Written by Collin McMichael, Education Coordinator
Hopefully, you have returned to Bug of the Month not in dire need of medical assistance due to the ever-growing Chestburster now dwelling within your ribs, but instead because you have found my overly colorful description of insect-on-insect brutality somewhat endearing. If you find yourself firmly planted outside of both these camps, remember: this here is Bug of the Month: After Dark and it’s about to get crazy. Also, since its basically contractually obligated at this point: At Bug of the Month, no one can hear you scream!
Scream you might, however, when you remember where we left off. I briefly touched on the ovipositor of the parasitoid wasps and its stab-tastic abilities. With these specialized egg-layers, the wasps puncture their target and pump several eggs, along with a substance known as calyx fluid )which I will get to shortly) into their target. In most parasitoids, these eggs develop similarly to any other holometabolous insect’s eggs: they hatch and the larvae go through several instars, leave their host, pupate, and eclose as adults. Some parasitoids, however, are polyembryonic. In these, the egg is laid and then undergoes mitosis, dividing repeatedly until there are hundreds of genetically identical eggs within the host. Usually, each of these eggs is able to develop into an adult wasp. Of course, we are talking insects so there are no rules and a smattering of iconoclasts.
The more derived polyembryonic parasitoids can exhibit caste systems in their larvae similar to, but not wholly resembling, the caste system seen in Xenomorphs. Lucky for all of us, these larval castes do not include anything weirdly dog-shaped, but let’s continue to ignore Alien 3 as a nation. Polyembryonic parasitoids are usually seen in areas rife with superparasitoidism, or when more than one wasp has laid eggs in a host. Some larvae can develop as a warrior caste whose sole purpose is to defend their other brothers and sisters from any interlopers that may intrude on their little slice of hemolymph.
Much to my glee, warrior larvae aren’t the scariest thing that end up in a parasitoid’s host. The previously-mentioned, somewhat-innocuous-sounding calyx fluid is actually much scarier. Calyx fluid has many components, most of which suppress immune system functions in hosts. One of the more interesting and terrifying ways they accomplish this is through a polydnavirus.
Polydnavirus, pronounced po-lid-na-vi-rus, gains its name because it is stored as multiple DNA strands across the wasp’s chromosomes. These viruses attack cells in the host by injecting genes to create proteins capable of dismantling the host’s own immune system. Polydnaviruses represent either the first cases that we know of in which a virus and a metazoan have coevolved or the first cases that we know of in which a metazoan has independently evolved its own viral component. Regardless, this mutualism between virus and wasp seems to have been alive and well for over 70 million years and may be independently derived in each of the lineages in which it’s found.
As scary as they are with all their evolutionary cookery, parasitoids are not at the top of their game. Not that I actually want to quote any of the first three Star Wars films, but I can’t make only one pop culture reference in BOTM so, “There’s always a bigger fish.” Parasitoids find themselves at the mercy of a higher trophic tier, the hyperparasitoids. Hyperparasitoids come from many walks of life, but wasps and flies are again the most common examples. These specifically parasitoidize parasitoids that are already inside their hosts. Apparently, the adults are able to sniff out the larvae within the host and puncture them with pinpoint oviposition. Much to the chagrin of the hosts, the enemy of their enemy is also their enemy. Hyperparasitoids cause death to both the host parasitoid and the host parasitoid’s host.
There have been a few examples of hosts fighting off their would-be, internal attackers. Generalist woolly-bear caterpillars of the family Arctiidae in the American Southwest have been shown to self-medicate when exposed to parasitoids. These caterpillars switch from plants that offer them the fastest growth and development times to plants that are somewhat toxic to their physiology. By slowly poisoning themselves, the caterpillars are able to fight off the parasitoid infections as the host is able to tolerate more of the nasty plant metabolites than the parasitoids. So next time someone tells you taking medicine is unnatural, tell them that even educated fleas do it! Okay fine they’re caterpillars, but that’s not a wildly off topic Ella Fitzgerald song.