How I Accidentally let 384 Parasite Wasps Loose in my Living Room

July 14, 2022

I have a small garden with a few vegetables in it. For the first time this year, I’ve grown cabbages, and at the end of June, I met my first cabbage pest. Little caterpillars. Pieris brassicae, The Cabbage White Caterpillar are extremely common, they’re the dainty little white butterflies you see fluttering around on a sunny day.

If you’re an entomologist, you probably already know where this story is going.

I carefully collected the caterpillars up, and set up a spacious enclosure for them in an Exo Terra vivarium, using buddleia sticks as wood and cabbage leaves for food. I’d collected 16 of these little critters and watching them eat was absolutely enthralling. These guys were ravenous, going through roughly 2 big cabbage leaves a day. I was getting worried about how long they’d need to eat for before growing into butterflies.

Pieris brassicae White Cabbage Butterfly Caterpillar Group Eating Cabbage Leaf

Not 3 days later, they’d nearly doubled in size. The group of 16 had eaten through 6 cabbage leaves and had left a remarkable amount of tiny caterpillar poop as proof. Sure, I’d read the literature (The Very Hungry Caterpillar, Eric Carle, 1969), but I spent longer than I’d like to admit watching the caterpillars methodically eat through the leaves in concentric arcs – like little eating machines.

White Cabbage Butterfly Caterpillar Silk on Window

Soon, they began climbing the walls of the vivarium, leaving the cabbage behind and preparing themselves for the next stage. A few of them started etching tiny little silk webs around themselves. My excitement was completely inappropriate for a 25 year-old man, and I was sending regular updates to friends and family.

After keeping the caterpillars for 4 days, I arrived home from work, briefly checked to see if any of them had formed into chrysalises yet and noted that they were all still caterpillars. Half an hour later, I noticed something strange.

There, at the back of the tank, one of them was covered in tiny little wiggling shapes. The shapes waved and rippled over the surface of the caterpillar, and as I took a closer look, I began to realise that they were larvae. Certainly not Cabbage White larvae, but something else.

It Was Parasitoid Wasp Larvae

Absolutely incredible. While not expected, these caterpillars were infected with Cotesia glomerata*, a type of wasp that lays its eggs in its victims. Just like the film Alien, and just as cruel, these larvae had been growing, from eggs, inside the body of the caterpillar. Up until now, they had been feasting entirely on the caterpillar’s blood and non-essential insides, keeping care to not hurt any vital organs.

After reaching the end of this stage of their lifecycle, they now begin to break out. The larvae have developed tiny, jagged teeth and begin chomping their way out of the host caterpillar. The caterpillar is paralyzed by chemical compounds produced by the wasp larvae, completely immobile, forced to wait for the larvae to eat their way out.

Cotesia glomerata parasitoid wasp larvae emerging from white cabbage butterfly caterpillar

The waving movement is the larvae climbing out. On average, there were around 24 larvae on each caterpillar. Over the course of 12 hours, all of the remaining caterpillars followed suit – in pairs, they paralyzed up against the glass and acted as a feast for their parasites.

It doesn’t, however, end here. The caterpillars all survived this experience.

The next day, I checked on the progress and found that the larvae had now spun themselves into silk cocoons. While the caterpillars would usually do the same, ready to chrysalise and turn into those aforementioned white butterflies, these caterpillars are different. This is disputed**, but the theory is that they now display motherly instincts. They use their own silk to coat the wasp brood in an additional protective layer. The caterpillars act as bodyguards, standing guard and protecting the wasps from other predators who may attack.

Cotesia glomerata larvae eggs caterpillar guarding

The outstanding thing here is that the parasitoid wasp, who had injected its eggs into the caterpillar around 2 weeks previously, had now controlled the caterpillar’s behaviour, altering it’s mind to look after her offspring.

Additionally, I did some research and found an incredibly interesting book, The Encyclopedia of Virology (Third Edition), 2008. Pages 250-256 go into more detail about Polydnaviruses, the abrogation of invertebrate immune systems. Those were a lot of long words, so I’ll break down what I understood.

When the wasp lays its eggs in the caterpillar, it also injects a virus. This virus infects the host, which stop the immune system from working properly. Anti-encapsulation means that the cells are coated with something like a mucous, which make the hemocytes unable to bind with the parasitoid eggs. In other words, the immune system becomes too slippery to probably destroy the parasite eggs. It’s likely that this affects the immune system’s functionality in other areas as well.

There are a few particularly nasty human diseases that this may remind you of. Yet another fact that solidifies these wasps as pretty cruel fuckers.

Sadly, the caterpillars do eventually starve to death. A grim end to their part in this story. Over the space of a couple of days, I watched as the caterpillar carcasses fell onto the floor of the vivarium, curling up and returning to the dirt.

384 Parasite Wasps

Now, I’m not a biologist or anything, but any experience I get to watch nature up close, especially in controlled conditions, I’m going to gobble that shit up. I may have lost out on the butterflies, but I had an opportunity to watch parasitoid wasps up close and learn more about another insect. Jackpot.

I actually sat down and did the math. I assumed a 25% mortality rate before breeding, and an otherwise successful wasp orgy in every generation. I also assumed that I could supply enough caterpillars (I’d have to find a caterpillar guy), and that there’d be no space limitations. Anyway, if I did the math right and got the lifecycles right, I could actually have 1,000,000 wasps by September. That’s right, I’d have 1 million parasitoid wasps, and a man can probably do a lot with an army of a million wasps.

Spoiler alert, I decided not to make a wasp army.

One morning a week later, I checked on the wasps and found the vivarium buzzing with tiny little black specks. The wasps had hatched. Pretty confident that they were all contained in the vivarium, with no escapees that I could see, I watched them and clarified what species they were, and made a few notes on their movement.

Cotesia glomerata parasitoid wasp near larvae coccoons

(I’m pretty sure they’re Cotesia glomerata, but if you’re more versed hymenopterology and that’s incorrect, please get in touch with me and let me know.)

Rapid, skittish flight almost similar to fruit flies. They’d jump around the brighter parts of their home, trying to reach nearby lamps. Some stuck around the cocoons, and there was even a level of interactivity as the recently hatched insects attempted to find mates as soon as possible.

After taking a few recordings, I noted that there were around 70 wasps in the vivarium, and decided to leave for work, and wait until the end of day to see if the rest had hatched.

This was a mistake

I returned home after a particularly long and hot day at work. I opened the door, entered the living room and immediately knew I’d messed up. Now I don’t have photos of this, because I was dealing with the wasps – but there were wasps, everywhere. The majority were at the windows, trying to escape, but the vivarium had clearly experienced some form of prison break. I reckon one of the wasps knew how to pick locks.

I managed to get most of them out pretty quickly and took the vivarium outside just in case any were yet to hatch.

Now, you may think that ethically, I have the duty of erasing as many of these cruel creatures as possible. I do, however, disagree. Despite the fact that we as humans look at their actions as pretty terrible, these guys act as natural population deterrents for a species of butterfly that, at the end of the day, is a pretty bad pest for a series of major UK crops.

I do love butterflies and all manner of wildlife, but there is a positive to Cotesia glomerata. Cauliflower, broccoli and cabbage are all decimated by the caterpillars of the Cabbage White butterfly. Having a natural predator that prevents the species from overwhelming the crop is probably beneficial. As something that stops further implementation of pesticides, it’s a necessary evil in my eyes.

Despite the messy ending, and the horrific information about these little cocksuckers I found, I immensely enjoyed keeping the experience. Maybe in the future I’ll reconsider the wasp army, but for now, I hope you’ve enjoyed learning about the cruellest insect I’ve ever had the pleasure of meeting.

*Special thanks to Tanya Bainbridge for clarifying the species as Cotesia glomerata. I had originally incorrectly named it Microplitis demolitor

** The jury is still out on this one, I’ve had a few chats with people about whether the behaviour changes are serendipity or not. Based on the information provided by Dr Gavin Broad from London’s Natural History Museum, the view is the following: the caterpillar begins spinning it’s silk pad as it usually would. It still plans to chrysalise, however due to its traumatic experience and possible anaesthesia, finds itself unable to produce more silk. While the silk does indeed protect the parasites from hyperparasites, it’s purely coincidental on the caterpillars perspective. This doesn’t account for the wasps. The wasps will likely have evolved to time their larval growth with the optimal emergence period, when the caterpillar will happen to provide additional protection. While, in this explanation, there’s no mind control, it’s still an absolutely outstanding example of evolution!