BODY SIZE RANGE. Female cicada-killers are much larger than males because their nest cells are provisioned with two or sometimes three cicadas by their mothers; their body size range is also larger than that of the males. The picture on the right shows the range of female wasp body size I have seen in the Easton, PA, area (smallest 356 mg, average 969 mg and largest 1564 mg). The size distribution of all of the wasps I have caught in the period 1990-1999 is shown in the graph on the left. The overlap in size between males and females is 10-15% and this clearly means that: a) many female larvae get provisioned with only one cicada by their mothers or b) that one of the two cicadas they got turned sour or c) that their mothers "intended" them to be males and that a mistake in contraception occurred. Female wasps make male eggs by witholding sperm during egg-laying. Thus, one would predict that laying female eggs would reduce the incidence of unintended fertilization of an egg (and, thus, the making of a very small female wasp given only one cicada) by laying female (= fertilized) eggs until they run out of stored sperm and then laying male eggs until they die. Constant alternation between laying male and female eggs would, at least in theory, increase the incidence of fertilization errors resulting in small females. Perhaps the 10-15% of females in the male size range were created by such mistaken fertilizations when their mothers were switching from making female eggs to making male eggs. I hope to be able to follow individual female wasps over an entire season in the summer of 2000 to learn whether or not there is a temporal pattern in the sex of eggs they lay; my preliminary work on this idea in the summer of 1999 is equivocal. See the discussion on the "Hunting" page for more on the subject of offspring sex ratios.

ANNUAL VARIATIONS IN FEMALE WASP SIZE. As did the male populations, female populations on Easton leks showed variations in body mass. Shown in the graph on the right are data from 1990,1991 and 1997, years in which I marked more than fifty female wasps. As was the case with the males, the mean mass varied from year to year, and the 1990 and 1997 females were significantly smaller than those in 1991. I have experienced much difficulty in laboratory rearing of wasp larvae because of mold growing on the cicadas on which the larvae are feeding and I have several times found larvae in artificial burrows (see Larval Development section) apparently killed by mold or, perhaps, disease. Dambach and Good (1943) note that they have found fly larvae eating the cicadas in wasp nest chambers and this would have reduced the amount of food available to the wasp larvae or might even have caused them to starve to death. Perhaps the annual variations in wasp size reflect differences in fly populations and reduced food availability to the wasp larvae as well as variations in the size of cicadas available to hunting cicada-killers and in the degree of fouling of the cicadas by mold, all of which would restrict food availability. Lin (1966) has reported that larval survival is directly correlated with total July/August rainfall.

FEMALE POPULATION SIZES ON EASTON LEKS. Shown on the left are data on female wasps marked and recaptured at the Van Wickle Hall lek at Lafayette in 1990. On three successive days (223, 224, 225; August 12-14) all female wasps present at the lek were caught and the ratios of marked to unmarked females were used to calculate the population size using the same calculations presented for the male wasps on the previous page. The mean value for the three population estimates (131, 118 and 124) was 124 wasps; the male population at the same location 20 days earlier was estimated at 74. Thus, and as was the case with male wasps, a large population of females used the lek, despite the fact that at any one time, only a few females were present on the lek. As of August 14, there were 17 burrows on the Van Wickle lek and this number increased to 32 by September 2, the date of the appearance of the last burrow I found on the lek in 1990. Females spend a large amount of time flying slowly over a lek, landing frequently to inspect prospective sites for digging new burrows and frequently entering and inspecting existing burrows. If males are present on the lek they will often try to mount the females looking for a nest site and may knock them onto the ground.

MOST MATED FEMALES LEAVE THEIR NATAL SITES. There is a report in the literature that newly mated females spend about two weeks exploring the area around them before beginning to dig burrows in which to lay their eggs. I conducted mark/recapture experiments in which virgin females were captured, mated in the lab, marked and released a few hours later at the site of capture. Only 2 of 20 (10%) marked and mated virgin females were seen again in 1997, as opposed to 44% of females caught and marked while they were looking for burrow sites. This may well be a way of ensuring outbreeding. Lin (1978) found just the opposite behavior in the cicada-killers he studied for ten years at playing fields in Brooklyn, NY; his marked females returned in what appears to be large numbers (Lin presents no data) to their natal leks a week or so after hatching.

FEMALES DIG AND SOMETIMES SHARE BURROWS. Female wasps spend lots of time searching for appropriate places to dig their nesting burrows and they often inspect active lek areas with patrolling males. When this happens the lek males try to mount the females and often knock them to the ground, where a struggle takes place. Females seem to favor for their burrows sloping, well-drained, grassy or ivy-covered areas with eastern or southern exposures and with large, deciduous trees nearby. Also favored are unmortared stone walls with gaps between the stones in which burrows can be dug. The picture on the right shows the mouth of a typical burrow. Click on it to see a montage of other burrows. The montage shows two unusual burrows: one is located inside a metal drain pipe and the other is located behind a narrow crack in the caulking between two granite steps of Van Wickle Hall. The crack was tall enough to let the female wasp enter, and she had dug a burrow in the dirt behind the step. However, the crack was too small to let her drag in paralyzed cicadas, which are slightly taller and wider than the wasp. She would bring back a cicada, try to drag it into the crack for 5 minutes or so, give up and hunt again. At one point there were ten cicadas abandoned outside the crack. On a similar note, one of the best ways to census cicada populations in an area is to find several active cicada-killer burrows and plug them with small sticks after a female is seen leaving them. In my experience, a female wasp treated this way will bring back paralyzed cicadas up to six times in a row before she gives up or darkness falls.

The process of digging a burrow is an involved one and in some cases, it begins the evening or night before hunting and egg-laying occur. I have frequently seen females digging burrows just after dawn and they may have begun in the pre-dawn hours. Once a female has found a suitable site for her burrow, she begins to dig a tunnel using her large mandibles, which can open to a distance half again as wide as her head. The soil which she digs out accumulates in the burrow and at intervals she backs up the burrow, pushing a pile of soil behind her using the large spurs on her hind legs (a 2.8 MB QuickTime movie of a burrowing female is available here). As the pile of dirt accumulates at the burrow mouth, the female continues to back through it pushing a pile of dirt and making a distinctive trench through the pile of dirt (see picture above on right). Digging can continue for over an hour and there is often a water-trap (the tunnel turns down and then back up, as in the diagram below on the left) placed near the burrow entrance. Tunnels over a meter long have been reported.

When the tunnel is completed the female digs an ovoid nest chamber at the tunnel's end and leaves to hunt for one or two cicadas with which to provision it. If the female has just dug the burrow she will memorize the area around the mouth of the burrow by making a series of up to 41 "orientation passes" over it. These oval passes begin with a brief walk on the pile of dirt at the burrow mouth, cleaning of the head with the forelegs and a short flight away from and back to the burrow mouth. Successive oval flights extend farther and farther from the burrow and the last pass often extends 15 meters or more from the burrow mouth. The female then leaves to hunt a cicada. If another female lands on the burrow mouth or enters the burrow while a female is making orientation passes, the female which is "orienting" will immediately enter the burrow for what I presume is an inspection. If the intruding female does not leave there is often an antagonistic encounter or a fight between the two wasps, probably over the use of the nest chamber at the bottom of the burrow. I have several times seen females sharing the same burrow, with both females bringing back cicadas to the same burrow and, presumably, using different nest chambers at the bottom of the burrow; this behavior has also been reported by Dambach and Good (1943). On the other hand, I have seen several vigorous fights between females at the mouths of several burrows. Pfennig and Reeve (1989) have reported that body size and possession of the burrow give an advantage to a female in these fights and that females are more tolerant of burrow intrusion by their near neighbors using adjoining burrows. In a later paper (Pfennig and Reeve,1993) these workers used DNA fingerprinting to show that neighboring females are related, accounting for their greater tolerance of burrow intrusion by near neighbors.

Females usually use the same burrow all day and often for several days in a row, digging new nest chambers off of the main burrow, provisioning them, laying an egg, and closing one nest chamber with the dirt from the digging of the next one and hunting again. By continuously watching a few active burrows on a given lek all day I have been able to determine how long it takes a female to enter a burrow with a cicada, lay an egg on it, close the chamber and dig a new one; these observations have also revealed the sex of the egg being laid because female eggs get two or three cicadas, but male eggs get only one. It takes a female about 45 minutes to take a cicada down her burrow, lay an egg on it, close the chamber with the dirt made by digging a new chamber and leave to hunt again. These data and others are summarized on the "Hunting" page.

FEMALES DIG BURROWS IN DIFFERENT AREAS. My mark/recapture studies in 1997 show that females range widely over Easton leks: 13% (29/220) of females were seen at least once on a lek other than the one on which they were captured for marking and 4% (9/220) were seen at least once on leks more than a kilometer from the lek on which they were captured for marking. As noted above, females spend a lot of time investigating active leks and other areas for potential burrow sites and often enter active burrows to inspect them. Occasionally two females will fight in or near the mouth of a burrow, presumably over the possession of a nest cell and there is a report in the literature that such fights occur more frequently when one of the females has an open nest cell in the burrow.

SMALL FEMALES MAY PARASITIZE THEIR LARGER SISTERS. I have seen several very small female wasps (those in the male size range) entering active burrows. It is possible that they may be looking for a burrow with an open nest chamber and one cicada left in it while the "owner" is out hunting for a second cicada in an attempt to make a female egg. In fact, Pfennig and Reeve (1989) present evidence that females which have left a cicada in an open nest cell return to their burrows, with or without a second cicada, sooner than females which have not left an exposed cicada in their burrows, perhaps to prevent usurpation of their open nest cells. In a limited number of observations, they also found that, when a burrow being used by one female was taken over by a new female, in 5 of 6 cases seen, the burrow had a cicada in an open nest cell. Only 2 of 7 intruders took over burrows from their "owners" when there was not a cicada in an open nest cell in the burrow.

Assuming that the very small females are too small to capture and carry cicadas back to a burrow, they may be trying to parasitize their larger sisters by entering the open chamber containing a single cicada, laying an egg on it and closing the chamber. If such small females lay female eggs on the single cicada, they may produce some of the next years' small females. Such small females might also lay only male eggs and they may account for a report in the literature that some female wasps specialize in laying only male eggs.