Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

Moths, Wasps and Germ Warfare

Wednesday, September 4, 2013

Last month's story, Moths and Wasps deals with small braconid wasps and hummingbird clearwing moth caterpillars. After writing this story, I read an article in Scientific American magazine that made me realize I had only told half the story, and that the closer one looks, the more one realizes how bizarre life can truly be.
Braconid wasps lay their eggs in the body of a caterpillar. The eggs hatch into larvae that proceed to feed off the body fluid of the caterpillar. When the larvae are fully grown, they eat their way out of the caterpillar and pupate (make a cocoon). Eventually they turn into adults, leave the cocoon, and begin the process over again.
The question raised in the Scientific American article is, why doesn’t the caterpillar’s immune system protect the caterpillar from the eggs?
Caterpillars, like humans have an immune system. The job of the immune system is to seek out foreign objects in the body -like bacteria or a parasite’s eggs, and destroy them. So, what prevents the caterpillar’s immune system from seeking out and destroying the wasps’ eggs?


As well as injecting dozens of eggs into the caterpillar, the wasp also injects a virus. This virus temporarily disables the caterpillar’s immune system, allowing the wasps eggs to grow within the caterpillar. The virus is very similar to the AIDS virus in humans. But there is one important difference. The AIDS virus disables the human immune system, allowing the virus to spread unmolested. However, a disabled immune system also allows other types of infections to flourish in humans, eventually resulting in death.
This would not serve the purpose of the wasp. The caterpillar must remain alive until the wasp larvae have grown and left the caterpillar. So the wasp virus only disables the caterpillars’ immune system for a matter of days, just long enough for the wasp larvae to grow large enough that the caterpillars’ immune system has no effect on them. After a few days, the caterpillars’ immune system is reactivated, protecting it from infections that might kill it before the wasp larvae are finished with it!
One might ask where the wasp gets the virus. Experiments have shown that the virus is a part of the wasps genetic makeup. Every wasp comes with its own virus, just as it comes equipped with its own legs, wings, blood, nerves and so on. It seems that the wasp and the virus are in fact one creature. How this merging of two life forms into one ever evolved must be the most fascinating story of all, but it will have to wait for many more scientific discoveries before it can be told.
Some might think it most unfair that the wasp uses germ warfare in its attack on the caterpillar. What a harsh cruel (though most interesting) world this is. But lest we cast the wasp as the villain in this story, remember that the wasp itself was victimized by a hyperparasite. Perhaps that hyperparasite also employed its own form of germ warfare to disable the wasps defenses.
This story serves to remind me that nature has so many wonderful stories to tell. Many of them are too small to observe with the human eye. When I walk through the woods or paddle the river and bemoan the fact that the birds seem to have disappeared, the insects are scarce and there are no mammals in sight, I just settle my eyes on any seemingly lifeless area and think of the prodigious life struggles between the bacteria, viruses, the molds and all the other living things that make up any watershed.
There are so many secrets of nature that I will never know. However, the anticipation of learning just one tiny little secret makes me look forward to every walk, every paddle with great anticipation.