first insects seen in spring, but as the season wears on they become early risers, taking advantage of the brief window of time when Sonoran Desert temperatures are not too cool, but not incinerating, either. They are looking for prominent outposts, often Palo Verde trees atop hills.

Once a male wasp claims a lookout tree, it vigorously defends it from competing males. Most interactions consist of the resident male flying off his perch in a brief foray to chase away the intruder. There is usually only feeble resistance, if any, from the interloper. Males that have been displaced from their own territories, however, are more likely to try and displace another male. This takes the form of long, ascending flights in which the wasps spiral around each other as they climb to heights of thirty feet or more. Sometimes they escalate to altitudes where they can no longer be observed without binoculars.

These aerial duels are contests of both endurance and speed. The faster and fitter male is thus the one more likely to win.

Female wasps cruising past an occupied tree elicit an entirely different response. The resident male gives chase and, if he successfully reaches her, attempts to couple with her in the air and descend to the ground. There, they mate briefly if she is receptive. The male then returns to his perch, which he hopes has not been claimed by another male while he was um, otherwise engaged.

This form of territoriality is called “landmark defense,” and it is the strategy of choice in situations where defense of a resource used by females is impractical or impossible. While blooming palo verde trees might seem to be a nectar source for female tarantula hawks (and they are, for Pepsis chrysothemis), females of Hemipepsis ustulata seem to prefer the flowers of creosote bush, which has a wide and scattered distribution in the landscape.

Once mated, a female tarantula hawk goes about the business of finding occupied tarantula burrows and baiting the spider into battle aboveground. These burrows are also widely scattered. So, male wasps can’t stake out a tarantula “colony” because there is no such thing, at least not for spiders the size needed by the female wasp.

Some of the hunting activities of these wasps may take place at night, or at least at dusk, for I have found females in southeast Arizona coming to blacklights at night. I have also found females feeding on oozing sap of Desert Broom (Baccharis sarothroides). Here in Colorado Springs I have seen both genders at a blooming saltcedar tree (Tamarix sp.), though the female simply circled the tree and flew off again.

Hemipepsis ustulata ranges from southern California east through Nevada to Texas, Oklahoma, and Kansas. It would be well worth it to accumulate more records of this species. More county records are certainly possible, and perhaps state records, too.

Sources: Alcock, John. 1984. “Ridgetop Rendezvous,” Natural History 93: 42-47.
Evans, Howard E. 1997. “Spider Wasps of Colorado (Hymenoptera, Pompilidae): An Annotated Checklist,” Great Basin Naturalist 57(3): 189-197.

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.