Posted by: lensweb | October 28, 2016

Where have all the butterflies gone?

I am very suspicious of flies that lurk. They are up to no good. When I saw a large fly sunbathing on top of wild carrot seeds by the pond at Forbes Hole I thought, Sturmia bella!

Sturmia bella, is a species of tachinid fly which is a parasitoid of small tortoiseshell and peacock butterflies and was recorded for the first time in Britain in 1998. It lays its microscopic eggs on patches of nettles where small tortoiseshell caterpillars feed. These eat the fly’s eggs which become larvae feeding inside the caterpillar, bursting out of their bodies just when the butterfly is beginning it’s transformation into a butterfly inside the chrysalis.

The small tortoiseshell is the labrador of the butterfly world, cheerful and content to live close to humans. It’s caterpillars devour ubiquitous nettles. As an adult butterfly, it feasts on the nectar of garden flowers and hibernates in garden sheds, flapping against windows to be let out in spring. Thanks to climate change, it has spread north and can be seen in remote parts of Scotland, but this ‘common’ garden butterfly  has been in short supply this year. Numbers have officially declined by 73% since scientific monitoring began in the 1970s.


Is Sturmia bella wiping out our butterflies? According to research by Dr Owen Lewis, an ecologist at Oxford University who is studying the impact of the fly. In 2011 Sturmia bella was recorded as far north as Lincolnshire the small tortoiseshell has declined significantly to the south of this latitude, but not to the north.

Sturmia bella was present in 26% and 15% of the larval groups of small tortoiseshell and peacock butterfly, respectively, and now kills more individuals of small tortoiseshell than any native parasitoid. Survival was 25–48% lower in batches of larvae where S. bella was present, the peacock butterfly being relatively unaffected, indicating that the fly causes host mortality in addition to that caused by native parasitoids.

As with many declining species, there is seldom just one cause and Sturmia bella is not now thought to be a major cause of butterfly population decline. In any case, after investigation using i-spot, it seems Linnaemya not Sturmia bella was lurking at Forbes. Another parasitoid of moths and butterflies. At the end of the day it seems that research may find that it is the foodplant that is the problem for butterflies,  nettles are now often  too dry and tough for caterpillars to eat.


Marion Bryce 20 October 2016


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