On Wednesday, I posted about a parasitic wasp that turns a caterpillar into both a living incubator and a zombie-like bodyguard for its larvae. Well, it seems to be a bumper week for wasp research; today, we have yet another demonstration of the amazing tactics used by these macabre parents to provision their young with food.
The stunning colours of the jewel wasp (Ampulex compressa) belie its gruesome habits. Its grubs feed on the bodies of cockroaches supplied by their mother. When a female wasp finds a roach, she stings it twice - once in its mid-section to immobilise its front legs, and the second directly into its brain. There, she pumps in a venom that stupefies the roach and changes its behaviour.
It's not paralysed but it moves sluggishly and shows no desire to flee from danger. In this befuddled state, the jewel wasp can grab the roach by its antennae and walk it around like a dog on a leash. The wasp leads its roach to its nest, where it seals it up and lays an egg on its belly. Even as the larva hatches and starts to eat the roach alive, the hapless insect doesn't struggle or fight.
Now, Ram Gal and Frederic Libersat from Ben-Gurion University in Israel have discovered how the wasp's venom keeps its victim so sedate but otherwise mobile and healthy - it's an incredibly precise tool that specifically reduces the cockroach's motivation to walk.
Shock and oar
Gal and Libersat placed stung cockroaches into a chamber where half the floor could be electrified. For a normal cockroach, a shock that's strong enough to make its leg muscles twitch is usually strong enough to make it walk to safety. When the roaches are first stung, they behave in much the same way, but their responses soon change.
An hour later, and it takes almost 5 times the voltage to make them scamper even though their leg muscles contract in a normal way. After 4 hours, the shocks need to be 10 times stronger. It also took as many as four shocks in a row to reliably get them moving. In the experiment, their responses only return to normal after 72 hours when the venom's effects started to wear off. Of course, in the wild, it would be far too late for them by that point.
These results show that it takes a lot more to get the stung roaches to move on their own accord. As Gal and Libersat put it, the stung cockroaches have "a deficit in 'reaching the decision' to walk". They're also less likely to keep on moving once they've started. Gal and Libersat demonstrated this by placing the roaches in a water-filled cylinder, a potentially fatal predicament that they need to escape within minutes.
Normally, cockroaches start swimming as soon as they hit the water, and spend about 90% of their time trying to escape from drowning. And while the vast majority of the stung cockroaches also started to swim, they didn't keep at it for long, kicking for just 10 seconds in every minute. It wasn't that they were more tired. When Gal and Libersat removed the slackers from the cylinder and turned them upside-down, they tried to right themselves by kicking out with their legs, as vigorously as normal cockroaches do.
The fact that stung cockroaches could still flip themselves back on their feet shows that the jewel wasp's venom doesn't affect their general motor skills. When they try to right themselves, their muscles show the same degree of activity as those of normal cockroaches in the same dilemma. Nor did the venom affect the roaches' ability to fly. When Gal and Libersat blew air over the wind-sensitive hairs on the insects' back ends, their wing muscles contracted at normal strength and frequency.
Gal and Libersat suggest that the wasp's venom could affect certain signalling chemicals such as octopamine and dopamine, that are known to affect the movements and motivations of insects. Indeed, the group has previously shown that the zombie cockroaches can be restored to their active ways by injecting them with octopamine (see video below).
Regardless of the method, it's clear that the wasp's venom is a precision weapon. It doesn't just indiscriminately target the roach's responsiveness or its ability to move. Instead, it's so well matched to the brain of its prey that it only affects the specific neural circuits that are involved in walking.
Reference: Current Biology 10.1016/j.cub.2008.04.076
Images from Frederic Libersat and Tuan Cao