Analysis of mud dauber venom can also provide cues to its defensive effectiveness. Stings from insects with effective defensive venoms hurt, cause damage, or both. Painful components include basic, that is alkaline, peptides, often with the small neurotransmitters such as histamine, acetylcholine, and serotonin. Common pain-inducing peptides among wasps are analogues of bradykinin, a small peptide that acts on the heart and causes intense pain. Mud dauber wasp venom lacks all of these components.2,15 A total lack of toxicity data to mammals or to arthropods hints that mud dauber venom also lacks meaningful toxicity. Stung spiders, though powerfully paralyzed, exhibit minimal indications of toxicity, and the heart and likely the digestive system and blood cells are unaffected. This implies no direct toxic or tissue-damaging effects are present.16
Once again, we reach an impasse. Are mud daubers dangerous? Do their stings hurt? We have little evidence concerning the pain, let alone potential danger, of a mud dauber sting. To make matters worse, I have never had the (mis)fortune to be stung in the process of normal fieldwork or research by a mud dauber (not that I haven’t been casual and lackadaisical around them). Seemed to be déjà vu cicada killers. I hadn’t been stung, predicted they wouldn’t hurt much (or they would have already stung me), and needed some facts. Only this time I was lacking a Joe Coelho to advise me on the sting: I had, at best, the tenuous comments written by Rod O’Connor more than a half-century ago. OK, time to bite the bullet—grab a mud dauber, get it over with, and go home with a data point and new wisdom.
It was a fine June day in Willcox, Arizona. No rain had fallen in months, and the only water around was in stock tanks for cattle. I was visiting a stock tank filled by an Aermotor windmill pumping water from underground into a large metal trough formed from half an old underground fuel tank excavated from a gas station. Fortunately, for me and the mud daubers, the check valve to the intake had failed, allowing the tank to overflow, forming a big puddle of mud. Many mud daubers were busily collecting mud from the puddle edges. Opportunity. I grabbed a big mud dauber and guided her posterior to my left forearm. With some struggling, she planted her stinger in my flesh and delivered her payload, after which I let her go, and she flew off. Underwhelming. That’s the only word to describe the pain. A minor, immediate sharp pain was detected. The pain did not jump out and grab my attention. Instead, it produced a pain somewhere between 0 and 1 on the pain scale. Shortly thereafter, a minor burning developed, earning 1 on the scale. This pain soon dissipated, leaving no visible swelling, redness, or sign of a sting. A trivially minor pain for a data point of major importance. Time to flee the heat of the day, have a cold beer, and relax writing notes.
THERE ARE TOO MANY SOLITARY WASPS to study them all. However, the large iridescent blue digger wasp, Chlorion cyaneum, commands our attention and respect, if not outright fear. Perhaps this wasp is intimidating because of its large size, 25–30 millimeters long; perhaps it is intimidating because of its flashy, iridescent brilliant blue body and purplish-black wings; perhaps it is intimidating because of its narrow thread-waist, something people seem to fear; or perhaps it is intimidating simply because of its rapid, jerky movement. Or maybe we are intimidated by all of the above.
Who is this colorful wasp who so catches our attention and had a 1970 El Salvador 30-cent stamp honoring her? She is a member of a small, elite genus of 18 thread-waisted wasps, most of which live in the Old World. The few studies of the group indicated that they are specialists on crickets, which they sting causing only transient paralysis in some species and long-lasting paralysis in others.1 One widespread African species, Chlorion maxillosum, takes parental care to a minimalistic extreme. Not only does mother wasp fail to dig a burrow to house her young and their cricket food, she doesn’t even transport the cricket to a safe location. Instead, she stings the cricket into short-lived paralysis, lays an egg on the cricket, and abandons it to fend for itself. The cricket soon recovers from its paralysis and either returns to its own burrow or digs a new burrow. The Chlorion larva now feeds safely and securely on its cricket in its ready-made cricket burrow.
Another species from North Africa provides slightly more parental care. She drives the cricket from its burrow, stings it into transient paralysis, lays an egg on the cricket, then drags the cricket back into its burrow and seals the burrow. North America has only three species of Chlorion; the two studied species both sting their prey into enduring paralysis. The common blue cricket killer digs legitimate 6- to 44-centimeter-long burrows, usually in sandy soil, and hunts crickets in the local area. Crickets are seized, stung in the underside of the thorax into total paralysis, and dragged back into the pre-excavated burrow. This is an ordinary enough behavior for solitary hunting wasps, but the blue cricket killer adds a unique twist. She often digs her burrows deep within the burrows of cicada killers. If wasps can be lazy, we might call this laziness; otherwise, we could call it efficiency, or safety and security disguised within another’s home. In any case, the cicada killer doesn’t seem to pay attention to the intruder and both coexist peacefully.2
Chlorion cyaneum, let’s call her the iridescent cockroach hunter, is the most unusual member of her group. She is particularly fond of sand dunes and other sandy areas, and she refuses crickets. She also can withstand extreme sand temperatures up to 50°C (122°F).3 What makes her so unique among her 17 relatives is her choice of cockroaches—sand, or sand-swimming, cockroaches. These cockroaches literally dig and swim through loose sand, and the females are wingless. The males are winged, flattened, tan cockroaches frequently attracted to lights at night. The iridescent cockroach hunter digs a burrow 15–30 centimeters long and provisions the cells with female, immature, and male sand cockroaches, which she stings into total paralysis.1
I became interested in iridescent cockroach hunters when I saw them in the field, strutting around flipping their wings, almost saying “see me, here I am” with the implied message “better leave me alone.” Wow, this wasp seems to be telling me something; but is it all bluff like a gopher snake hissing and shaking its tail in the leaves to imitate a rattlesnake, or is it real? Oh, these solitary sphecid wasps are getting tiring. So many seem intimidating, yet none follow through with stings. Worst of all, records of stings are rare to nonexistent in most and totally lacking for Chlorion. The best we have is Eric Eaton’s comment on his blog, Bug Eric, that one cannot easily examine live specimens “without getting painfully stung” (bugeric.blogspot.com, August 18, 2010). OK, time to get it over with. The wasp is not going to voluntarily sting me or anyone else, so I reached into the insect net and grabbed a fine female, who stung my fingertips twice during the removal process, and applied her to my right forearm. The pain was sharp, with a nettle-like flare. Anyone who has walked through an eastern North America nettle patch knows the feeling. The pain, fortunately much less than real nettle stings, lasted about 3–5 minutes before the last rash prickles disappeared. Pain rating of 1+ on the pain scale, more than a mud dauber, but certainly less than a honey bee sting. Another ordeal survived.
PAPER WASPS ARE WELL-KNOWN, pain-causing, stinging social wasps that build open papier-mâché nests attached under roof, door gable, or other sheltered places. Nobody, or at least nobody with intact nerves, doubts the painfulness of their stings. They live in social colonies with overlapping generations, task specialization, and parental care. They also have solitary relatives in the same family, the Vespidae, that look similar and prey mainly on caterpillars. Paper wasps evolved from a lineage of these solitary wasps, thereby providing possibilities of looking into the origin of the paper wasps’ painful defensive venoms. Did the paper wasp ancestor already painfully sting, or did painful stinging evolve after paper wasps branched from their solitary relatives? Fortunately, many solitary relatives of paper wasps exist and provide a convenient means to probe this chicken-or-egg question.
Meet the “walk on water” wasp, sometimes referred to as the “Jesus Christ” wasp. Actually, several wasps, especially in the tongue-twistingly na
med genus Euodynerus, routinely land on water to take a drink. But here the main focus will be on the species crypticus that especially resembles some paper wasps. These wasps do not literally walk on water; instead, they come to open-water surfaces like mini-helicopters lowering themselves from the sky, landing gently on the water with legs widespread and wings held obliquely back and raised as if ready to take off at any moment. Without moving or walking, they take a deep drink from the water surface for 12–15 seconds, after which, like heavily laden tanker helicopters on their way to fight a forest fire, lift off slowly from the water surface and fly away.
This behavior raises several questions: Why do the wasps perform this behavior of landing on open water, risking drowning? Why do they want so much water? The first question is harder to answer with certainty but appears to be a way to reduce the risk of predation. In natural situations, the wasps are rarely found drowned under the surface. Perhaps the risk in these normal situations is not that great, unlike in the artificial situations of man-made swimming pools where they are sometimes found drowned, especially after active kids cannonball into the water. By landing on water, the wasps avoid ambush by various predators, especially frogs, lurking near the water’s edge.
The answer to the question of the great need for water by water-landing wasps comes from their life history. Dwight Isely in 1913 described in detail the natural history of E. crypticus in Kansas.1 The female wasp selects very hard, dry surfaces in bare ground to dig her burrow. Such hot, dry areas likely have fewer intruders, predators, or parasites than other areas, but the ground is rock hard. The wasp solves the problem by moistening the soil and then removing large mud balls dropped a short distance away, littering the bare area around the burrow. Her vertical nest goes down about 10 centimeters and has one or two cells. Isely described one wasp making 16 water trips and removing 86 clods of earth in 40 minutes while excavating her nest. When the digging is finished, the wasp hunts skipper butterfly caterpillars that she extracts from their tough silken retreats within crumpled leaves and stings each caterpillar three or four times in the throat and toward the nerve ganglia that control the legs and jaws. Five to seven mostly paralyzed caterpillars are stored in a tangled mass in a cell, an egg laid, and the cell closed. In Arizona, the yellow paper wasp, Polistes flavus, also floats on the water’s surface while collecting water. It also looks remarkably similar to crypticus. The main difference is that crypticus is stockier. Is this a case of mimicry or simply an ancestral and a derived species that look similar?
Back to the question of sting pain: did sting painfulness or the sociality of paper wasps come first? Two points are noteworthy. First, the social wasp nest is exposed and vulnerable to all sort of predators, especially large predators, whereas solitary crypticus has very little to defend, especially from large predators unlikely to dig into rock-hard ground for such a small reward. Second, crypticus needs to keep its caterpillars alive, fresh, and paralyzed. Polistes kills its caterpillars by chewing them into meat balls to feed immediately to the young. Intuition would say that crypticus has little need for painful or damaging venom active on large predators and, to the contrary, damaging venom might be disadvantageous by killing the prey, causing it to spoil. In contrast, Polistes has no need to preserve prey alive but does have great need to have painful and damaging venom to deter predators. Thus, we would predict that selection pressure would cause pain to form during the process of social evolution in Polistes, not before in the purely solitary phase. The chicken comes before the egg.
Testing time. Once again crypticus and other solitary relatives of paper wasps are unlikely to ever sting anybody voluntarily or in defense. Ready. Grit your teeth. Let’s go. Back to the stock tank, only this time the wasps were collected while floating on the greenish water. Three different crypticus were placed on my arm and enticed to sting. Each produced a low-level burning, somewhat analogous to a micro-bit of paper wasp venom. Being charitable, the pain rated at best a 1 on the pain scale. Not content with just one species of eumenid wasp (the solitary relatives of paper wasps), I captured a large potter wasp in Shepherd’s Tree caravan park near Ellisras in South Africa and had her sting my wrist (she wouldn’t/couldn’t sting through my fingertips). Again, at best a 1 on the pain scale. I was content with these two experiences, but fate apparently was not. One day while I was walking sandal-clad through a mesquite flat, I felt a sharp pain under the middle toe of my left foot. It was sharp and somewhat itchy, but not burning like a paper wasp sting. This one rated higher, a 1.5 on the pain scale. The culprit turned out to be a yellow eumenid wasp. Looks like solitary wasps, either eumenids or sphecids, are not able to produce meaningfully painful stings.
MY SON KALYAN, WHO HAD JUST TURNED 8, asked, “Dad, are there any insect tanks?” “Well, if you mean hard as an army tank, fast as an army tank, and with the firepower of an army tank, then, yes. They are called velvet ants.” Velvet ants? What are velvet ants? No, they are not ants, they are wasps that look like sturdy ants, often covered in dense velvety red, orange, yellow, white, or black fur, hence their name. Unlike ants, they do not live in social colonies, have no queens, and live strictly solitary lives. Commonly recognized velvet ants are female wasps that have no wings, nor even a hint of wings. They can (and we might add, readily do) sting and have the distinction of possessing the longest, most agile stinger found within the insects. Female velvet ants literally are micro-tanks on six strong, short legs. They are hard as a rock, sometimes so hard that entomologists bend the steel pins used to mount insects for study. For young children, who are always up for a challenge, one of my favorite challenges is to spot a velvet ant running on the ground and say, “I’ll bet that you can’t smash that insect.” Fighting words. The universal response is to stomp on the velvet ant. It simply makes a velvet ant impression in the ground; the velvet ant picks itself up and runs off. Stomp, stomp, stomp. The same result. But don’t try this barefoot.
Unlike the females, male velvet ants do not look at all like ants or their sisters. Instead, they sport fine functional wings, are usually black or brown, sometimes with splashes of color, and appear more like slow, furry, ill-defined flying insects. Although they are wasps, they don’t have that sleek, agile appearance of most wasps, resembling mini-flying, meandering teddy bears. “Teddy bears” they are. They cannot sting like females, cannot really bite, and when captured sing and release perfume. Quite cute and harmless. Females, too, are cute, but far from harmless.
In 1758, Carl Linnaeus, the father of modern taxonomy, described several species of velvet ants, among them Mutilla europaea. This uncommon velvet ant is also one of the most unusual. Of the roughly 6,000 velvet ant species, only this species and perhaps one or two other closely related species are known to use highly social insects as hosts; all the rest of known hosts are solitary insects or, at most, primitively social. The habit of parasitizing bumble bee and honey bee colonies appears to be the reason M. europaea attracted early attention. In the eighteenth century, sugar and sweets were expensive and in scarce supply, and honey was highly prized for a variety of reasons in addition to its sweetness. No wonder something that attacked honey bees was noted and described early. The invading M. europaea targets mainly colonies of numerous different species of bumble bees. Struggles between guarding bumble bees and the velvet ant are rare, perhaps good for the bees, as attacking bees tend to end up dead. Once in, the velvet ant makes itself at home, moving about unimpeded by bees and parasitizing bee postfeeding larvae and pupae within their silken cocoons. The velvet ant egg hatches into a larva that feeds as an ectoparasite on its bee host, molting four times as it grows, and finally, having finished its food supply, spins its own cocoon within the bee’s cocoon, pupates, and emerges as a fresh adult 30 days after the egg was laid. These velvet ants can be serious threats to bumble bees with as many as 76 velvet ants produced in one colony.1
Mutilla europaea will on occasion enter honey bee colonies, thereby ensuring its fame. Some honey bees, as with bumble b
ees, attack invading velvet ants, suffering the same lethal outcome. After a few minutes, the honey bees avoid the intruder, who seeks bee larvae spinning their cocoons and lays an egg inside with the spinning larva. Several literature reports described scary scenarios of bee losses within beekeeper colonies, something, which if accurate, seems to have disappeared with modern beekeeping, and likely was greatly exaggerated at the time. These stories fit nicely with an enduring human tendency to embellish stories to make them more interesting. Not to be outdone by the Europeans, a variety of late nineteenth- and early twentieth-century American writers described dire situations, such as velvet ants being “serious insect enemies of honey bees,” statements best regarded as fanciful or old wives’ tales (all written by men), for no valid documented reports of invasion and damage to honey bees in the New World are known.1
The most famous North American velvet ant is the cow killer, so named because those stung by one felt the sting could “kill a cow.” This velvet ant, Dasymutilla occidentalis, vies for the most attractive species, with its short, neatly aligned, red and black velvety coat and two pleasingly arcing, large red spots on its abdomen. This beauty is memorable to anyone who sees it and adorns most nature guides that cover insects. The cow killer was prominently figured in 1703 by James Petiver, making D. occidentalis the first North American velvet ant species accorded such recognition. Misinformation surrounds any conspicuous organism, and such is the case with the cow killer. C. V. Riley, the first official U.S. government entomologist and first Smithsonian Institution curator of insects, published in 1870 a letter from a Texas man who reported a cow killer entering a beehive and killing bees that attacked her.2 This began the vilification of the cow killer, which lasted until 1932.
The Sting of the Wild Page 19