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Although we may not like them, nettles are the foodplant of the small tortoiseshell caterpillars.
The butterfly’s wandering habit aids its distribution, but this is more governed by its choice of food plants. These are the Common Nettle (Urtica dioica) and the Small Nettle (Urtica urens) – which explains the second part of the butterfly’s Latin name - ‘urticae’. Nettles grow in all kinds of situations, even in intensively farmed landscapes, and this is the main reason for the healthy population of this butterfly. There are many less fortunate species that are not so common or widespread because they require specialist habitats and food plants, like the Marsh Fritillary or the Silver-studded Blue. Furthermore, as well as the hairs on nettles protecting the plants from grazing animals, enrichment of the soil by fertilizers encourages growth and may benefit the Small Tortoiseshell, although it is not good for plants and wildlife in general.
Our butterfly entered the house in late August to hibernate. If it has survived, awakening time, depending on the weather, would be about the end of March. Once on the wing, it would seek to mate and, if successful, lay eggs if it were a female, but this was impossible to know, as the sexes look the same. You have to look hard on the undersides of the nettle leaves to find the tiny, glass-green eggs. They are laid in clusters of a few score on the sunny, south-facing edge of a nettle patch. Smaller, younger plants are preferred. So many eggs are laid because predation rates are high, particularly by bugs and beetle larvae, as is the infestation of parasites. They have to survive for twelve days before they hatch.
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