Friday, 4 May 2007

Wasp regulates butterfly population

One of the joys delivered by the arrival of spring is the opportunity to sit out in the garden and watch wildlife closer to home. One of the creatures to make an early appearance in the garden this year has been the holly blue butterfly. Over recent days several individuals have been seen together flitting around the holly tree and over the old boundary wall, covered in a mass of ancient ivy. This small butterfly is the only blue on the wing at this time of the year and it is also the one most likely to be encountered in gardens. Pastel blue in colour, it can be separated from the common blue (which may visit more rural gardens later in the year) by the lack of any orange markings on the underside of its wings. The holly blue also differs in its behaviour; while other blues fly low over the ground, the holly blue flies high around bushes and shrubs.

In spring, the butterfly lays its eggs on holly, the caterpillars timing their emergence to coincide with the availability of tender new buds. Later in the year a second generation (and in some years also a third) will be reared on ivy. The number of generations each year is determined by temperature; long, warm summers will allow a third brood in the south of the country but only a second brood in the north of England, where a single brood is more typical.

The larvae, feeding on flower buds, berries and terminal leaves can be found with a little bit of effort, as can signs of the damage caused by their feeding behaviour. At this stage, they are vulnerable to a parasitic wasp called Listrodomus nycthemerus. This wasp almost exclusively attacks the holly blue and, in some years, can inflict substantial damage on the population. The female wasp deposits an egg into the holly blue caterpillar, without killing it. The egg remains within the body, unhatched, until the butterfly enters its pupal stage. At this point, the wasp egg hatches and the emerging larva proceeds to eat the entire contents of the pupal case and, by doing so, kills the butterfly, before completing its own life cycle. The occurrence of such parasitization is thought to be behind the strong cyclical fluctuations seen in holly blue populations. As the butterfly becomes more abundant, so wasp numbers build up to the extent that they eventually cause the butterfly to decline in abundance; a wasp decline follows and the cycle starts over again. These fluctuations, and the long-term increase in this species, are monitored by Butterfly Conservation. For more information on their work, contact the Norfolk Branch through Hilary Beynon 01493-740499.

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