The emerald cockroach wasp, Ampulex compressa, also known as the jewel wasp, is a solitary wasp mostly found in tropical regions.
How did the cockroach wasp get its name?
The emerald part of the name comes from their green metallic coloring, the cockroach part because they catch cockroaches for its offspring to feed off. But it does not serve up these cockroaches dead. They turn cockroaches into ‘zombies’ leaving the killing to its larvae. A dead cockroach will begin to decay, a paralyzed one does not. There is a problem, however. The best cockroaches are too big to move back to the nest in a paralyzed state.
So these wasps have evolved an intriguing solution. Rather than totally paralyzing the cockroach the first sting only temporarily paralyzes it before they inflict a second sting into the head. This disables the cockroach’s ability to send motor signals but preserves the ability for the legs of the cockroach to move. The cockroach wasp can now walk the cockroach by its antennae to its nest. The cockroach is now trapped within its own body and totally subject to the control of the wasp, unable to move by itself. It remains alive for up to 2 weeks while the wasp larvae consume it, saving the essential organs to last.
How do emerald cockroach wasps know where to sting a cockroach to disable it?
The wasp’s stinger can actually feel around inside the cockroach’s head to locate the exact place to inject the venom. Studies have found that if the ganglia of the cockroach is removed then the emerald cockroach wasp will be confused, and spend considerable time trying to locate the missing part. (Source)
What effect does cockroach wasp venom have on cockroaches?
Once the venom is injected into the cockroach’s head it prevents its ability to walk, but it doesn’t stop the ability of the cockroach to groom itself. It has been observed that following a sting to the head the cockroaches aggressively groom themselves for up to half an hour. Reasons for this are unclear. All the while their fate is sealed. The impact of the sting on the cockroach seems to wear off after a week, but unfortunately for the cockroach by that time the larva will have feasted upon it.
Why do their stings disable rather than kill cockroaches?
The reason the emerald cockroach paralyses a cockroach, rather than kill it, is primarily to prevent it from decomposing. A decomposed cockroach isn’t much nutrition for a growing wasp larva. The sting actually slows the metabolism of the cockroach meaning that even without the consumption of food and water the cockroach will live long enough to make a fresh meal for the wasp larva.
Why are males smaller than females?
Males are given smaller cockroaches to eat. The smaller the cockroach the smaller the wasp. Males are provided with smaller cockroaches than the female larva, so consequently, with less food, they grow to less than half the size. Since the male wasps don’t do any of the hunting their size is less important. The female emerald cockroach wasp will save the biggest cockroaches for its female offspring.
The female knows which of its eggs are male and which are female as only the female eggs are inseminated. She stores semen after mating and decides which eggs she will inseminate and which not. Only the female wasps can sting as the stinger is actually part of the reproductive system of the wasp.