Thursday, 5 February 2009

Wasp ways

My garden is full of wasps of all stripes*. They are mostly solitary species, looking for a little something to feed to the youngsters. Many spend their formative days inside other insects, dining on the victim's internal organs while it is still alive. Eventually the wasps emerge, killing their hosts. Because of these deadly consequences, such wasps are termed parasitoids — their life cycles are similar in many ways to that of parasites but the outcome for the host is significantly different. True parasites usually don't kill their host; true parasitoids do.

Depending on the species, wasps either lay their eggs on or in another insect while it goes about its daily activity or relocate the victim to a nest. In the second case, the soon-to-be-larval-larder isn't always encountered in a convenient spot. The wasp has first to disable the victim and then lug around the large and awkward lump, sometimes over many metres. Life isn't easy for a parasitoid.

The emerald cockroach wasp (Ampulex compressa) avoids the problem of heavy lifting by making the take-away take itself away. Instead of paralysing the victim, Ampulex modifies its behaviour to make it compliant. The doomed cockroach can still move, but is acquiescent. All the wasp as to do is lead it away. It's one step away from the Dish of the Day.

The elements of the Ampulex life cycle follow the typical parasitoid pattern. An egg laid on the host hatches after a few days. The larvae spends time on the outside, feeding on haemolymph (insect 'blood') through the cockroach's body wall. As it grows, it chews its way in, spending the rest of its larval life feeding on the host's viscera. When it metamorphoses into an adult wasp, the inevitable happens. The host finally expires. And it may well be grateful for an end to that gnawing feeling.

The truly remarkable part of the life cycle is the way in which Ampulex alters the cockroach's behaviour. It's a two part process. First, the wasp temporarily paralyses the victim's front legs. It achieves this by inserting the sting into the prothoracic ganglion and injecting a venom — a mixture of the neurotransmitter GABA, taurine and β-alanine — that blocks nerve impulses to the first pair of legs. The paralysis only lasts for a couple of minutes, but that gives the wasp enough time to perform the next bit of surgery.

Now Ampulex injects its venom directly into the brain and the suboesophageal ganglion. The sting goes in from the underside, which is not as heavily armoured as the top of the cockroach's head. The venom induces a progressive change in behaviour. On being stung in the brain, the cockroach first grooms itself for about 20 minutes. (This could explain those makeover shows on television.) Then the venom permanently blocks its escape response by inhibiting the associated nerves. The victim no longer receives 'danger, run away!' signals from the antennae or the sensitive hairs on the cerci. (Those pointy bits sticking out of the posterior of a cockroach are covered in fine hairs that detect the slightest air movement. Each hair has its own nerve. When a nerve is triggered, the cockroach goes into scuttle mode. That's why the little beggars are so difficult to approach.)

The wasp grabs hold of an antenna with its jaws and leads the cockroach to a burrow. Once the victim is in place, the wasp lays an egg and then seals the burrow entrance with pebbles. A month or so later, a new wasp emerges and all that remains is a hole in the ground filled with the husk of a cockroach.

Not surprisingly, Ampulex compressa is used as a biological control for cockroaches. It is not as effective as other wasps, which target cockroach eggs, but it has a certain élan, wouldn't you say?


* Except European wasps, for some reason. Perhaps the drought has hit them hard? Not that I'm complaining.


nut said...

Amazing! Just dropped in for a hit of nature talk. Nice entry.


Snail said...

There's an acanthocephalan (thorny-headed worm) that does similar things as an internal parasite of cockroaches. It reduces the host's escape response so increasing the cockroach's chance of getting eaten --- the acanthocephalan can then move into the next host to complete its life cycle.