Public Domain image by Alex Wild from the Insects Unlocked project at the University of Texas at Austin
Adaptation is limited by trade-offs. One common trade-off observed in nature is that when parasites adapt to attack one group of hosts, their adaptations may reduce their range to parasitize hosts.
This trade-off has been used to explain, in part, the tendency for parasites to specialize in a subset of available hosts, such as herbivorous insects on plant hosts or blood-feeding insects on animal hosts.
However, there’s only been a few studies considering the ranges of behavior-manipulating parasites. This is particularly true for parasitoids — insects that parasitize and eventually kill their hosts.
Now, research shows evidence that a parasitoid wasp attacks and manipulates the behavior of diverse hosts based on taxonomy — groups of organisms classified based either on shared characteristics or on evolutionary relationships. This finding contrasts with the expectation that this behavior-manipulating insect would have a taxonomically limited host range.
Mind over matter
We, humans, are defined by large brains compared to body size. This adaptation comes at an extreme cost to the rest of our physical traits. We’re certainly not at the top of the food chain because we’re the ultimate physical predator — we’re slow, uncoordinated, unhurried to develop, and unfit for the wild. Yet, we can think our way to the top of the animal kingdom.
This is because there are only so many resources to go around to enhance our traits. There’s only so much energy to go around for mind and matter. Resources are finite, and maximal optimization of all traits at once is impossible.
For example, when energy is devoted to growth, it may not be available for reproduction. This is one reason why so much growth occurs during childhood before sexual maturity. Compare that to the many animals who are born hitting the ground running, literally, like giraffes.
Tales from the crypt-keeper
One interesting instance of this tug-of-war over adaptation can be seen in parasitoid wasps. As parasitoids, they lay their eggs on or in the bodies of other arthropods, sooner or later causing the death of these hosts.
Different species specialize in hosts from different insect orders; some select beetles, flies, or bugs, and others exclusively attack spiders. For example, there’s a spider that under manipulation by a parasitic wasp larva builds a special “cocoon web” that will house the larval wasp until it grows up.
Recent papers describe the discovery and life history of the parasitoid “crypt-keeper wasp” (Euderus set). When discovered, it was so named because of its parasitism and behavioral control of the “crypt gall wasp” (Bassettia pallida) in live oak trees.
The crypt gall wasp has a curious life cycle. This process begins with the female crypt gall wasps piercing a leaf or stem. This induces the formation of a swollen internal gall — known as a “crypt” — inside the stem. Here, inside these tiny nursery domes, crypt gall wasp eggs are deposited into the trees where the developing crypt gall larval wasp will then feed and grow. Typical adult crypt gall wasps later chew a small exit hole in their crypt made of woody plant material and then fly away.
However, when parasitized by the crypt-keeper wasp, the chewing behavior of the developing crypt gall wasp changes. This causes the crypt gall wasps to chomp significantly smaller exit holes, blocking their ability to breach from the gall. These stalled crypt gall wasps eventually get their heads stuck in the small hole, like a dog with its head stuck in a fence.
This leaves the crypt-keeper parasitoid larvae to then freely feed on the disabled body of the host crypt gall from within the gall to safely grow. When the crypt-keeper larva matures into an adult wasp, it chews through the “head plug” and absconds the gall.
Since the crypt gall itself manipulates oaks to develop crypts in their young branches, the crypt-keeper is a “hyper-manipulator” — a parasite that manipulates a parasite that itself manipulates a host.
Given the potential trade-off between specialization and host range, does this strategy of behavioral manipulation limit the range of parasitism for the crypt-keeper wasp?
The parasitoid communities of most oak gall wasps are understudied or unknown. So, it is premature to say that the manipulation of the crypt gall wasp by the crypt-keeper is a unique relationship.
Surveying the crypt-keeper kingdom
For these reasons, researchers examined the host head-plugging manipulation by the crypt-keeper wasp and related family members.
They looked at two key questions. They wanted to understand if crypt gall wasps were limited as the only hosts for parasitism by crypt-keeper wasps and their close relatives, and they wanted to know if the behavior of hyper-manipulation is found across multiple wasp species related to the crypt-keeper wasp.
From approximately 100 species of oak gall wasp, the researchers identified hosts and nonhosts of crypt-keeper wasps using wide-ranging collections. From 2015 to 2018, they collected more than 23,000 crypts from more than 10 oak species in North America.
They then assessed associations between wasp infection and “head-plugging” behavior in the parasitized wasp hosts.
Enemy of the state
Among these collections, the scientists reared crypt-keeper wasps from six different gall wasp host species. Interestingly, each of the wasps’ hosts has converged upon similarities in their extended traits. Specifically, the galls they induce on oaks share characteristics that may make them vulnerable to attack by crypt-keeper wasps.
The researchers predicted that crypt-keeper-like wasps from different hosts should be pretty unrelated and different because they are tightly tied with their hosts. However, their findings show that all parasitoid wasps reared are crypt-keeper wasps.
All known hosts also lack the elaborate structural defenses found on many other galls. This can include spines, fuzz, or larval cells suspended deep inside otherwise empty chambers. These defenses grow more substantial as the gall grows, which may make many galls inaccessible to all parasitoids of gall wasp larvae.
It is unknown how the crypt-keeper wasp is manipulating crypt gall larvae. Scientists don’t know if the crypt-keeping manipulates the brains of the hosts it infects. One looming question is how the crypt-keeper larva makes the gall wasp stop chewing at such a precise point.
There is yet much to learn about these mind-controlling creepy crawlers.
A deeper dive — Related reading on the 101:
- Nature’s horror stories include mind-controlling parasites that turn bugs into zombies | Science 101
It’s not just the crypt-keeper wasps that take part in this mind-controlling …
Will mind control soon be used on humans?!