Sunday, March 16, 2003		Nature

This wasp is an ungrateful guest, indeed!
Nutan Shukla

ICHNEUMON fly, despite being a solitary creature, is one of the most successful wasps that parasitises juicy caterpillars for laying her eggs. It is a tiny insect with a delicate hovering flight. The tails of the females have a long proboscis-like projection, which is, in fact, a needle-sharp ovipositor or egg-tube, used to deposit eggs inside the body of caterpillars.

The female lays only one egg in each caterpillar. She is able to make out by smell whether the potential host has already been parasitised or not. If satisfied, she injects her fertilised egg deep in the caterpillar’s tissues in such a way that the egg gets the ideal conditions to hatch in a protected environment — inside the caterpillar. After coming out of the egg the young larva starts feeding on the living tissues of its host, which later on kills the caterpillar, but not until it is mature and ready to leave its host.

Apart from finding precisely the right environment for its developing young to grow in, there is nothing particularly social about the ichneumon fly’s behaviour.

The larvae of rhyssa, member of the family of ichneumon wasps, are parasites that feed on larvae and pupae of wood wasps. This does not appear to be easy for the wood wasp larvae lead a highly sheltered and secure life by staying in tunnels, one to two inches deep beneath the tree bark of fir trees and feeding on wood. Despite this they are parasitised by rhyssa that has evolved ways and means to overcome the problem.

It is compulsory for the female rhyssa to search for the host larvae otherwise she will not be able lay her eggs. Naturally she starts probing the trees likely to have the larvae by tapping their barks with its antennae which are so sensitive that they can pick up any vibration caused by larva inside the tree. Once she is sure of the food’s presence it circles round, tapping the bark rapidly and repeatedly. This is perhaps either to reassure itself about the presence of the larva or to force it to freeze with fear. Then she lifts her abdomen high in the air, preparing to insert her ovipositor — tube through which eggs are laid.

This superbly adapted egg-tube is no thicker than horsehair, but is almost as long as the rest of the female’s body. When not in use the tube is hidden safe and well protected by two stout sheaths, but whenever the insect prepares to use it the sheaths are withdrawn and the tube is ready to use. Once the wasp is sure about the presence of food inside the wood it drills its ovipositor into the wood to seek out the wood-wasp larva. The hole that is drilled is not always directly above the larva, but once it is touched by the ovipositor’s tip, a single egg is delivered, to be placed next to, or on top of, the doomed host.

When the egg hatches rhyssa larva soon starts feeding on the wood-wasp larva and then pupates, that is it spins a cocoon in which to develop into an adult. It doesn’t take very long for new rhyssa wasp to emerge out on the surface of the fir tree trunk fully prepared to plague a new generation of wood wasps.