Let me establish something right off the bat: I do not like parasites. I find them creepy and unsettling; thoughts of bot-flies and ticks haunt my dreams. Despite studying insects, I have staunchly avoided any classes that could put me in contact with something that wants to burrow under my skin. But, even if I would like to social distance from just about all parasites, I have to admit, some of them are kind of cool.
As an aspiring hymenopterist — someone who studies bees, wasps and ants — parasites are plentiful and prolific in my field. There are over half a million species of parasitic and parasitoidic wasps, each stranger than the next. Parasites are organisms that live on or in a host or host colony and benefit at their expense. Parasites range from blood sucking ticks and lice to kleptoparasites who steal the hard-won food of other organisms and social parasites that kidnap organisms to support their community. The key thing about parasites is that having one doesn't necessarily mean you’re going to die. It’s in the parasite's best interest to keep you alive as long as possible. If you have a parasitoid, however, you’re out of luck. Parasitoids will eventually kill their hosts, generally when their larvae — oftentimes laid inside the host — grow into adults. Thankfully, parasitoidic organisms are (usually) insects that target other insects.
As we delved into parasites in my Intro to Entomology class, I had a lot of questions. Chief among them being: Could parasites have parasites? The answer is, absolutely! In fact, even a parasitoid can have a parasitoid, who in turn has a parasitoid … You get the picture.
These Russian nesting dolls of the parasite world live on the caterpillars of cabbage butterflies. Cabbage butterflies, while native to Eurasia, were accidentally introduced to North America in 1860, and spread like wildfire. Cabbage butterflies are extremely common in Wisconsin; you’ve probably seen them flit around flower bushes and tall grasses. While these butterflies look like nothing more than harmless, stumbling white cotton balls flying around in the summer breeze, their caterpillars, known as cabbage worms, chow down on broccoli, cabbage and cauliflower. The standards for these plants are notoriously high, and cabbage worm damage causes swaths of fresh produce to be unsellable.
Enter Cotesia glomerata. C. glomerata, otherwise known as the glomerata wasp, is a small wasp in the family Ichneumonidae and a primary parasitoid of cabbageworms. When a plant is being gnawed on by a cabbageworm, it sends a chemical distress signal that attracts glomerata wasps. Then, like a scene from a low-budget horror movie, the glomerata wasp stings the cabbageworms, laying her eggs inside the defenseless caterpillars. The wasp larvae develop inside the caterpillar, slowly eating it from the inside out, but make sure to keep the caterpillar alive. When they get big enough to pupate, the larvae use sharp teeth to poke holes in the side of their cabbageworm host. They then form cocoons on the cabbageworm’s body — cocoons that the cabbage worm will never be able to build themselves. At this point, the cabbage worm is still alive, under a form of insect mind control. The cabbage worm will protect the glomerata wasps cocoons until it’s final hour.
The plant's distress signal, however, doesn’t just attract glomerata wasps. Other opportunistic parasitoids are looking for a place to lay their eggs. Lysibia nana, another species of parasitoid wasp, are on the hunt for glomerata wasp cocoons. While the adult females exclusively feed on nectar, the larvae are secondary parasitoids, developing on glomerata wasp larvae while they are parasitizing cabbage worms. While the original host is still alive, the L. nana female will lay her eggs inside the glomerata wasp cocoons. As she lays her eggs she releases a paralytic agent into the glomerata wasp pupae, halting its development. These new moms, however, have to be fast. The survival rate of their young drops drastically when the glomerata wasp larvae are more than two days old. If she can lay her eggs before that point, the L. nana larvae will have an 80% chance to survive to adulthood. When the L. nana adults emerge from the glomerata wasp cocoons, they will be a similar size to their glomerata wasp counterparts.
If you know anything about trophic levels, this may seem confusing. As energy moves through an ecosystem, through plants, to herbivores, to predators, some of that energy is lost. About 10% of the energy is stored as body mass, while the rest is used for bodily functions and expelled as heat. For example, a cabbage plant may store 1,000 Kcal of energy. The animal that eats it, such as a cabbage worm, acts as the primary consumer and will retain 100 kcal of energy. The organism that eats the cabbage worm, the secondary consumer, will retain 10 kcal of energy. Therefore, the tertiary consumer will only retain 1 kcal of energy, a huge loss. If L. nana are only getting a small fraction of energy their glomerata hosts are getting, they logically shouldn’t be able to grow to their size.
Parasites, however, have a way of getting around this pesky energy problem. The first is simple; they don’t do a whole lot besides, yanno, destroying the sanctity of their host’s body. Parasitoid larvae are largely sessile, fixed in one place and unmoving, and practically have a whole buffet in front of them. Inside a host, all they have to do is eat; no energy is wasted on movement, development or defense. What a dream.
Another reason why parasitoids are incredibly efficient is they actually change the chemical composition of their host to suit their needs. During the process of parasitism, the nutrients, fats and minerals found in the parasite's host are altered to uniquely benefit the parasite or parasitoid. This is how the L. nana wasps can grow as big as their host on such limited resources.
These strategies open up a whole new world of parasitoidic possibilities, not just for L. nana. For cabbage worms, it means they get another layer of parasites stacked on top of them. Gelis agilis are tiny, wingless wasps that spend their adult lives pretending to be ants. This disguise allows them to travel the insect world largely unharmed, as ants are considered aggressive and unpalatable. G. agilis can be either secondary or tertiary parasitoids; the female will either lay her eggs in the primary parasitoid, glomerata wasp pupae or the secondary parasitoid, L. nana larvae. These wasps are so evolutionarily fit for dealing with limited resources that the secondary and tertiary parasitoidic larvae of G. agilis will grow to similar sizes, indistinguishable as adults.
We still don’t know how deep the parasitism wormhole goes. There is a very real possibility that there could be even more layers of parasitism — imagine finding a parasitoid within a parasitoid within a parasitoid within a parasitoid; an astonishing feat of evolution. Fascinating, as long as they're not burrowing under your skin.