The Crypt-Keeper Wasp: An Interesting Parasite and Its Effects
An Interesting Hyperparasite
The crypt-keeper wasp (Euderus set) is an interesting and attractive insect that was discovered in 2017. It’s a hyperparasite, or one whose host is also a parasite. It parasitizes an insect known as the crypt gall wasp (Bassettia pallida). The crypt-keeper wasp controls the crypt gall wasp’s behaviour, forcing it to do the parasite’s bidding and killing it after it has served its function.
For many people, the word "wasp" probably conjures up the image of a large, black and yellow insect that is capable of giving a painful sting. Though these insects can be very annoying, they are only a small component of the wasp group. Most wasps are parasites that don't sting. Parasitic wasps are one of the largest groups of insects. Some species can be very helpful in controlling pests.
Classification of the Insects
Family Eulophidae (includes the crypt-keeper wasp)
Family Cynipidae (includes the crypt gall wasp)
Origin of the Crypt-Keeper Wasp's Name
The species name in Euderus set comes from Set, the name of the Ancient Egyptian god of evil and chaos. Set was also known as Seth. One of the stories about the god describes him trapping his brother Osiris in a crypt and then killing him. A crypt is an underground chamber that is often located under a church and used for a burial.
The crypt-keeper wasp enters the gall or "crypt" of the crypt gall wasp, where the latter insect dies from the effect of E. set's actions. The species name is apt because the sequence of events is reminiscent of the god's behaviour.
Euderus set was discovered and named by scientists at Rice University in the United States. The insect was discovered in Florida but has now been found in other states in the southeastern U.S.
Facts About Euderus set
Insects are classified in the phylum Arthropoda and the class Insecta. Wasps, bees, ants, and sawfiles belong to the order Hymenoptera within the insect class. Euderus set belongs to the family Eulophidae within the Hymenoptera order.
The crypt-keeper wasp has an attractive body with an iridescent turquoise and green sheen. It's a tiny insect whose length ranges from little over a millimetre to slightly over two millimetres. The size might explain why it wasn't discovered until 2017.
It's intriguing to think about the hidden and often energetic activities of creatures too small for us to notice or appreciate without magnification. Being small doesn't necessarily mean that a creature is insignificant. The crypt-keeper wasp might be important.
A parasite is an organism that lives in or on another organism (the host) and obtains its food from this organism. It generally doesn't kill its host. Organisms in the parasitoid category of parasitism do kill their hosts at some point, however. Euderus set is a parasitoid because it infects and eventually kills the crypt gall wasp.
Galls are areas of abnormal tissue on or near the surface of plants. Their creation is a response to the presence of insects, mites, fungi, bacteria, and viruses. Their exact cause or causes are not completely understood. The abnormal tissue may result from irritation of the area or from stimulation by the parasite. The study of plant galls is called cecidology.
Galls generally appear as swollen structures on the surface of plant parts, as shown in the photo above. They have a wide variety of appearances and are often attractive as well as interesting. They are sometimes likened to benign (stationary and non-cancerous) tumours in humans. They are usually harmless, but when they're numerous they may create a problem. The gall sometimes benefits the invader in some way, such as by providing a protected area for an insect's eggs to develop.
Although the crypt gall wasp belongs to a family that often triggers the production of noticeable galls on plant surfaces, it produces atypical effects. An affected twig or branch may look a little swollen overall, but it has no external galls. Instead, the twig or branch contains small holes. Each leads to a compartment known as a crypt (or a gall) where the insect's egg develops. In a heavy infestation, a branch may have many holes and crypts. The crypt can be seen in the video above.
Scientists have recently discovered that the crypt-keeper wasp can also infect other species of gall wasps, including ones that create typical galls on oak trees. These galls are sometimes known as oak apples.
Facts About Bassettia pallida
The crypt gall wasp is orange-brown in colour. Like its crypt, it can be seen in the video above. It lives in the southeastern United States and is a parasite of sand live oak trees, or Quercus geminata. It sometimes infects the southern live oak, or Quercus virginiana. The first tree is evergreen and is found on the coast of the southeastern United States. It grows in sandy substrates. The southern live oak is another evergreen tree that grows in the same region but is usually found in a different habitat. It grows in grasslands and forests.
The female wasp deposits her eggs in young stems. The eggs hatch and turn into larvae inside the crypts that form. Eventually pupae are produced followed by the production of the adult insects. The new adults chew a hole in the bark and escape. The immature crypt gall wasps obtain nutrients from the tree and are therefore categorized as parasites.
The Relationship Between the Two Insects
A female crypt-keeper wasp (E. set) deposits her eggs in a compartment occupied by a crypt gall wasp (B. pallida). The relationship that develops between the insects is sometimes known as hypermanipulation because one parasite affects the behaviour of another.
A parasitized B. pallida chews an opening to the outside world in the wall of the crypt, as it would do if it was going to escape. The hole is too small, however. When the gall wasp tries to travel through it, its head becomes stuck in the hole and blocks the opening. A crypt-keeper wasp feeds on its body and eventually creates a tunnel in its head so that it can escape. The parasitoid somehow controls the ability of B. pallida to chew a hole of the correct size. E. set can escape from the hole but B. pallida can't.
The parasitoid appears to affect the adult stage of B. pallida. Both the larva and the pupa of the parasitoid are able to infect the adult crypt gall wasp. Researchers have discovered E. set individuals half inside and half outside their host's body. The parasitoid eats the gall wasp from the inside out, leaving bits of the exoskeleton (the outer covering) in the crypt.
The image below shows a male Biorhiza pallida, a relative of the crypt gall wasp. In real life, the wings are folded over the body. Like the crypt-keeper wasp, the insect can be seen with the unaided eye but is tiny.
Control of the Host
The relationship describe above may sound more like predation than parasitism and the gall wasp may sound more like prey than a parasite host. E. set is categorized as a parasite and B. pallida as its host, however, because E. set appears to manipulate the host's behaviour and doesn't kill it immediately. This manipulation causes the host to behave in a way that benefits the parasite.
An interesting fact has been discovered experimentally. E. set is three times less likely to be able to create a successful emergence hole when it's left to its own devices than when B. pallida creates the hole. It seems to be beneficial to allow B. pallida to create the hole before killing it.
Some researchers suspect that the crypt-keeper wasp turns the crypt gall wasp into a "zombie", perhaps by means of a specific chemical or mixture of chemicals. Chemical control has been found in some other parasites. So far, however, none has been found in the crypt gall wasp. Reports about one insect "brainwashing" the other or about E. set being able to use "mind control" to influence B. pallida are premature and may not be accurate.
Another possibility is that a specific behaviour on the part of the host or a specific condition in the crypt may trigger E. set to attack its host at the right time. The timing of the attack may enable the host to create a large enough hole for E. set's exit while ensuring that the host is too weak to finish the task and escape itself. The precise control of the exit hole size is impressive, whatever factor or factors are responsible.
Further research is needed in order to discover how the host is controlled and to learn more about the sequence of events in the crypt.
Studying the Crypt-Keeper Wasp
Since the crypt-keeper wasp has been discovered very recently, there is probably a lot more that can be learned about it. The new report about its ability to parasitize additional hosts besides the crypt gall wasp is an example of how little we know about the insect.
E. set is interesting biologically and may be important in other ways besides simply increasing our knowledge of parasitic wasps. Its effects on other insects may tell us something about their activity or even help to keep them under control. This may be very helpful if the other insects have benefits or disadvantages in our lives. The crypt-keeper wasp may be more significant than we realize.
- Discovery of the crypt-keeper wasp from Rice University
- Description of a new species of Euderus from ZooKeys and Pensoft
- A parasitoid manipulates the behaviour of a parasite from The Royal Society Publishing and the US National Library of Medicine
- The story of Set and Osiris (PFD document) from the University of Texas
- Information about plant galls from Trees for Life (a registered Scottish charity)
- The crypt gall wasp and its manipulation from New Scientist
- The crypt-keeper wasp can control seven additional species from New Scientist
© 2019 Linda Crampton