Hollowed Out for Halloween

We’ve heard a lot about viruses these past few years, with the corona virus front and center. We’ve all been scared at some level about getting COVID. We worried about how sick it would make us and whether we might spread it to someone else. So when I think of a virus, I think of something I caught somehow. It makes me sick for a while before my body kills it, using my healthy immune responses. I’ve had COVID twice now, fortunately with no long-term effects. 

Biological warfare enters the conversation at some point when the origin of the COVID19 virus is discussed. Biological warfare has been outlawed by international treaties. However, it’s not confined to humans. It is an integral part of the life all around us, and viruses with specific tasks are part of the arsenal. Consider, for example, the horror show that is the tomato hornworm, (Manduca quinquemaculata) AKA the larva of the five-spotted hawkmoth; versus the little braconid wasp named (Cotesia congregates). These two creatures are locked in a struggle to the death, and the little wasp seems to have the advantage. She has powerful weapons in her arsenal. First of all, she has her wasp waist. This may have been elegant in a Victorian lady, but it is more practical for our little wasp. This “waist” is a constriction that divides her abdomen into two sections. Her ovipositor or egg-laying device is at the end of her abdomen. The wasp waist gives her the flexibility to point her ovipositor precisely where she wants it to go so she can inject her eggs—65 or 70 of them at a time–into the body of her chosen victim, the tomato hornworm. She’s not satisfied with just depositing her eggs, though. She includes wasp venom in the package and a sneaky virus called a PolyDNAVirus. The virus has evolved with the wasp over millions of years, creating a mutualism between the wasp and the virus. The virus lives only in a certain region of the wasp’s reproductive tract, and has no other purpose than to invade the hornworm. Once there, the virus does several things to the hornworm. It interferes with its immune system by disabling the hematocyte cells. These cells had the job of engulfing and killing foreign matter like the wasp’s eggs. The virus also destroys the cells that produce hormones and interferes with the hornworm’s ability to get rid of dead or damaged cells. It also inhibits the hornworm’s ability to wall off the invasive eggs. So once infected with this wasp virus, the hornworm is out of options. Meanwhile, the wasp eggs hatch into wasp larvae, and start devouring the hornworm from the inside out. The hornworm is still alive through all of this. Eventually it will die, an empty shell. The wasp larvae will pupate and emerge as adults from cocoons on the hornworm’s back. Then, all innocence, they’ll fly off to pollinate tiny flowers.  

Charles Darwin was horrified by this process. In an 1860 letter to the American naturalist Asa Gray, Darwin wrote: “I cannot persuade myself that a beneficent and omnipotent God would have designedly created parasitic wasps with the express intention of their feeding within the living bodies of Caterpillars.” And yet …

Just as the tomato plant had no real defense against the hornworm plundering its leaves, the hornworm was out of options once the wasp laid her eggs. Before that, it tried using its coloration as camouflage or hiding in inaccessible places. Hiding or removing its frass (or body waste) removes clues of its whereabouts, and avoiding plants it’s already chewed on helps to cover its tracks. The hornworm does not lie docilly while the wasp attacks, either. It may drop off the plant, twist and thrash to dislodge or kill the wasp, wriggle vigorously, and even regurgitate onto the wasp to entangle it. The hornworm is much larger than the wasp—four inches long to the wasp’s 1/8-inch size—but once the eggs are installed, the hornworm is toast.

This is only one example of the battles going on out there. They’ve been raging for the past 247 million years between 200,000-plus species of parasitic wasps and their chosen prey. Left to their devices, the combatants maintain a delicate balance and keep each other in check. 

Photo by Author. Alt text: Horror on the porch.  A bright green caterpillar hangs from the defoliated stem of a tomato plant, which the animal has been feeding on.  Its back bristles with more than 70, tiny white cocoons, each housing a wasp pupa.  The tomato hornworm is still alive, but is doomed.  It has been successfully hijacked as host for the little parasitic wasps and will not complete its own life cycle, to become a five-spotted hawk moth.