This newly molted dog day cicada waits patiently as its exoskeleton hardens.
The dog days of summer have arrived early in Washington and with them the dog day cicadas. Unlike their periodical cousins, who make an appearance once every thirteen or seventeen years (see “St. Mary's survivors – Cicadas of Brood XIX, June 6, 2011), dog day cicadas join us every summer. The immature stages, called nymphs, spend years underground feeding on sap in the roots of plants. Eventually the fully developed nymphs emerge, usually after sunset, and move from the soil to upright structures such as trees and shrubs. The nymphs hold fast to a plant or other upright structure and their old exoskeleton splits along the midline, allowing the adult cicada to emerge. During this vulnerable time, the darkness of the night protects them from daytime predators such as squirrels and birds. After several hours, the new exoskeleton hardens, and by sunrise the dog day cicadas are ready to fly. Ensconced in treetops, the males begin to vibrate a drum-like membrane called a tymbal to create their shrill song. Each species of cicada has a unique song, and the females use the chorus to locate and select a worthy mate. Periodical cicadas rely on strength in numbers to overwhelm the capacity of predators to consume them; the emergence of huge numbers of periodical cicadas allows hungry predators to eat their fill, yet enough cicadas will remain to successfully reproduce and carry on. However, dog day cicadas must rely on stealth and speed to survive. Their green and black coloration creates camouflage that enables them to blend into their arboreal environment. They are hard to spot, and when you approach they take off like F-14’s. Your best bet for getting up close and personal with a dog day cicada may be to wait until nightfall, grab a flashlight, head for your favorite cicada tree, and watch for the nymphs to arise from their subterranean crypts near the base of the tree.
The cicada killer is a large, strikingly marked wasp.
Dog day cicadas have been singing up a storm for several weeks. While bug-lovers find endless delight in their serenades, others are saying “Enough already, when does this end?!” Well, cicada-phobes take heart. You have an ally in your wish to be done with cicadas in the form of an awesome assassin called the cicada killer wasp. A sandy flower bed in the front of my home has become a haven for cicada killers, and for the last few weeks male cicada killers have entertained me with aerial sorties as they battled to establish territories. Each morning, these large wasps perch on prominent vegetation such as shrubs or bushes to chase away other males that dare to enter their territory, as they await the appearance of their mates. The female cicada killer exploits dog day cicadas as a source of food for her babies. Her life cycle is closely synchronized with that of her prey. After spending most of her life underground as a larva and then as a pupa, she completes development in summer and emerges near the same time as dog day cicadas. After emerging and having a brief romantic interlude, the female cicada killer constructs a gallery in the ground that will serve as a nursery for her developing brood. This chamber consists of a tunnel that may be over a foot deep, with as many as 10 or more individual brood cells constructed off the main gallery. When the nest chamber is complete, the female cicada killer searches branches for the boisterous annual cicadas. Upon finding a suitable victim, she grapples with the cicada and stings it. Her remarkable venom does not kill the cicada, it merely paralyzes the insect, rendering it unable to fly or otherwise defend itself. The cicada killer then straddles the cicada, which may exceed her in weight, power-lifts the victim, and flies back to her burrow. Both amateur and professional insect enthusiasts have reported instances where a cicada killer straddled an immobilized cicada and climbed a vertical structure, such as a tree trunk, before takeoff. Apparently, a height advantage is necessary to attain flight while transporting their massive load. Once the heavy lifting is over and the female arrives back at the nest, she quickly wrestles the cicada into the brood chamber. In a fascinating display of gender selection, the female cicada killer chooses the sex of her spawn. If a male wasp is to be born, the female lays an unfertilized egg on the paralyzed cicada. If the cicada killer decides to produce a daughter, she will return to the treetops, collect one or two more cicadas, place them together in a nest chamber, and then lay a fertilized egg on one of the victims. The fertilized egg will hatch into a daughter that will consume the cicadas and develop into a female wasp. The explanation for this fantastic behavior is in part because female cicada killers are about twice the size of males (up to 2 inches), hence, they need twice as much food as larvae to complete their development – at least two cicadas instead of one. After depositing the egg, the cicada killer seals the nest, excavates a new brood nest and resumes her search for cicadas.
Paralyzed cicadas are sometimes abandoned near the nest site.
Back in the nest chamber, the egg hatches and the developing larva attaches itself to the skin of the paralyzed cicada and literally eats it alive. Trapped in an underground tomb and paralyzed while being eaten alive…..what a sad end for the dog day cicada. After a few weeks, the cicada killer larva completes its development and forms a case in which to pupate underground. It spends the remainder of the summer, autumn, winter, and spring in the chamber before emerging as an adult the following year to hunt cicadas. While cicada killers look ferocious, they do not attack humans, males cannot sting at all, and stings from females are very rare. I have only heard of two reports of cicada killer stings, one of which happened when someone inadvertently kneeled on a female wasp as she worked in the garden. I have photographed cicada killers at very close range, within inches, without any problems. A curious male buzzed me as I entered his territory and a female approached to ogle me when I put my nose in her gallery, but I came away unscathed. Homeowners may be dismayed when cicada killers construct large numbers of nests in their lawns, with the attendant mounds of earth surrounding each entrance hole. Attempts to control cicada killers with pesticides may be futile. While you can kill the current batch of cicada killers in your lawn in the short term, it is the texture, exposure, and drainage of your soil that attracts a new crop of cicada killers to your lawn each year. Improving the density of your turf may help by reducing the amount of exposed soil that may attract females seeking nest sites. The noted cicada killer expert, Professor Chuck Holliday of Lafayette University, recommends keeping the soil at the nesting sites unusually wet, as nesting females are attracted to well-drained soils. I had an agreeable conversation with a Maryland farmer who was considering options for dealing with a bumper crop of cicada killers in his vegetable garden. In the end, he decided on a live-and-let-live strategy, and conceded that these critters were not a serious treat and actually pretty entertaining to watch. Besides, “the holes they make in the dirt help aerate the soil.” You will hear no argument with that on my part.
A nervous male cicada killer dashes from his perch to defend his territory.
References
The inspiration for this episode came from James the farmer, Dawn my former student, Marty, and several folks who contacted me recently regarding these fascinating creatures. Information about cicada killers came from Chuck Holliday’s magnificent cicada killer web site.
For more information about cicada killers, including videos of them in action, please visit the following site:
http://sites.lafayette.edu/hollidac/research/biology-of-cicada-killer-wasps/