When Cicada-Killer Wasps Become Cicada-Stealer Wasps
By Edward Ricciuti
Now a legendary ghost town, Ruby, Arizona, was once a brawling mining community, where residents kept guns handy to ward off murderous bandit gangs raiding from nearby. Banditry still continues at Ruby today, but not by humans—rather, in the form of aerial piracy by birds stealing cicadas from wasps.
These birds, kingbirds and roadrunners, snatch nearly half the annual cicadas carried by female cicada-killer wasps (Sphecius spp.) to feed their young upon hatching in their underground brood chambers.
Even when a cicada-killer wasp manages to run the gauntlet of avian attackers, however, researchers afield at Ruby have found its catch remains at risk of yet another kind of theft. If the wasp leaves its burrow to hunt again without laying an egg on the cicada it has stored, another female wasp may sneak in and surreptitiously lay its own egg on the host. It is a strategy by which females that may have difficulty obtaining their own prey may pursue so as to still be able to reproduce, according to a study published today in of the Journal of Insect Science. The research was conducted by Joe R. Coehlo, Ph.D., of Quincy University in Quincy, Illinois; Charles W. Holliday, Ph.D., of Lafayette College in Easton, Pennsylvania; and Jon M. Hastings, Ph.D., of Northern Kentucky University in Highland Heights, Kentucky.
Losing cicadas to their shady sisters is not a phenomenon restricted to the Ruby vicinity, according the paper, which compared freeloading females of the ghost town’s Pacific cicada killers (S. convallis) with those of eastern cicada killer (S. speciosus) on the bucolic campus of Lafayette College. Both places a have large colonies of cicada-killer burrows, at Ruby in sandy tailings of an abandoned lead mine and at Lafayette in fine-clay soil of architectural berms next to a building.
Observations at both locations, conducted in Arizona in 2009 and Pennsylvania in 2008 and 2010, suggest that when competition for prey among female cicada-killer wasps is high or when prey numbers are low, dishonest provisioning of the nest may be the way to go from an evolutionary standpoint, allowing the wasps to expend less time and energy resources for the same result. After all, hunting cicadas and hauling them back is tough, labor-intensive work. Typically, after a female cicada-killer wasp mates in early summer, she burrows about a foot deep and 18 inches long into loose soil then digs nest chambers off the tunnel where she lays eggs. Next, she must search out a cicada in the trees, sting it, and lug it home. The load is often larger than the load-bearer, sometimes so heavy the cicada-killer wasp must haul its prey to a launch point well up in vegetation to get airborne.
A wasp loaded down by a cicada payload is easy to spot and about as maneuverable against kingbirds as a cargo plane attacked by fighter jets—a big, fat target, in effect. For the kingbirds, stealing wasps’ cicadas is much easier than hunting for these insects, whose cryptic coloration can make them hard to find. Seventy percent of the kingbird raids observed at Ruby were successful, suggesting that bird predation could be a major drain upon cicada-killer populations where it is common. However, since there was virtually no robbery by birds at the study site in Pennsylvania, the researchers surmised that, while birds’ thievery may influence wasps’ host stealing, the practice can occur whether or not birds are raiding.
At Ruby, kingbirds congregated around the wasp colony, as the concentration of cicada killers made for easy pickings. Typical of flycatchers—the family to which kingbirds belong—kingbirds perched atop low bushes, surveying the vicinity for wasps. Once a target was sighted, they sallied forth with a quick, darting flight pattern, seized the cicada with the bill and wrestled it away. Rarely, a kingbird may kill the wasp.
Taking another creature’s prey is known as “kleptoparasitism.” Similarly, the researchers have christened the behavior of wasps appropriating cicadas from their fellow wasps as “provisioned nest cell kleptoparasitism” (PNCK).
“At high female population densities and/or in times of low prey availability—i.e., Ruby in 2009—PNCK should be far more advantageous than ‘honest’ provisioning of a wasp’s own nest cells and, thus, selected for,” say the researchers.
“Pirating cicadas may be the only viable reproductive outlet for females that are small or in environments with few prey. We suggest that provisioned nest cell kleptoparasitism may have evolved in cicada killers as an alternative strategy to standard provisioning, given the dual uncertainties of adult body size and prey availability,” they write. It could be, say the researchers, that all females are capable of performing PNCK when they can, but that may be the only way those too small to carry cicadas may reproduce.
The researchers were able to snoop on the trespassers by tempting them into an ingenious “trap nest” device that mimics burrows and the nest cells where they placed a PVC pipe to simulate the burrow and a plastic cup to simulate the cell. “If we had not utilized trap nests, we would not have known that it was occurring at all,” they say.
The study started after the researchers noticed female wasps entering the burrows of other females, which had been left often while the owner was hunting. Sometimes the owner would return and evict the trespasser. The researchers decided to spy on burrows to determine what was happening underground and came up with their trap nest as a way to do it.
As a rule, each nest chamber holds one egg and gets one or two cicadas. Female wasps know the sex of the egg before they lay it. A male egg often gets only one cicada, given that male wasps grow to be much smaller than females. Females usually get two, lunching on the second after finishing the first. When more than one cicada is to be placed in a nest cell, the female leaves the burrow open with the first cicada within while she is away hunting for additional cicadas. About half of open burrows are victimized by intruding females.
It is possible that frequent PNCK favors an over-population of male wasps. As to whether this is fact or not, Coelho, lead author of the study, says “We would predict so, but have no evidence to back it up.”
As long as they were collecting cicadas paralyzed by wasps, the researchers decided to put them to use. Many people find the insects tasty, with a nutty flavor. Indeed, canned cicadas are available commercially. In their paper, the researchers noted, “The authors confirmed the palatability of T. parallelus [a cicada] by themselves consuming a number of freshly paralyzed specimens, roasted on a gas grill, without ill effect.”
Adds Coelho, “I made kabobs with tomato and bell pepper and grilled them. We had paralyzed cicadas left over after the experiments, so it seemed like the natural thing to do.”
Journal of Insect Science
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.