A caterpillar, eaten hollow, stays to guard the wasp's young

After parasitic wasp larvae chew out of a caterpillar and pupate beside it, the dying caterpillar thrashes at anything that approaches them. Caterpillars that do this lose fewer of the pupae to predators.

Some parasitoid wasps lay many eggs in one caterpillar. The larvae feed inside, carefully, on tissues the host can spare until the end, and then they chew out and spin cocoons on the leaf. In the species Grosman and colleagues studied, the caterpillar does not leave and does not die at once. It stays on the cocoons and strikes at predators — bugs, in the experiment — that come to feed. When the researchers removed the bodyguard, more pupae were eaten. When they left it, the pupae survived at a higher rate. The behaviour is induced. An unparasitised caterpillar does not stand watch over a cluster of cocoons.

How the wasp writes that last instruction is only partly known. A virus the wasp injects with the eggs, and the way the larvae feed, are both suspects. The benefit is the part that was measured, in one system, with one predator. Other parasitoids kill the host and leave. This one keeps a ruin on duty. The ruin is not choosing. The choice was made by something that has already moved out.