30 Dec 2005

the ichneumon fly

Bio-warfare - USDA fights invasive fire ants with flies

Ichneumonidae wasps (also called “assassinator wasps”, “ichneumon fly” ) are natural wonders of parasitism and biological population control. The wasps actively hunt for their hosts, sting them into a paralytic stupor and lay their eggs on (or inside) their body. These wasps typically have very long ovipositors and some species even use them to bore deep into wood, seeking out host pupae to inject their eggs.

Larvae hatching from the eggs feed on the host and grow inside them. It appears that the larvae practice selective feeding - they feed on the host’s fatty tissues and muscles earlier than devouring vital organs. The host provides the larvae fresh food while it is being eaten alive from the inside. Studies have reported butterfly chrysalis twitching in agony as they are devoured by the pupae inside. Observation of such instances created great debates in the medieval Church on the subject of the Benevolent Creator - how could God possibly create an animal which so cruelly ate alive another of His own creation ?

i first observed these wasps, years before i read about them, while eating a post-lunch orange on our rooftop one balmy winter afternoon. There was this insect jumping (more like floating) funnily across the roof. Walking nearer, i found it to be a cricket. Now, crickets either crawl or jump, but this one was sort of skipping along! Inspecting closer, i found that a slender wasp was actually carrying the cricket between its legs. The cricket, seemingly dead, was about three-times the wasp’s size and weight. So, it was only able to fly a distance of about a foot at a time. And i had never heard of wasps eating crickets !

The wasp lugged the cricket across the roof to a crack in the wall. Deposited it at the entrance of the crack and dove inside. It soon emerged and with dexterous handling, dragged the cricket into the crack. As i sat there watching, it soon emerged, sans cricket and flew away. Maybe, to hunt for the next hapless prey.

A few years later we saw a Walt Disney movie on hunting animals. And here was this wonderful footage of a duel to death - between a large, hairy, tarantula and a nimble, slender, wasp. The tarantula charges and the retreating wasp falls to the ground on its back. As the tarantula crawls over the wasp to deliver its deadly bite, it is stung in its exposed underbelly by the wasp. The wasp proceeds to drag the huge tarantula to a hole in the ground, lays a pearly white egg on the spider’s abdomen and covers it up with loose sand.

While i have, since then, observed ichneumon wasps on numerous occasions during field trips, one event stand out in my memory. We have a madhabilata (Hiptage benghalensis ?) plant next to the verandah in our compound in Calcutta.
It is a hardy, vigorous climber with clusters of lightly-fragrant, slender-stalked pink-white flowers, which typically bloom at night. Walking out to the verandah one summer night, i was taken aback by a wave of little popping sounds coming from the garden. Walking out to the garden torch in hand, i was astounded to find the madhabilata plant covered by a cloud of slender, hairy, caterpillars - bright red, white and black. The popping sounds came from the caterpillars shaved off leaves from the climber. The noises continued into the night. 

Next morning the caterpillars had vanished almost as magically as they had arrived. Only the madhabilata, now stripped bare of all greenery, bore mute testimony to the invasion overnight.

A few days later, the scrub below the madhabilata was splattered with ghostly exo-skeletons - scaly cases in the shape of caterpillars, complete with the hairs but hollow inside. Some ichneumonidae at work again ?

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