Halloween Special: Parasites and Parasitoids

As you lay petrified in bed the night of Halloween after watching a scary movie, you begin to wonder if you will ever be able to sleep again. Your mind races as it plays back scary image after scary image from the festive film. Once you are finally able to drift into an unsettled sleep, you have a vivid nightmare. In your nightmare, you encounter a giant fly which injects its babies into your flesh. As the fly larva travel throughout your body, threatening to take over your organ systems, you rush to the hospital to seek medical aid. But the fly babies work quickly to seize control of your brain, controlling your mind and forcing you to lay on the ground in the fetal position. Eventually the fly babies eat their way out of your body, ready to begin their fruitful lives of finding a mate, finding a host to lay their babies in, and dying.

The image above depicts a particular wood wasp that uses its long tail-like organ in the back (called an ovipositor, don’t worry I will cover that later in this post) to inject its babies into a host organism. Horrifyingly beautiful isn’t it?! I am still perplexed as to why Hollywood has not created a horror film about the various parasitic organisms on our planet. In my opinion, it would beat any horror film out there! Blood-sucking vampire, boring. Smelly green swamp monster, overrated. Incredibly fast and deadly alien creature that has no eyes and a freakishly acute sense of hearing, not that cool. (Actually, that was one of my favorite horror/suspense movies!) A tiny flatworm that can live inside of a snail, take control of its mind causing its head to pulsate (similar to a strobe light) and attract birds to feed on its zombie corpse… now that’s scary!**

My experience with parasitic organisms is fairly extensive. In college, I had the pleasure of working with a particular type of ichneumon wasp which injected its fertilized eggs into a beetle larval host. The wasp larva would eventually eat their way out of the beetle larva.
A passion of mine is studying and collecting bugs. While I was collecting bugs one day, I had a surprise encounter with a parasitic fly. I was tracking a red-winged grasshopper in a dusty field at the base of a mountain holding a bug catching jar in one hand and the lid in another. The grasshopper had been on to my tactics for the past 400 yards. Every time I got close it would promptly fly off. But, like a hungry lion relentlessly hunting a zebra, I would not be outrun! I was creeping up directly behind the grasshopper as quietly and as stealthily as I could until, with ninja-like reflexes, I trapped the red-winged grasshopper in my jar. I know what you are thinking, bug catching can’t be that epic… well it is!

On the drive home, my entomology partner was handling the wheel while I was in the passenger seat admiring my haul of bugs. I peered into the jar with the red-winged grasshopper in it and promptly let out a yelp. To my great astonishment, there were eight tiny worms, or what I initially thought were worms, crawling around in the jar with the grasshopper. I made my friend take his eyes off the wheel to witness this crazy discovery. There definitely were no worm-like creatures in the jar before I had captured the grasshopper, so what was going on? I felt like Henry Walter Bates and other great early naturalists who went on adventures to make new biological discoveries. I took the grasshopper and worms back to the lab I worked at for further analysis. After talking to my boss, who happened to be an entomology professor, and rearing the worms in a Petri dish, I discovered that the worms were actually fly larva. An adult female parasitic fly had deposited her young into the poor red-winged grasshopper before I came along to snatch the hopper. When the grasshopper was fighting for its life in my capture jar, the larva sensed the danger the grasshopper was in and decided to jump ship.

The lives of billions of species is constantly in flux all around us everyday. Predators hunt for prey and prey are constantly trying to stay one step ahead of the hungry predators. But hidden from our plain sight, there are other types of predators on the hunt. Parasites are generally invertebrates that depend on a living host for its survival. This typically means the parasite will feed off of the insides of the living host, though sometimes they will live on the outside of the host or feed on the exterior of the host (lice and mosquitoes are considered parasites). One bizarre example of parasites is the flying insect known as Stylops melittae. Female stylops will mature inside of sand bees and they will remain inside of sand bees throughout their lives. Once the female stylops matures, they will break through a section of the sand bee’s abdomen for mating purposes. Mature male stylops are attracted to a scent that the female produces and the male will find her and mate with her while she is partially inside of the sand bee. Yes, the sand bee is still alive throughout this process. Talk about a bizarre and twisted nightmare if you are a parasitized sand bee!

Parasites may be terrifying, especially for the sand bees, but parasitoids are even more interesting and scary to me. My examples of the fly parasitizing the grasshopper and the wasp parasitizing the beetle larva are examples of parasitoids. There are some major differences that set parasites and parasitoids apart. Parasites will usually spend the vast majority of their lives inside of the host and need the host to remain alive so they can receive nutrition benefits throughout their adult lives. Parasitoids on the other hand only spend their infant, or larval, stage of life inside of the host and then they will usually eat through the host once they (the parasitoid) become mature adults. This kills the host. I am not sure what is worse, sharing a portion of your painful life with a parasite inside of you or only feeling the pain for awhile until the parasitoids eat through you and kill you (For the record, I am definitely leaning towards the latter scenario).

The female parasitoids are very specific as to which host they will insert their babies. The host has to be a specific species and the host has to be in a specific stage in its life for the female to deposit her young. So how do the female parasitoids find just the right host? The female parasitoids have learned specific sounds and chemical signals, called kairomones, that the host makes as well as habitats or locations the host will travel to. Once the female parasitoid finds the perfect host, she deposits her babies into the host using a tube shaped organ called an ovipositor. Once the babies are fully developed inside the host, they will emerge from the host as full grown adults.

So what? Why do we care about this somewhat morbid cycle of life that many invertebrates experience? Parasitoids are very important in maintaining pest populations around the world to a minimum. This can be very beneficial to farmers in maintaining healthy crops. In fact, pest infested plants have even been known to send out chemical signals that attract parasitoids to their location to combat the growing numbers of insect pests. Besides plant eating pests, parasitoids may also carry diseases that limit animal pest populations in certain areas.

So when you wake up from your nightmare, be grateful you are not really a host species for a parasitic fly, or wasp, or worm. Next time you step outside, look around you. Look for a fly or a wasp that may actually be part of this crazy cycle of locating a host (or a mate sticking out of a host) to help its offspring survive. Look around for other invertebrates (or even some vertebrates) that may be struggling to survive due to life sucking parasites and parasitoids. Realize that there is way more to this world than just your Halloween party plans. Somewhere, some poor parasitized beetle can go to the Halloween party because they have bigger problems on their hands, like baby wasps eating away at their insides.

If you read this whole blog post, you are my hero! Thanks for reading. Please feel free to leave me a comment and feedback below. Also, let me know if there is any particular science topic you would like me to look at “through green tinted glasses” and I will write about it.

Science word of the week: “Kairomone”—a chemical signal emitted from a particular host organism and interpreted by a parasitic organism. This interspecific interaction harms the emitter of the chemical and benefits the interpreter.

**For more information on the crazy, disco-pulsating snail, follow this link: https://www.wired.com/2014/09/absurd-creature-of-the-week-disco-worm/

**Also, For more awesome examples of parasites and parasitoids, follow this link: https://news.nationalgeographic.com/news/2014/10/141031-zombies-parasites-animals-science-halloween/

[Work Cited] The ideas in this post came from my crazy head, but a lot of the information was adapted from the following sources:

• https://australianmuseum.net.au/predators-parasites-and-parasitoids
• https://news.nationalgeographic.com/news/2014/10/141031-zombies-parasites-animals-science-halloween/
• https://www.wired.com/2014/09/absurd-creature-of-the-week-disco-worm/
• Insect Parasitoids. 13^th Symposium of the Royal Entomological Society of London. Jeff Waage and David Greathead. The Royal Entomological Society of London 1986.

This is the red winged grasshopper that had fly larva emerge from its body after I had captured it.

A wood wasp, and example of an insect that inserts babies into its host.

My wife, baby and I at a recent Halloween party. Butterfly, ladybug, and bug catcher respectively. Aren’t we cute!