Although the weather has become quite autumnal in the last week or so, the mosquitoes which have been flying around my house in the evenings in unusually high numbers in recent weeks (fortunately without biting me much) have not yet disappeared, and have reminded me of an interesting article1 that I came across earlier in the year about another biting insect. The blackflies (Simuliidae) are a significant nuisance in many parts of the world — my aunt who lives outside Montreal in Canada has friends in the city who do not visit her during the blackfly season — and in some places they are an even more serious problem. Adverse reactions to large numbers of bites have killed many livestock, for example around the Mississippi and Danube rivers prior to changes in river management that eliminated the flies’ breeding sites2. The most serious effect of blackflies on human health is the transmission of river blindness (onchocerciasis) caused by the nematode worm Onchocerca volvulus in parts of Africa and (to a lesser extent) South America.
In Britain, blackflies are not usually a major problem, and there is just one species that causes significant trouble to people, namely the Blandford Fly, Simulium posticatum. This is moderately well known for biting people in parts of Dorset, and takes its common name from the town of Blandford Forum, where the local brewery has even named a beer after it. The article that prompted me to write this piece indicated, to my surprise, that it has been recorded over much of southern and eastern England, including a second concentration just to the north of Oxford, not many miles from the CABI offices; apparently it is known locally as the ‘Stonesfield Stinger’ or the ‘Woodstock Fly’ after a village and a town in the area. I didn’t visit this area during the blackfly season (late April to mid June) this year; if I do next year I will have to decide between covering myself with insect repellent to avoid the unpleasant consequences of a bite, which can include severe irritation, pain, swelling and blistering3, and leaving an easily visible area of skin uncovered in the hope of seeing this insect that I have, in writing this, read so much about.
My search of CAB Abstracts, the CAB Abstracts Archive and one or two other sources for information about S. posticatum, and other Simulium species in Britain, revealed a number of other interesting facts that I hadn’t previously known. In 1989 there was a case of meningitis suspected (but not proven) to have been linked to a bite from this species4; the biological pesticide Bacillus thuringiensis var. israelensis is now used in Dorset to control the larvae of the fly3,5,6 (one of a comparatively small number of instances where this widely used biopesticide has been used in the UK) and has reduced human biting rates by 99%3; the great majority of S. posticatum females in fact bite species other than humans7 (just as well for us); onchocerciasis, transmitted by different species of blackfly, is found in cattle in Britain8, but does not appear to cause any significant symptoms9; and in the early twentieth century, before it was found to be a nutritional deficiency disease, pellagra was suspected to be transmitted by blackfly bites10. Perhaps I or one of my colleagues might investigate one of these further for a future blog entry.
1: Crosskey, E. W. et al.: On the geography of Simulium posticatum in Britain. British Simuliid Group Bulletin (2007) no. 27, pp. 5-14.
2: Kettle, D. S.: Medical and Veterinary Entomology. CABI, 1995.
3: Ladle, M.: The Blandford Fly. Published on North Dorset District Council website in 2004.
4: Hamlet, N.: Acute meningism and the Blandford Fly. Journal of Infection (1989) vol. 19 no. 2, pp. 187-188. doi:10.1016/S0163-4453(89)92114-2
5: Welton, J. S. and Ladle, M.: The experimental treatment of the blackfly, Simulium posticatum in the Dorset Stour using the biologically produced insecticide Bacillus thuringiensis var. israelensis. Journal of Applied Ecology (1993) vol. 30 no. 4, pp. 772-782.
6: Ladle, M. and Welton, S.: An historical perspective of the ‘Blandford fly’ (Simulium posticatum Meigen) problem and attempted control of the pest species using Bacillus thuringiensis var. israelensis. Integrated Pest Management Reviews (1996) vol. 1 no. 2, pp. 103-110. doi: 10.1007/BF00142829
7: McCrae, A. W. R. and Hill, N.: Host selection by the Blandford Fly (Simulium posticatum Meigen), with blood-meal identifications. British Simuliid Group Bulletin (1994) no. 3, pp. 23-27.
8: McCall, P. J. and Trees, A. J.: Onchocerciasis in British cattle: a study of the transmission of Onchocerca sp. in North Wales. Journal of Helminthology (1993) vol. 67 no. 2, pp. 123-135.
9: Eichler, D. A. and Nelson, G. S.: Studies on Onchocerca gutturosa (Neumann, 1910) and its development in Simulium ornatum (Meigen, 1818). I. Observations on O. gutturosa in cattle in south-east England. Journal of Helminthology (1971) vol. 45 no. 2/3, pp. 245-258.
10: Sambon, L. W.: The natural history of pellagra. With an account of two new cases in England. British Medical Journal (1913), July 5, pp. 5-12.