Caterpillar Parasitism & Parasitoid Wasps! | Tobacco & Tomato Hornworm | Koaw Nature


So dace_88, one of Koaw Nature’s viewers and generous jaguar Patrons from Patreon, sent me a question and some pictures. “Hi. Today I was picking tomato worms off of my plants in the garden and came across three worms that had parasitic white eggs all over their body. I assume these will eventually kill the worms for me. Can you provide some more insight into the nature of this parasite?” Well dace, you are definitely correct in that your hornworm has a parasite. And I find this sort of parasitism to be fascinating, so let’s definitely clear up a few things for you. [bumper music] [upbeat music] Firstly, to clarify for a bit of accuracy— your tomato plants are in your garden are not being eaten by tomato hornworms, rather that is a tobacco hornworm you have there, which, is also the tobacco hawk moth or Caronlina sphinx moth. Both species of hornworm eat from the same family of plants, thus a tomato hornworm can be found on a tobacco plant and a tobacco hornworm on a tomato plant. The two species look very similar but you can tell the difference by looking at the horn and stripes. The tobacco hornworm has a reddened horn while tomato hornworms have a black or blue horn. Also, the tobacco hornworm has seven diagonal white stripes with black borders while tomato hornworm has eight V-shaped white markings with no border. But let’s look at those interesting white objects growing out of that fat green larva. So those aren’t eggs, but they are cocoons of wasp larvae that had been eggs which hatched into larvae inside of the hornworm developing within for about two weeks. So that hornworm is a host for a parasite – a parasitoid wasp to be more precise. And what is unique about parasitoids from other parasites is that they will kill their hosts or at least render them sterile. There are thousands upon thousands of parasitoid wasp species that specialize upon different insect orders. There are two main strategies for this parasitism – an endoparasitic type where the parasite develops inside the host, letting that host continue to feed and live for a period of time— and an ectoparasitic type, where the host is paralyzed immediately. The moth larva infected here has been parasitized by an endoparasitic type, that is, within its body, by what is likely the species of wasp (Cotesia congregata), from a family of parasitic wasps known as braconids. But what is utterly fascinating about this parasitism, is that these parasitoid wasps have coevolved with certain viruses that are injected along with the eggs inside of the host— a mutualistic symbiosis existing for tens of millions of years. These viruses are known as polydnaviruses and are integrated within the wasps’ genome; [upbeat music] when injected, a polydnavirus works to protect the eggs and larvae from immune suppression by the host’s hemocytes. A venom is also injected that works to enhance the effects of any virus injected. The parasitic eggs will hatch within a couple of days and undergo two molts within the hemocoel, or the body cavity containing all the blood cells and other interstitial fluids, all the while that parasitoid is feeding off of the nutrients provided by the host that is still alive and feeding, which, in dace_88’s case, is the foliage of a tomato plant. The hornworm will continue feeding until about a day before the larvae emerge. Then the larvae will form those white cotton-candy looking cocoons from which adult wasps will depart roughly a week later— seeking to once again find a host to parasitize. So if you care about your tomato plants in your garden, you might as well just leave those parasitized hornworms where they are as nature will take its course. And those wasps emerge from those cocoons. And those hornworms won’t pupate, that is, they won’t become moths and lay eggs. So you have a natural pesticide going for ya. And I mean overall when you think about it—it is a gruesome occurrence. Even Charles Darwin found this to be quite disturbing as he wrote in a passage about a different but very similar family of parasitoid wasps. “There seems to me too much misery in the world. I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars.” – Darwin Yes Charles, that is a little messed up but that’s life. And thanks again to dace_88 for posting that question and sending me those pictures. Good luck with your garden! And I’m going to post another YouTube video with some better footage of some actual wasps parasitizing these hornworms that was taken in a lab. So check that out if you’re a bit more curious. Spread some knowledge and be nature heroic!