I began studying cicada-killer wasps in the summer of 1989, just after reading in translation Fabre’s wonderful books on French sphecid wasps (“The Hunting Wasps” and “More Hunting Wasps”, Jean-Henri Fabre; Dodd, Meade & Co, New York, 1915 and 1921). Although he used a florid, anthropomorphic style and he thought Darwin absolutely wrong about descent with modification, Fabre’s observations on hunting wasps are acute and very interesting. That summer I discovered several areas on the Lafayette College campus which are used as sites for the communal burrows made by the eastern cicada-killer wasp, Sphecius speciosus, Drury. This seemed an opportunity too good to pass up (a relatively uncommon wasp nesting in profusion nearly at the door of my laboratory) and I have studied the animal’s biology nearly every summer since 1989. What follows is a summary of the published literature on the cicada-killer, to which are added some of my own, mostly unpublished data, pictures and observations.
The picture above on the right is of a female wasp entering her burrow with a cicada. Females weight about one gram and males weight about half a gram. This size difference occurs because female eggs get at least two cicadas to eat, while males usually get only one. Note that females have a pair of long tibial spines on each of their hind legs; they use these in pushing out the dirt they excavate from their burrows (Coelho & Holliday, 2008; Wiedman, 1999).
EMERGING. Cicada-killers hatch out in high summer, sometime between June and July, depending on the latitude and prevailing climatic conditions. At Lafayette (in Easton, PA, USA) they usually emerge within a few days of July 15, when male cicadas begin singing to atract their mates. The previous year’s young have lain underground all winter in unusual cocoons, one of which is shown on the left. The males begin to hatch about two weeks before the females, as is common in many solitary or weakly social wasps and bees; this allows them to be available to mate with the females when they emerge. Because cicada-killers often dig many burrows in favorable areas and several females often make nest cells in the same burrow, a large number of unrelated wasps will emerge the following year in each burrowing area. The wasps developing in each group of burrows, therefore, probably form isolated breeding populations.
MALES HATCH FIRST. Males establish territories in areas where nests were made the previous year (“emergence areas”) and where females will emerge in the current year. It seems very likely that the males simply stay in the area where they themselves emerge because last year’s nesting areas are widely scattered in the environment and the area in which the males emerge is the only place they are likely to find emerging, virgin females with which to mate. This hypothesis has not yet been tested, but I plan to capture and tag newly-emerged males to see if they stay where they hatch. Using tagged wasps, I have learned that males rarely move more than 20 meters from their home ranges and that they will return there when captured and released up to two kilometers away. Thus, it is clear that, although they patrol for potential mates in only one area, they know the surrounding area very well, probably from feeding forays.
Once they emerge, the males either establish territories on the emergence area which they actively patrol or they rest on trees or vegetation within about 15 meters of the emergence area. I call this second group of males “satellite males” because they swoop in to patrol the emergence area for virgin females and are often attacked and driven off by the males with territories directly on the emergence area. This system closely resembles the “leks” or breeding territories set up by a variety of birds and mammals (e.g., the leks of grouse and kob antelope and rookeries of sea lions; see Hoglund’s Leks,1995) and on which males fight for control of small territories in which females are willing to mate with any male which can hold a territory. Thus, I will call cicada-killer emergence areas leks. The highlighted word in this sentence will link you to a 224 KB QuickTime movie of a male leaving his Pachysandra perch to chase another male from the lek and then returning to his perch.
The graph on the right shows the dates of tagging and subsequent sightings for 169 male and 222 female wasps on all seven leks I studied in Easton in 1997. Forty-four percent of marked females and forty-one percent of marked males were seen again at least once; females were nearly three times as likely (13% of marked females vs.5% of marked males) to be seen on a lek other than the one at which they were first captured and marked. The average “life” of a male after marking was 4-5 days and it was 6-7 days for females. The longest times between marking and last sighting were 15 days for one male and 24 days for one female and these numbers probably represent the upper limits of the wasps’ reproductive lives.
FEMALES HATCH AND ARE MATED. Females begin emerging onto the lek one to two weeks after the first males appear. I have preliminary results which indicate that virgin females emit a pheromone which attracts males to them: when placed on an active lek, screen mesh bags containing virgin females attracted many more males than bags containing males or empty bags (the “male visit ratios” are 8:2:1, respectively for two trials). Given that several males are often on a lek when a virgin female emerges, one might think that they would compete to be her mate and sometimes this is the case. If such competition is common, then one might think that larger or more aggressive males might have an advantage and Joe Coelho and I have conducted experiments to see if this is the case. These experiments are presented in the page on male behavior. After being mated, females do not allow a second male to mate and, in mark-recapture studies, I have shown that they usually leave the area where t hey hatched and were mated (only 2 of 20 marked and mated virgin females were seen again in 1997, as opposed to 44% of females caught and marked while looking for burrow sites). This may well be a way of ensuring outbreeding.
DIGGING A BURROW. After emerging and mating, female wasps spend about two weeks searching for appropriate areas in which to dig their burrows. They usually choose southeast-facing, well-drained slopes with lots of large deciduous trees nearby. This means that if cicada-killers nest on your property they will come back year after year. Spraying insecticide to kill them is nearly useless because this year’s wasps hatched elsewhere and chose your lawn because it meets their requirements. The same thing is very likely happen again next year (click here for an excellent statement on cicada-killer control). Females dig burrows up to three feet long and two feet below the surface. At the end of the burrow they dig a spherical nest chamber in which they will put one, two or rarely three paralyzed cicadas. To remove the dirt which they excavate, the females back up the burrow, pushing the dirt behind them with special “spurs” on their hind pair of legs. (2.8 MB QuickTime movie of a burrowing female is available here). A wasp will often dig several nest chambers branching off of the end of the same burrow, using the dirt from the excavation of each new nest chamber to seal the previous one. Females regularly investigate the burrows of other females and there is considerable sharing of burrows by more that one wasp as the season progresses. The picture on the right is a typical burrow; click it to see a large plate of different burrow types and locations. Females sometimes fight in and near burrows, presumably over the use of a completed nest cell. Both females and males are known to take shelter overnight in the nesting burrows. Females sometimes start digging a new nest chamber in their burrows at night, behavior which would maximize the daylight time available for hunting cicadas.
HUNTING CICADAS AND LAYING EGGS. Cicada-killers and other wasps, bees and ants (hymenopterans) have an unusual genetic system of sex determination. Mated females keep sperm in a special structure called a spermatheca and they use it as needed to add or withold sperm when laying individual eggs. Unfertilized eggs develop into males and fertilized eggs develop into females, a system called haplodiploidy. This system of sex-determination was formerly theorized to be responsible for the evolution of sociality in hymenopterans, but the theory is now strongly questioned. What it means for the cicada-killer female is that she must chose the sex of each egg before she lays it. Because cicada-killer females are roughly twice as large as males, during development they need much more food than males do and female larvae are usually given two or sometimes three cicadas by their mothers, while male larvae get only one cicada. Of course, this means that a female wasp intent on laying a female egg must hunt, paralyze and bring to her nest cell at least two cicadas. Thus, it is costly in terms of time and energy for a wasp to lay a female egg and much cheaper for her to lay a male egg. Cicada-killers may well be an excellent model system in which to study the factors affecting choice of offspring sex by females. More data on this subject are presented on the page on the biology of cicada-killer females. The picture on the left is of a male annual cicada, Tibicen chloromera. Click here to see a 164 KB QuickTime movie of a wasp bringing a cicada back to her burrow).
It was originally thought that female wasps hunted mostly male cicadas and that they located them by the loud calls they make to attract female cicadas, but this is not the case. Both male and female cicadas are caught, paralyzed by a sting to the nervous system and carried back to the burrow, where an egg is laid on them. This is quite an effort for the wasp because the cicada can weigh twice as much as the wasp and this makes flying back to the burrow while carrying it a difficult task. Cicadas are sometimes abandoned by the female wasps which have paralyzed them and they can be found on the ground, where they stay alive and flexible for several days until they are found and eaten by ants or birds. I have observed one female house sparrow and her fledgling chick ambush a cicada-killer returning to her burrow with a cicada and I lost sight of them chasing the wasp in what I believe was an attempt to make her drop the cicada. In Easton, I have only observed cicada-killers to capture Tibicen chloromera and T. linnei, although at least three other Tibicen species have been reported in the area.
The picture on the right shows me offering a hand to a female who had landed short of her burrow and was dragging her cicada through the grass with great difficulty. It is not uncommon for this to happen and wasps will sometimes climb up a tree trunk, a bush or, in this case, a biologist, to gain altitude for another takeoff to return to the burrow. Females are known to abandon their cicadas if they get close to the burrow but cannot find the entrance and spend too much time wandering around dragging the cicada. This is thought to be adaptive because there are several species of flies which will lay maggots on the cicada while the wasp is handling it near the burrow entrance, where the flies lay in wait for them. The maggots develop rapidly and they either kill the developing wasp larva or eat so much of its cicada that it dies or develops into a very small adult.
As shown in the picture on the left, female cicada-killers always lay their 4-5 mm long eggs under the left or right second leg of the cicada, presumably because it puts the emerging wasp larva in the best position to begin eating the cicada. As noted by Fabre, insects paralyzed by hunting wasps continue to live and they stay “fresh” (that is, mold-free) much longer than if they are dead. The growth of mold on the cicada and larva has been a major source of mortality in my atempts to rear wasp larvae in artificial burrows in the laboratory and may well be an important factor in larval mortality and/or malnourishment resulting in small larvae and adults in wild wasp populations.
LIFE’S A BITCH AND THEN YOU DIE. It’s impossible to relate to what might be going on in a nervous system as small as a wasp’s, but one must imagine them to be happy, as Camus imagined Sisyphus to have been. The males live for two weeks or so of intense lek patrolling, fighting and mating and then they die. I have often found them dead near a lek with wings worn so short that they must have had difficulty flying (picture on right). Females live longer, about four weeks after emergence, but they work even harder than the males, digging many burrows and nest chambers and bringing back an estimated 108 cicadas on which to lay 42 female eggs and 24 male eggs. I was sitting in the shade early in September one year watching an area of active burrowing when a female cicada-killer fell out of a tree above me like a falling leaf and died soon after hitting the ground. Since all of the adults die each year, the next generation comes from overwintering larvae which develop underground and hatch out the following July.
DEVELOPMENT AND HATCHING. Within a day or two of being laid, the egg rapidly develops into a small larva which begins eating the cicada at the thin skin of a leg joint. It molts frequently and develops a long neck behind its small head and is able to reach farther and farther into the cicada as it grows. The larval development has been described by Evans and Lin (1956) and I have been able to film it in laboratory-reared larvae. Several QuickTime movies documenting larval feeding and growth are available on the page on larval development and hatching.
The picture above on the left is of a 30-millimeter larva; it is nearly ready to make a cocoon and has its head and long neck stuck into the cicada on which it is feeding. Female larvae develop from fertilized eggs and they are given two or more cicadas by their mothers; when they have finished the first cicada they move to the second one. Male wasps develop from unfertilized eggs and they are given only one cicada by their mothers. Development at room temperature in the lab is rapid and the larvae take only about 10 days to grow to full size and weave a cocoon in which to overwinter; unfortunately, I have not been able to get any lab-reared larvae to survive over the winter in the lab. The picture on the right below is of a lab-reared larva which was taken from its cocoon not long before it would have hatched. Shown on the left is a newly-hatched male wasp; the white substance has been defecated by the wasp and is probably rich in uric acid and other water-saving nitrogenous waste products, as is the meconium voided by other insects on emergence.
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