Monday, September 16, 2013

Life Cycles in the Bush

This bush in front of my house is where the entire drama plays out..

Praying mantis egg case from 2012.
I hope you like complex stories with lots of twists and turns full of treachery and just a touch of gore. If so, you have come to the right place. That place is a single bush in front of my house. This year the bush has gotten a reprieve from the hedge clipper, mainly because I didn't want to disrupt what I thought might be happening there. It did not disappoint.

Mantis shedding exoskeleton.
Last year I saw several praying mantis egg cases attached to this and other nearby bushes so I was expecting a big year of mantis sightings. well, that didn't happen. Instead I was treated to something I had never seen before, a single female praying mantis molting for the last time, her final instar, the one that would reveal her final majestic form and give her the power of flight with long newly revealed wings. There she was slowly working her way out of her penultimate exoskeleton. She would finally work herself free then wait for those wings to unfurl and for her soft shell to harden into a protective armor. For the next hours, though, she was vulnerable to anything that spotted her and wanted a soft insect meal. It was a risky time. I watched for a long time before I realized something was going wrong. Her transparent wings never seemed to completely unfold and the beautiful green and brown outer wings didn't fold neatly against her body.

The molt that didn't go right.
This mantis was never going to be the beauty I had anticipated but she would be the only one I would see on this bush this year. After a few days she disappeared and I was left to face a mantis-free Summer, at least as far as that bush was concerned. Sure, I would see others elsewhere but it was not the same as seeing one in my special bush.

The mantis may have gone somewhere but that didn't mean that the bush had nothing to offer. If the mantis is the top arthropod predator on the bush, the spiders, dragonflies, and robber flies are close behind. The bush is teaming with tiny spider webs housing an unbelievable variety of spiders whose names I don't know. There are also larger ones. One day came across a green lynx spider feasting on a brown stinkbug.

A green lynx spider consumes a brown stinkbug.
I had seen these beautiful spiders hiding on the undersides of larger leaves or lying in wait in some other place on the plant. The are lightning quick and rely on ambush attacks and even chasing their prey. No web traps for these guys. They are the real deal. With eight eyes facing front, up, and sideways, their depth perception is exceptional. Formidable indeed. This one, however had company. If you look closely you can see a swarm of flies helping themselves to the bounty of the stinkbug. They are parasitic flies who are particularly fond of spider kills, especially stinkbugs These kleptoparasitic (Isn't that a great word?) bugs feed almost exclusively off the captures of spiders. I am sure there is much more to this story than I know now but rest assured I am going to find out. In any case, this would not be my last encounter with parasitic flying things.

One day on my way to make my daily check on the bush, I noticed a large moth hanging out near my front door. In some ways, no surprise since I keep my porch light on all night for security purposes. It attracts all sorts of bugs and consequently many spiders as well but that is another story.

This moth, though, was quite something. As it turns out, it was intimately connected to my bush. It  was a hawkmoth, a beautiful creature that spends part of its life as a tobacco hornworm caterpillar (closely related to the tomato hornworm). Well since I have no tobacco and no tomato plants in my yard and none that I know of anywhere nearby, what to make of this moth? The bush provided the answer.

A tobacco hornworm denuding a stem.
I may not have had any tobacco or tomatoes growing in my yard but I certainly did have a tobacco hornworm. I found this one pausing to swallow his meal and look over his work as he made his way down one stem of my favorite bush, denuding it as he went. That stem was stripped clean from the tip down to where he was. Other nearby stems had already been stripped. Should I kill it to save the bush? Maybe later. For now my reflex was to get the camera and so I did. I resolved to just watch for a couple of days to see what would happen. After all, how much damage could a single caterpillar do? I looked for others just in case but found none so this one was safe, at least from me. Some bird might consider it to be a tasty morsel but not likely. Most caterpillars either taste bad to birds or resemble those that taste bad. This guy should be there for a while.

Well he might have been safe from me and the birds but he was far from safe. In fact when I first saw him, his fate had probably already been sealed. It was just not apparent to me. A few days later, I looked for the caterpillar and found him or one like him having a really bad day. He was covered with tiny white egg-like structures that made him stand out from everything else around, a flash of white in the sea of green.

Tobacco Hornworm with Empty Wasp Cocoons
 They were not eggs though, They were cocoons from which dozens of tiny wasps would emerge and fly
away to continue a bizarre life cycle. This caterpillar had been host to these wasps and paid for it with its life. After some time, the wasps did emerge from the tiny cocoons and most flew away to begin the cycle all over again.

Wasps and cocoons to show actual size.
The whole story is like something out of a science fiction horror story. Let's start with the tiny female wasp. At just over an eighth of an inch long, she flies around until she finds a well developed tobacco hornworm in which to deposit her eggs. With her sharp ovipositor, she penetrates the tough skin of the caterpillar and lays dozens of eggs beneath the skin. That would be bad enough but there is more. While she is depositing the eggs, she is also infecting the hornworm with a virus that attacks its immune system so it cannot protect itself from the newly laid eggs and the larvae they will produce. The same virus also attacks muscle (to slow the caterpillar's movement) and the central nervous system (to change its behavior). The wasp larvae hatch inside the caterpillar and begin feeding on blood and other tissues, all the while avoiding damage to any vital organs or systems. When they are mature and ready to become wasps, they chew their way through the tough outer skin, weave those tiny cocoons around themselves and start the metamorphosis into flying wasps. Now the caterpillar changes behavior (remember that virus?} and becomes the defender and protector of the firmly attached cocoons. It will strike violently and repeatedly at any disturbance to its body. It is probably enough to ward off at least some predators who might approach.  In some other species of caterpillar/wasp parasitism, the caterpillar actually uses its own silk supply to weave a second protective cocoon around the emerging larvae. There is something very scary about viral control over behavior. Great fodder for the sci-fi writer but absolutely based in the real world.

Needless to say I was being naive to think that there was only one hornworm in that bush. They tend to crawl up the undersides of stems and blend in very well with the colors. Stripped stems are the main giveaways. Well, the naked hornworms may be hard to see but those that have been parasitized are easy to spot. It is like seeing small white flags waving in the green foliage. There were several more in the bush so I decided to watch the story unfold again. I already knew the sequence. The caterpillar was going to have some very bad days then die, my bush would suffer some leaf loss, and the little wasps would emerge from their cocoons, fly away, and start the cycle all over again. They would do something like "live happily ever after."

Then out of nowhere an old friend showed up, bad wings and all. The mantis was back. It was like in the movies. A character is casually introduced at the beginning then fades away for the entire flick only to reappear at the end looking for something, . . . .in this case, an easy meal. She consumed the caterpillar and most of the silken cocoons including the tiny wasps inside. A few of the cocoons fell off the hornworm and into delicate spider webs below below but all the rest were eaten along with the host. Who knows? Maybe she is why I hadn't seen many other hornworms on the bush. Now, who gets to live happily ever after? The apex predator of course.

Neither the hornwom nor its parasites will survive this time.

End of story? Well not quite. You see, there is another kind of small wasp with long grasping legs who latches onto the backs of praying mantises, out of reach of those grasping claws. There she sits and simply waits. When the mantis is ready to lay her eggs, the wasp lets go and deposits her own eggs in the newly formed but still soft mantis ootheca (egg case). Her young will hatch early and feed on the developing mantis larvae . . .  and so it goes.

By the way, that first caterpillar continued to chew on leaves even after the little wasps had emerged. It was a futile effort to develop into a pupa. One day it just let go and fell dead from the stem, empty cocoons and all. I picked it up for a closer look. Like so many other things from my front yard adventures, this ended up in my studio as part of a new still life.

Still Life with Tobacco Hornworm and Parasitic Wasps.       21" x 14"  Print