a sheep in wolf’s clothing

July 27, 2007

in Insects,Natural history

Cickill

Eastern cicada killers (Sphecius speciosus) are big wasps. The “dainty” males are about 3 cm long, and the females up to 4 cm. Adding to the intimidation factor is the behavior of the males, which gather in “leks” of up to 70 individuals — all buzzing around and defending small territories, awaiting the appearance of virgin females to mate with. We are lucky enough to have an annual colony of cicada killers right against our building at work, and it is unsettling to stand in the midst of these busy insects even for the knowledgeable among us. I’ve had plenty land on me as I sat in the middle of the lek, and I still get creeped out when they settle on my back where I can’t see them. Cicada killers are truly frightening to folks who don’t know what they are, but rest assured for all their size and hectic activity, they are wussies.

Males — the most conspicuous — cannot sting. Females can sting, but must be strongly provoked (or stepped on). Even so, as members of the wasp family Crabronidae, they have weak stings that, unless you are allergic, would feel like a pin prick and diminish within an hour.

It is high time for cicada killers now. They emerge in coincidence with cicadas, having overwintered underground, first as larvae in cocoons, then as a spring-developing pupae. Males emerge first, a week or so ahead of females. Unlike many insects, a female cicada killer will only mate once. Presumably once mated the females give off some scent that males detect when close by. I’ve seen males approach, knock over, or tackle females, but immediately fly off rather than try to mount them. Virgins, on the other hand, may find themselves in the midst of a determined, enthusiastic ball of males, one of which will win her over.

Once mated, females find an appropriate area, often a well-drained slope near a woodlot, and build a burrow up to three feet deep (not a typo). Our colony is in a border of fist-sized rocks on top of sandy soil next to a sidewalk. Burrows may have several branching nest chambers. In each, the female will place one or two paralyzed but living cicadas on which she will lay her eggs. Eggs are always laid under one of the second legs of the cicada,a pparently the best spot for a little wasp larva to start eating.

The female cicada killer decides the sex of her offspring, because she stores sperm in a structure called a spermatheca, and uses the sperm to fertilize her eggs as she chooses. She will lay an unfertilized egg on a single cicada for a male, and a fertilized egg for a female; the fertilized eggs also get a second cicada. Since it is the females that have to hunt and carry around cicadas (which are much heavier than the wasp), they are bigger than males and need the extra provisions. Each female cicada killer will lay about a hundred eggs in a season, most of which will be males.

Even among males, there is size variation, depending on the size and condition of the cicada they were provided with as a larva. Paralyzed cicadas usually remain “fresh,” but the situation is complicated by rivalry for the cicada itself. Certain species of fly wait at the entrance of the cicada killer’s burrow in order to lay their own eggs on the paralyzed cicada. In this case, the fly larvae compete for the food source.

In less than three weeks after they emerge, males die. The females live four to five weeks, giving them time to construct and provision their nest burrows. Underfoot, cicada killer larvae slowly consume their rations. It will be another year before a new generation appears, intriguing some of us, and fooling others to fear them.


Many thanks to Prof. Chuck Holliday’s Cicada Killer Page, a one-stop source for all kinds of life history and research data on these fascinating insects.

Please don’t molest a colony of cicada killers. If you need to know more about why they are harmless and why dousing them with a load of pesticides is ineffective in the long run, check out Joe Coelho’s web site; he’s a colleague of Holliday.

Top photo: Male cicada killer, on the lookout. Center: Potential hapless victim, a Swamp Cicada (Tibicen chloromera).
Bottom: Female cicada and co-worker’s hiking boot.

Femckshow


{ 10 comments }

Comments on this entry are closed.

Previous post:

Next post: