Jewel Wasps and Zombie Cockroaches: An Intriguing Relationship

The American cockroach and the jewel wasp are interesting insects in their own right. Their interaction is especially intriguing, however. The wasp stings the cockroach, changing it into a zombie. It then leads the roach to a nest and lays an egg beside its body. The larva that hatches feeds on the living roach. Researchers have recently discovered that the cockroach has a defence mechanism that often but not always works. The roach uses a “karate kick” to repel its attacker.

A jewel wasp (Ampulex compressa)
Photo credit: Muhammad Mahdi Karim, GFDL 1.2 license

The Jewel Wasp

The jewel wasp is also known as the emerald cockroach wasp and has the scientific name Ampulex compressa. The insect has a blue-green body with a metallic sheen. The “thighs” of its second and third pair of legs are red. It’s not hard to see why one of the animal’s common names is jewel wasp. Its colouration is very attractive. The wasp is a solitary insect that lives in tropical parts of the world, including Africa, South Asia, and the Pacific Islands. It’s particularly interesting for its method of subduing and controlling a cockroach without immediately killing the animal.

The American Cockroach

The scientific name of the American cockroach is Periplaneta americana. Despite the insect’s common and scientific names, it’s native to Africa and the Middle East, not the United States. The animal is believed to have been introduced to North America in the seventeenth century, where it has become a pest. Though it’s often a highly disliked insect, especially in buildings, some people may feel sorry for its fate when it meets a jewel wasp that is ready to lay an egg.

The American cockroach
Photo credit: Jan Drewes, CC BY-SA 2.5 license

Creation of a Zombie Cockroach

The jewel wasp first grabs hold of the cockroach with her mouth and then aims her stinger at the thorax between the first pair of legs. The secretion that she injects quickly paralyzes the cockroach, enabling the wasp to take more care with her next jabs. She carefully searches for the right places to inject her venom in the roach's brain, or ganglia. Amazingly, she can detect the location of these areas by touch as she moves her stinger around inside her victim’s head. She chooses two specific spots for the injections. 

Although the venom for the head injections is apparently the same as the one used in the thorax, different effects are produced. When the wasp has finished her work, the roach starts grooming itself. It continues to do this while the wasp flies away to find a dark and secluded spot for a nest. Researchers have discovered that the cockroach isn’t paralyzed at this stage. It could still move away, but it doesn’t. In addition, if the insect is rescued, it stops grooming itself in about thirty minutes and its behaviour slowly returns to normal. This doesn’t normally happen in nature because the wasp soon returns. 

The Journey to the Nest

When the jewel wasp returns to its victim, it break off part of the roach's antennae and feeds on the hemolymph (the insect equivalent of blood) that emerges. This apparently gives the wasp energy for her next task. She grabs hold of the remains of one of the roach’s antennae and leads the insect to the nest. The roach may be six times as large as the wasp, but it doesn't resist the wasp's control. It moves willingly without stumbling. 

Once the pair have entered the nest that the wasp has created, she lays an egg on one of the roach's legs. She then seals the opening to the nest with items such as pebbles and bits of wood and flies off, perhaps to find another victim. She has many eggs to lay. The video below shows the wasp attacking and subduing the cockroach, leading it into the nest, and then laying an egg. 

The Wasp Egg and the Larva

The remarkable venom slows the cockroach’s metabolism so that its body is still in good condition when the egg hatches into a larva. At first the roach is alive as the larva chews a hole in its abdomen and enters its body to feed, but eventually it dies. The larva eats the roach's organs in a specific order so that it stays alive as long as possible.

The larva becomes a pupa inside the cockroach. Lab studies have shown that the roach doesn't respond to the attack, even as it's being eaten alive. The new wasp that is produced is able to remove the blocked opening of the nest and escape, leaving the remains of the cockroach's body behind.

Roach Karate Kicks

Though cockroaches are generally not well liked, some people may be sympathetic for what the insects experience during a wasp's attack. A researcher at Vanderbilt University has recently discovered that the roach is not completely helpless, at least in some cases. Ken Catania used slow speed videography to record cockroach behaviour as a jewel wasp approached. He found that the roaches often applied a swift kick to the wasp with one of their spiny hind legs. The process is shown in the video above. 63% of the time, the roach successfully repelled the wasp and survived. The wasp left to look for another victim.

The good news for the cockroach: the defense worked for 63 percent of adults that tried it. The bad news: juveniles almost always failed and got stung in the brain. Quotation from Heidi Hall, Vanderbilt University news release

There's probably a lot more to learn about the relationship between the wasp and the roach. The nature of the venom that's injected would be fascinating to explore in more detail, since it has pronounced and unusual effects. The effects of the venom on the cockroach are also intriguing. Scientists are gradually identifying some of the chemicals involved and their effects, but more studies are needed to fully understand the venom. Nature never ceases to be interesting. 

References

How cockroaches turn wasps into zombies from Scientific American

The wasp that enslaves cockroaches from Wired Science

Karate kicks keep cockroaches from becoming zombies from Vanderbilt University