When I was a young boy I used to rear any caterpillar that my mother found while cleaning vegetables. They were mostly caterpillars of the small and large white butterflies which live on the leaves of cabbages, cauliflowers and related plants. Many of my first attempts ended in a disaster. Just before changing into pupae the caterpillars shrivelled and died. Next to them there used to be several light yellow cocoons the size and shape of a small rice grain. I soon discovered that these were the pupae of the ichneumon fly, a parasitic insect which I later found out was not a fly but a wasp. This species has a very important role as it keeps the numbers of some species of butterfly down. Ichneumon flies are very small insects belonging to a large family of parasites of varying shapes and colour most of which resemble bees and wasps. Several species are found in the Maltese islands but the exact number is not yet known.

The female wasp lays its eggs inside a small caterpillar. The eggs soon hatch and the maggots live inside the caterpillar feeding on its blood without damaging the internal organs as this would kill the caterpillar on which they depend. After about three weeks the caterpillar becomes fully grown but just before it changes into a pupa the maggots eat their way out of its skin spin a cocoon around themselves and become pupae.

Up to the end of World War II, war farmers relied on these parasites to control cabbage white butterflies. The presence of these parasites in the field was closely monitored. Soon after the war new chemicals such as DDT started to be used to control pests.

These chemicals were initially very successful and managed to control the pests but after a short while the pests returned with greater numbers than ever before because the insecticides were also killing the insects that had been controlling them.

portelli.paul@gmail.com

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