Horror story plots in pretty places

by Emily Stone Naturalist, Educator Cable Natural History Museum

Not long after my encounter with a parasitic wasp on the Mississippi River bluffs, a Museum member brought in a photograph of a mystery wasp. It’s skinny, two-inch long body was ringed by rusty brown, black, and yellow stripes. We couldn’t see the ovipositor, because it was curled up under the wasp and boring down into wood, but after a quick Google Images ID search, we determined that this was a “giant ichneumon wasp” (Megarhyssa macrurus) and its ovipositor can be four inches long! (Macrurus means “long tail” in Greek.)

While wasps with such huge projections on their abdomens can be quite scary, they are actually harmless to humans. Their ovipositor is specialized for piercing through wood, or into certain prey. Some are able to sting us, some aren’t, but why would they want to? These solitary wasps don’t build up an enormous empire to defend.

The “giant ichneumon wasp” (Megarhyssa macrurus) can have a four inch long ovipositor, which it uses to drill into wood where the pigeon tremex horntail wasp has already hatched it larvae. 
Photo By Bruce Marlin. The “giant ichneumon wasp” (Megarhyssa macrurus) can have a four inch long ovipositor, which it uses to drill into wood where the pigeon tremex horntail wasp has already hatched it larvae. Photo By Bruce Marlin. (In contrast, social wasps build large colonies that allow for a division of labor. The queen lays all the eggs, and the worker females can use their ovipositors for stinging. They are more likely to sting in defense, since all of their eggs are in one basket….. uh…nest.)

The giant ichneumon wasp uses its super long ovipositor to drill into a dead or dying tree – right into the tunnel being carved out by the larvae of another wasp – the pigeon tremex horntail wasp (which doesn’t sting at all). The ichneumon wasp will sting the horntail larva to stun it, then squirt an egg or two through its ovipositor onto what will be the ichneumon larvae’s first meal.

As we flipped through the Museum member’s other photos, sure enough, we came to a picture of the reddish-brown body and yellow and black abdomen of the pigeon tremex horntail.

The horntail feeds on dead wood, and it has forged a special relationship with a fungus. As the female horntail lays her egg, she also introduces a white rot fungus. This fungus grows within the wood ahead of the horntail larvae and helps them digest their fibrous food. This type of symbiotic relationship, in which both parties benefit, is known as a mutualism.

The sneaky and dedicated giant ichneumon wasp has discovered the fungi’s relationship to the horntail, though, and uses the fungi’s presence to find trees that house the ichneumon’s prey.

Parasitism is a type of symbiotic relationship. In parasitism, one party benefits and the other is harmed, but isn’t killed. Ticks, fleas, and mosquitoes are parasites. These wasps are actually parasitoids. Parasitoid wasps spend a significant portion of their lives with a single host victim in a relationship that is essentially parasitic. Unlike a true parasite, however, a parasitoid ultimately kills the host.

Even the parasitoid itself is not safe, however, since other ichneumon wasps, who lack such a formidable ovipositor, may re-use the giant ichneumon’s holes to lay their own eggs. Then their larvae destroy the original eggs, and eat the horntail larva themselves. If you like big words, these pirates are known as hyperparasitoids.

Once you remove the big words, all we’re saying is that the wasp larvae eat other creatures alive. Nature can be more gruesome than any horror film, but the plot lines are more complex, and the scenery is more beautiful.

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