Wasp Works Its Will on a Captive Spider By Nicholas Wade
July 25, 2000
There are few things more creepy than alien possession, the notion of one creature taking over another's body and bending it to different purposes. Though this may happen every day on other planets, an egregious example has come to light on earth too, and as close to home as the forests of Costa Rica.
Here lives an orb-weaving spider, so called because of the perfect roundness of the web it industriously rebuilds every day. A serious hazard of the spider's busy life is that it is hunted by an ichneumon, or parasitic wasp.
If the wasp's attack is successful, it temporarily paralyzes the spider and lays an egg on the tip of its abdomen, where it is out of reach. For two weeks the spider spins its web and catches insects every day as if nothing were amiss, except for the growing larva that clings to its belly and sucks the juices that drip through small punctures it makes in the spider's body wall.
So far this is just the usual grim script of parasitism. But then comes a strange twist. The night before the wasp larva kills its host, it somehow induces the spider to build a most unusual web.
Instead of its delicate orb, the zombified spider constructs two stout silk cables with thick cross-braces in between. This durable platform stands up to wind and rain better than the spider's ephemeral web. The wasp larva then kills the spider, and spins its cocoon on the platform constructed for it, safe from the ants that patrol the ground below.
Dr. William G. Eberhard, the spider expert at the University of Costa Rica who first noticed the bizarre phenomenon, says the wasp larva manipulates a particular subroutine in the spider's web-building program. The subroutine, an early part of constructing the web's frame, usually has five steps. Under the larva's direction, the spider just performs the first two steps over and over again.
This reprogramming of the spider's routine is presumably achieved by some chemical the wasp larva injects into its host. Dr. Eberhard has found that if he removes the larva from the spider on the final evening, the spider will build a platform-style web that night and the following night, but will revert to making its usual orb thereafter, as if recovering from some strong drug.
He has no idea what the chemical might be, but hopes first to identify what gland in the larva may be secreting it. The wasp's behavior is described by Dr. Eberhard in a report in last week's Nature.
Dr. Jay Rosenheim, an expert on parasitic wasps at the University of California at Davis, said many parasites were known to shape their host's behavior in various ways. "But what is really amazing and wonderful about this example is that the host's behavior is manipulated in such a detailed way," he said.
As another example he cited the case of a wasp, Cotesia glomerata, that parasitizes the large white butterfly. When the larvae emerge from the caterpillar, they spin their cocoons right next to it, and the stricken insect then stands guard by weaving a web over the cocoons and threatening attackers, behavior that is presumably induced by the larvae.
The Costa Rican wasp discovered by Dr. Eberhard preys on an orb-weaver called Plesiometa argyra. The wasp, a new species, is awaiting a name from Dr. Ian Gauld, a wasp taxonomist at the Natural History Museum in London.
Dr. Gauld said this was the first time he had seen a wasp with the ability to manipulate its host's behavior in such a way, and that nothing was known about how the manipulation was accomplished.
"I think biology is one of the last great frontiers," he said. "We have got no idea about what there is on earth with us, let alone what it is doing or how it does it."