This is a guest post by our Research Associate Elsa Youngsteadt
North Carolina’s steamy July days bring out some of our most spectacular solitary wasps. These sleek and streamlined hunters are quite docile toward humans, but are to be feared by other insects and spiders. The largest of these wasps in North Carolina is the cicada killer (Sphecius speciosus). Females can be up to an inch and a half long and weigh about a gram—as much as a shelled almond.
Cicada killers are solitary, meaning that each female typically builds her own nest and hunts prey to feed her own offspring. (In this habit, they differ from yellowjackets, hornets, and paper wasps, which live in social colonies.) Nevertheless, cicada killers do tend to be found in groups, since they all agree on what makes a good nesting area: sparsely vegetated, southeast-facing slopes or unmortared retaining walls, with plentiful cicadas in nearby deciduous trees.
NC State’s campus hosts a few such cicada-killer neighborhoods, and we started to see the wasps out and about last week. But they haven’t quite gotten around to killing cicadas yet.
Males emerge a week or two before females, and spend their time feeding at flowers or sap and establishing territories, where they perch on vegetation or stones to keep watch. They make sorties to chase off other males, pursue females, and investigate intruders—including you, your pet, or your camera lens. Once you know that males cannot sting, these hovering advances (described by one pair of biologists as “businesslike”) can seem quite charming.
When the females finally emerge, they mate—only once, but for a long time (30 to 50 minutes). When they settle on a good spot for a nest, the earth-moving begins, and herein lies the occasional conflict between cicada killers and humans. One female wasp can excavate nearly a half-gallon of soil for a single burrow, which can be up to 40 inches long! And she makes about four burrows in her lifetime. She piles the tailings in a neat but largeish U-shaped mound at the entrance of each burrow, and this pileup can damage turf and other plants. But keep reading.
Cicada killers also benefit trees by hunting large insect herbivores—namely, dog-day cicadas in the genus Tibicen. These are the camo-colored, thumb-sized insects that spend their brief adult lives in July and August shrieking and buzzing in the treetops or sawing into branches to lay their eggs.
In her lifetime, one female cicada killer can gather 100 or more cicadas (a hundred!)–each of which weighs about twice as much as she does. She paralyzes the cicada with her stinger and hauls it laboriously back to her nest. She brings one cicada for each of her male offspring, and two or three for each female. (The wasp knows in advance the sex of the next egg she will lay, and stocks the nest accordingly). Then she lays an egg at the base of the cicada’s middle leg and seals up the nest chamber that contains it, never to meet her offspring.
The eggs hatch in a day or two and the long-necked larvae develop quickly, each one polishing off a whole cicada in less than four days. Upon completing its meal, the larva spins a silken cocoon and enters diapause, remaining in a suspended state until the following May or June, when it pupates. That brings us back to the present season, when adult wasps emerge and dig their way out of their nursery tunnels in July—males first.
If you have cicada killers on your property and find that you really can’t endure them, despite their docile manner and benefits for trees, there are few proven options for control. One study in West Virginia eliminated cicada killer activity on a golf course by spraying the appropriate insecticide directly into active burrows, or immediately around their entrances. (Broader area sprays were not particularly effective.)
However, because the developing larvae may survive this treatment, and because good nesting habitat remains appealing to new females year after year, wasp removal will likely be an annual battle. In most situations, the benefits controlling cicada killers with insecticides are unlikely outweigh the financial and environmental costs.
Other wasp biologists have recommended fertilizing and liming the soil in the affected area to promote thicker vegetation that will eventually deter the wasps; one has informally demonstrated a laborious but effective control method that involves plugging burrows and using a badminton racket to swat the wasps. (Again, this would be an annual undertaking.)
I hope that you don’t have to go to all that trouble, and that you can enjoy watching these insects, instead. And stay tuned for updates on our campus wasps. Once we find females dragging cicadas to their nests, there will be another photo shoot!
Sources and further reading:
Evans, Howard E., and Kevin M. O’Neill. 2009. The Sand Wasps: Natural History and Behavior. Harvard University Press. pp 37 – 43.
Hastings, Jon M., et al. 2010. Size-specific provisioning by cicada killers, Sphecius speciosus, (Hymenoptera: Crabronidae) in North Florida. Florida Entomologist 93: 412 – 421.
Holliday, C. W., and J. R. Coelho. Improved key to new world species of Sphecius (Hymenoptera: Crabronidae). Annals of the Entomological Society of America 99: 793 – 798.