HUNTING FOR CICADAS: will the next egg be a male or a female? Female cicada-killers must decide the sex of each egg before they lay it. This is so because they must hunt twice, rarely three or even four times, for each female egg they lay and only once for each male egg. In turn, this means that one can learn the sex of each egg a female lays by continuously watching her at work over a day and counting how many trips she makes to hunt. The picture on the right shows my equipment arranged next to a stone retaining wall which had 28 active burrows along the visible length of the wall. At several active leks like this one I observed 5-10 burrows all day at intervals over three summer breeding seasons (1996, 1997 and 1998). Hunting females are reported (Dambach and Good, 1943) to circle slowly up through the foliage of a tree, methodically searching each branch for cicadas. Several workers have reported that the wasps catch more than twice as many female cicadas as males, so it is unlikely that the wasps use sound to locate male cicadas.

Females hunt and lay eggs continuously when conditions are good (a warm, sunny day with relative humidity less than 90% and lots of cicadas singing) and their use of their burrows falls into a typical pattern. A female will leave the burrow after digging a nest chamber, hunt for a cicada, return to the burrow with it (Here's a 164 KB QuickTime movie of a wasp bringing a cicada back to her burrow) and stay in the burrow for three minutes or so and leave to hunt again, or she will stay down the burrow for 45 minutes while she lays an egg on a cicada and closes the nest chamber with the dirt made by digging a new nest chamber. Then she will leave to hunt again. A female will return to her nest chamber from an unsuccessful hunt to inspect it and leave to hunt again (could she be trying to protect her nest chamber from her smaller, possibly "parasitic" sister wasps noted on the female biology page?). The following is a typical sequence of observations from my field notes in which a marked female wasp spent just over two and a half hours to provision her nest with three cicadas and, thus, laid a female egg:

8/15/97, lek at Burke and Mixsell Sts.:

11:48:09 am, female 42 out of burrow, 19 orientation passes and away to the East
12:12:15 pm, female 42 back with a cicada (21 minutes 44 seconds, successful hunt)
12:14:09, female 42 out and away (1:54 down burrow, "quick turnaround")
12:27:58, female 42 back with a cicada (13:49, successful hunt)
12:30:15, female 42 out and away (2:17 down burrow, "quick turnaround")
13:04:07, female 42 back with no cicada, entered burrow (33:52 unsuccessful hunt)
13:05:33, female 42 out, 5 orientation passes and away at 1:06:03 (1:26, "quick check" of burrow)
13:19:13, female 42 back with a cicada (13:10 successful hunt and 3 cicadas down burrow)
14:25:14, female 42 out and away (1 hour, 6 minutes and 1 second to lay egg, close nest chamber and dig a new one)
Female 42 was not seen again until 16:25 (could she have taken a break for a late lunch?), after which she spent an hour and a half hunting two cicadas and laying another female egg. The observer then went home for a beer and dinner.

By continuously observing many active burrows over three summers I have accumulated the following data:

• Successful hunts last 24.0 +/- 1.8 (standard error of mean) minutes (155 or 62% of 249 hunts observed)
• Unsuccessful hunts last 46.7 +/- 3.9 minutes (94 or 38% of 249 hunts observed)
• Short times down the burrow with a cicada last 2.81 +/- 0.25 minutes (78 "short-times" observed)
• "Quick checks" after an unsuccessful hunt last 2.90 +/- 0.28 minutes (94 "quick checks" observed)
• Egg-laying and chamber closure last 45.1 +/- 1.7 minutes (96 eggs laid)
• 49 female eggs (74% of eggs of known sex), 17 male eggs (26%) and 30 eggs of unknown sex (observations began with the return of a female with a cicada and an egg-laying, so cicada number in the nest chamber was unknown) were observed.

GIRLS ARE EXPENSIVE; BOYS ARE CHEAP: The data above can be used to calculate a rough time budget for estimating the "cost" to a female of making male and female eggs and to estimate how many male or female eggs she might lay in her lifetime. To do this, several assumptions must be made:

a) The female lives for 34 days, 14 of which are spent wandering just after mating, per reports in the literature.
b) The female hunts from 8 am to 8 pm, with 2 hours spent feeding and 25% of the available days not suitable for hunting (rain or hot with high humidity),
c) 62% of hunts are successful and last 24.0 minutes each, plus a 2.8 minute "short-time" putting the cicada into the nest chamber,
d) 38% of hunts are unsuccessful and last 46.7 minutes each, plus a 2.9 minute "quick check" of the burrow.
e) Only one cicada is given to male eggs and two are given to female eggs.
f) Each egg-laying requires 45.1 minutes.
g) Time to dig a burrow is ignored because it is not known how many females dig a new burrow for each group of eggs laid and how many use existing burrows dug by other females.

The following times would be required for a wasp to lay 100 female eggs:

a) 200 successful hunts (200 x 24) = 4800 minutes
b) 100 "short-times" down the burrow with a cicada (100 x 2.8) = 281 minutes
c) 123 unsuccessful hunts and 123 "quick checks" ((46.7 + 2.9) x 123) = 6101 minutes
d) 100 egg-layings/chamber diggings (100 x 45.1) = 4510 minutes
e) TOTAL to make 100 female eggs = 15692 minutes or about 157 minutes per female egg or 3.82 females per 10-hour day

The following times would be required for a wasp to lay 100 male eggs:

a) 100 successful hunts (100 x 24) = 2400 minutes
b) 61 unsuccessful hunts and 61 "quick checks" ((46.7 + 2.9) x 61) = 3026 minutes
d) 100 egg-layings/chamber diggings (100 x 45.1) = 4510 minutes
e) TOTAL to make 100 male eggs = 9936 minutes or about 99 minutes per male egg or 6.06 males per 10-hour day

FINALLY, if we assume that 74% of the eggs laid are female and 26% are male, as noted above, in a 20-day reproductive life marred by 5 days unsuitable for hunting, then 11 days are spent making (11 x 3.82) = 42 females and 4 days are spent making (4 x 6.06) = 24 males, for a total of 66 wasps made in each female's lifetime. Further, each female kills (42 x 2) + (24 x 1) = 108 cicadas in her lifetime. Another way of looking at these calculations is that a typical breeding aggregation on a lek with an average population of 100 females during the month of August would lay (74 x 3.82) = 283 female eggs and (26 x 6.06) = 156 male eggs each day. The female eggs each get two cicadas and the male eggs get only one, so (2 x 283) = 566 cicadas are killed for the female eggs and 156 are killed for the male eggs each day, for a total of 722 cicadas killed per day. If we assume that the entire month of August is a normal breeding season for cicada-killers and that only 23 of 31 days are suitable for hunting, then a typical 100-female breeding aggregation will clear (23 x 722) = 16,606 cicadas from the surrounding trees during the month of August. Since cicadas are known to damage the new branches of trees by making holes in the tender bark in which to lay their eggs, a gaggle of cicada-killers is a good thing to have around if you value the deciduous trees on your property! Dambach and Good (1943) dug up 13 burrows and all of their nest cells in an Ohio burrow aggregation and calculated that the 373 burrows in the aggregation contained about 11,300 cicadas, so my calculations above are roughly comparable with theirs.

It would be very interesting to study the effects of cicada availability, wasp age and other factors on the decision by female wasps whether to make male or female eggs at each laying. Do they make males if cicadas are hard to find? Do they make females when cicadas are plentiful? Do they start out making females until they run out of sperm and then make males until they die? If so, how do they know when their "sperm tanks are on empty"? If they have one cicada and the sun is setting, do they say, "To Hell with it, this one's going to be a boy!", or do they stay in the burrow overnight and try again in the morning to get a second cicada to make a female egg? To answer questions like these, I have tried to rear cicada-killers in my laboratory so that I can control prey availability, wasp nutrition and other variables to learn their effects on egg sex. My results so far are few; I have had much trouble with lab-reared larvae being killed by mold and I have been able to induce only two female wasps to lay eggs on cicadas in artificial habitalts in my laboratory. I have had better results using a cicada-killer version of the "trap nest" technique. The picture on the left is of a female wasp in a burrow which she dug in the earth inside a piece of 4" plastic sewer pipe put in the ground. The picture on the right is of a trap nest installation at Van Wickle Hall on the Lafayette campus. Using this technique I was able to obtain the developing larvae and cocoons shown on the next page on larval development and hatching.