One day last summer, I was making a feeble stab at cleaning the deck (or as I like to call it the “backyard basement!”) There was a long, thin plastic planter (junk) with dirt in it sprouting some grasses that had happened there on their own. I noticed a commotion in the soil and saw a treehopper partway stuck in a little hole, wiggling around to try to get in the rest of the way. Strange bug behavior! I realized this was actually the prey of a ground nesting wasp. There are many species of wasps that nest in the ground and they each provision their nests with different prey.
This one had a treehopper that it had paralyzed with its stinger and venom. The treehopper was small and triangular – more like a pyramid. It was brown and because of the way it was angled, the wasp couldn’t pull it down the hole. When she finally did, she would lay her eggs on it and the treehopper would be food for her babies.
This reminded me of an amazing field experience I had in the Sierra Nevada Mountains in Northern California, working for a great researcher named Dr. Jay Rosenheim. We were studying a beautiful little wasp called Ammophila (or, sand-loving). These wasps are about an inch long and very thin. They are metallic red and blue. Beautiful! They dig single-celled nests in the hard ground of the abandoned logging roads in the mountains. If you saw a nest in cross-section it would look like a golf club. The chamber at the bottom holds the eggs and their provisions.
When an Ammophila wasp digs, she sets her large jaws, or mandibles, in the hard ground and vibrates her flight muscles without beating her wings. This shakes the dirt loose. You can hear her! If you walk slowly and quietly down the dirt road, you may hear a little buzzing noise that sounds like a little drill. She ends up digging down about one and a half body lengths. Then, she grabs a jaw-full of loose dirt, flies over to nearby bushes and tosses it in! It takes about a half an hour for her to finish off her nest.
When she is done with the initial dig, she needs to go find provisions. But first, she finds a small pebble that is slightly smaller than the opening to the nest. The tunnel down to the egg chamber is tapered from the entrance, so the pebble sticks in the opening. This temporary closure will protect the nest while the mother wasp hunts. This wasp provisions its nest with caterpillars, inchworms. Some are brown, but many are bright green. It can take her an hour or a couple of days to find food for her young. It is usually a couple of hours.
When she finds prey, she stings it, injecting paralyzing poisons into it. She doesn’t want the caterpillar to die, because then it would begin to decompose and wouldn’t feed her babies. Next, she needs to get it back to the nest, so she carries it the length of her body, underneath, and holds it with her thin legs. In some cases the caterpillar will outweigh the wasp and she has a dangerous, slow flight to get it to the nest. It is a dramatic site to see a brilliant blue and red wasp carrying a large bright, almost neon green caterpillar!
Her work isn’t done, though. Next she has to remove the temporary closure pebble and check out the inside of the nest to make sure it’s still clean. On her final check, she turns around at the bottom and sticks her head out enough to grab the caterpillar and drag it down. She is in there for a few minutes laying her eggs and getting everything all set up.
When she comes back out, she finds a different closure rock that is slightly smaller and goes a bit deeper into the entrance. Then she piles in dirt and packs it down with her little drill. This is another time you can hear her working. By the time she is finished there is almost no trace of the nest location – it is well concealed.
So this is an incredible process to observe and study, but it is just the foundation for a truly amazing set of interactions. While the Ammophila wasp is originally digging her nest, another wasp (called Argochrysis), much smaller and rounded, appears on grass perches nearby. This little wasp is metallic green and red and quite striking, but small. It watches the nest digging. When the Ammophila wasp has her head down in the nest opening, the little Argochrysis will fly over to another perch to triangulate the location. It will remain very still while Ammophila removes dirt, and find another perch when she digs back in. Eventually, the little wasp will learn the precise location of the nest.
When Ammophila leaves to find a caterpillar, Argochrysis will go scout out another digging wasp. But she will continue to visit the larger wasp’s nest site every twenty minutes or so until Ammophila returns from the hunt. It takes about half an hour for Ammophila to open, provision, lay eggs in, and organize her nest, so the twenty minute rotation means the little wasp won’t miss her. Argochrysis has been observed monitoring at least four different nests!
When Ammophila returns with her caterpillar and prepares to provision the nest, Argochrysis can barely contain her excitement. She waits until the larger wasp pulls the caterpillar down, then, knowing Ammophila will be down there for a while laying eggs and such, she flies into the nest! She lays her eggs on the top of the nest chamber and quickly flies back out, hopefully unnoticed. If Ammophila senses the intrusion, she chases away the marauder and then feverishly cleans out her nest, sometimes even abandoning it and all the energy she has put into it.
If Argochrysis is successful, her eggs will hatch first, drop from the ceiling of the chamber and eat the Ammophila eggs and the caterpillar! It is a parasite parasitizing a parasite!
And to top it off, while Ammophila is provisioning her nest, a large fly (Hilarella) zips back and forth across the open entrance flicking her abdomen over the nest. She is actually flipping living baby flies, maggots, into the nest. She has incubated them inside her body, so they are essentially born alive. This gives Hilarella a major head start to eat everything else in the nest. This is a third level of parasitism! They're called "cleptoparasites" because they steal the prey from other parasites. All of this is based on the hard work of the Ammophila wasp.
Back on the deck, the ground-nesting wasp I saw wrestling with the treehopper had to push it all the way back out, readjust it and then drag it in. I watched as she did whatever she needed to underground until she came out and filled in the entrance. Her last move before she flew off was to drag a couple pieces of a fallen leaf over the top of the nest to hide it better. The babies will live there through the cold Winter and dig their way back out in the Spring. If I’m very lucky, I may see them come back up since I’m sure I’ll still have plenty of work to do in my backyard basement.