By Lydia Tate
Around Halloween, thoughts of spooky creatures like ghosts, vampires, and zombies haunt our minds. As humans, we can rest easy assured of the fact that none of these fearsome threats exist outside of scary movies. However, cockroaches aren't so lucky. They can turn into real-life zombies when stung by a female Jewel wasp which injects a neurotoxin venom directly into their ganglia, the cockroach’s version of a brain. This zombification allows the wasp to temporarily paralyze the cockroach, attach her egg to its leg, and lead it to a hole where it has no will to escape and eventually becomes the first fresh meal of the hatched wasp larva.
The process is reliant on the components and effects of the Jewel wasp's venom according to Christie Wilcox (2016). The venom itself contains a neurotransmitter (chemical substance that transmits a signal from one brain cell to another) called Gamma-aminobutyric acid (GABA) that is primarily inhibitory and targets the cockroach's central nervous system. GABA dampens the ability of a neuron to be activated by signals called action potentials. An action potential is an electrical impulse that occurs when there is a change in charges on either side of the neuron's membrane. There is usually a more negative charge, known as the resting membrane potential, inside a neuron due to negatively charged proteins on the inside and positive sodium ions concentrated on the outside. Negatively charged chloride ions are able to diffuse through the membrane towards the outside of the cell since they diffuse along the concentration and electrochemical gradient that is formed by a more positive charge on the outside. Due to their charge and inability to diffuse through the double membrane, sodium ions need a channel to allow them into the neuron. When GABA binds to its receptors, they open channels that allow negative ions such as chloride to cross the membrane more easily. When combined with the opening of sodium gated channels and the flood of positive ions into the neuron after a nerve impulse, the influx of negative chloride ions can prevent the threshold (a more positive voltage needed to start the action potential) from being reached and inhibit the action potential. In summary, a component of the venom prevents the neuron from firing and transmitting a message further by preventing the membrane from reaching the necessary voltage for an action potential.
When the wasp first stings the cockroach's thorax with an organ called an ovipositor, the GABA-containing venom temporarily shuts down motor neurons and paralyzes it, allowing the second sting to be positioned into the ganglia. This second sting is positioned directly into the regions of the ganglia that will allow the venom to function as a neurotoxin, and the wasp uses her ovipositor to feel around using chemical cues to locate the proper region. The venom does not numb the cockroach's senses; instead it mutes the neurons to be much less responsive to stimuli, taking away its sense of fear and will to escape.
The first stage of the zombification is marked by 30 minutes of obsessive grooming. This could be due to a possible dopamine-like compound in the wasp venom that functions to make sure the cockroach is clean for the wasp larva and also to distract the cockroach while the wasp goes to find a suitable location to leave the cockroach and her egg later. Dopamine has links to addictive behavior and is released when pleasurable activities are completed, so this compulsive grooming is linked to the properties of the venom as well.
After the grooming stage, the wasp attaches itself to the cockroach and directs it towards the hole it previously prepared and attaches its egg to the leg of the cockroach. The venom is effective for up to a week, leaving the cockroach with a slowed metabolism so it will stay alive and be an easy, fresh meal for the larva once it hatches soon after. There are over 130 species in the same wasp genus that are considered parasitoids since they feed off of other animals (such as spiders, caterpillars, and ants) as larvae.
The Jewel wasp’s utilization of a zombifying venom is scary, at least from a cockroach’s point of view. Luckily, wasp venom can’t control our human minds and turn us into zombies as far as we know. However, this spooky part of nature does make me wonder just how far the abilities of neurotoxins extend, and whether one day mind-control could be a human concern.
Wilcox, C. (2016). ZOMBIE NEUROSCIENCE. Scientific American, 315(2), 70–73. https://www.jstor.org/stable/26047070