What happened to our lawn? OR Curtains for cicadas

October 7th, 2016

What happened to our lawn? OR Curtains for cicadas


Last year we found it fascinating while these large wasps created holes and piles of dirt here and there in our lawn. This year, the wasps and their piles of dirt are back in numbers. However, if you aren’t looking at these nesting sites during mid-summer (which happens to be both cicada and cicada killer season), you might not even realize that a population of the next generation of cicada killers is in the soil beneath your feet. But, if you are around when these adult wasps are active, the eastern cicada killers are very noticeable. They are up to 4 cm long with bright light yellow bands on their abdomens. These big wasps are said to feed on nectar but I only see them on the top of the soil, associated with the nesting behavior. People are generally frightened of them but these large wasps are not a big threat to humans. Males are territorial and they buzz people standing in their territories but are not aggressive unless provoked and even then, the males do not have stingers; the males are really most interested in finding females and other males to fight in order to defend their territory. I was buzzed by a female who noticed me with my camera looming over her while she was starting a nest but she eventually just flew away. So, the females have stingers but they are not aggressive either; they are most interested in digging nests and then finding and stinging cicadas.

A very obvious result of the activity of cicada killers are the piles of dirt that they create at the opening of a nest; these piles can be 5-10 cm tall, 25 cm long and 15 cm wide. So, while they are being made, the piles are very noticeable. The female does all the work digging the nest by pushing out the dirt like a little backhoe. Each nest belongs to only one female but there can be fairly dense aggregations of burrows, with one area in Brooklyn hosting hundreds of wasps, each year for at least 10 years. It seems that once a cicada killer population finds a site that they like (like our grass, struggling in sandy soil) they come back year after year.

Female eastern cicada killers, Sphecius speciosus, in the northeast of North America are great hunters, finding cicadas in the tree canopies that they then paralyze by injecting venom; they then fly to their nests, while carrying the cicada upside down beneath them. They drag the cicada down the long burrow and into one of the chambers and then lay an egg on it. Females don’t seem to be too picky about the species of cicadas they attack but it can be quite a challenge for a mother cicada killer to carry a cicada, often 2-3x her size, from a tree canopy to her nest. Females only hunts after they’ve created a burrow. Burrows run obliquely downward until they’re about 10-20 cm beneath the surface of the ground, and the burrow then extends more parallel to the ground surface, often for a total length of > 1 m. I did not dig up my lawn, but others have excavated nests, presumably in areas with burrows but without lawn on the surface. Along the burrow and off the far end of it, the wasp creates up to 16 side tunnels ending in chambers that are each 2-3 cm in diameter. One cicada killer female provisions each chamber with 1-4 cicadas for each egg that will become a larva, with more cicadas provided for female than male eggs, as female cicada killers are somewhat larger than males. That makes for a very busy female, finding all of these cicadas for her kids to eat.

 

When a female cicada killer has found a cicada that she wants to feed to one of her offspring, she injects it with paralytic venom. After being paralyzed, cicadas can still be alive (but ‘almost or mostly dead’, kind of like Sleeping Beauty or Westley, the Dread Pirate Roberts) for 148 days if not fed upon, which is much longer than non-paralyzed cicadas would have lived. So, these cicadas are in suspended animation until the egg hatches and the larva starts eating them (at which point they really die). It seems that being ‘almost dead’ does not really need to be effective for so long, as the cicada is consumed by the wasp larva during the several weeks after oviposition and doesn’t need to be ‘mostly dead’ for a third of a year.  

Want to read more?

Howard Ensign Evans, a professor at Kansas State University, then Cornell, then Harvard, and finally Colorado State University, wrote excellent books on wasp behavior, especially wasps living in the sand, in easy-to-read writing styles. He was a finalist for the 1964 National Book Award for “Wasp Farm” but if you really want to read more about cicada killers, you can find more in:

Evans, H.E., O’Neill, K.M. 2007. The Sand Wasps: Natural History and Behavior. Harvard University Press, Cambridge, MA

Evans, H.E. 1966. The Comparative Ethology and Evolution of the Sand Wasps. Harvard University Press, Cambridge, MA

Ann Hajek--Associate Curator of Invasive Insects