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The Blandford Fly

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The Blandford Fly (Simulium posticatum) is a small (2-3mm) biting fly belonging to the family Simuliidae or 'blackflies'. It is widely distributed throughout Europe and European Russia, while in England it is patchily distributed in an arc running from East Anglia through Oxfordshire into Dorset. It is also found in small numbers in the New Forest.

Simulium is the most diverse blackfly genus, with almost 1,200 species occurring throughout all the zoogeographical regions of the world. The most important species is S. damnosum, the vector of 'river blindness' (onchocerciasis), which is caused by a parasitic worm, and occurs mainly in tropical Africa.

S. posticatum, although being a 'nuisance species' with a bite which can be painful and distressing, is not known to transmit any diseases. In the United Kingdom, it only reaches numbers sufficiently large to cause a public health problem near the town of Blandford Forum in Dorset; hence its sobriquet, 'The Blandford Fly'.

The Blandford Fly first came to the attention of public health officials in the 1960s and reached a peak in the late 1960s and early 1970s. In a four-week period during the spring of 1972, some 600 people were estimated to have visited their doctors in Blandford to be treated for insect bites.


The juvenile stages of S. posticatum have a requirement for running water and normally inhabit lake outfalls, so the adult female lays its eggs about 45cm above the water level in steep banks in June or July when the weather is warm. The eggs lie dormant until late winter when the water levels have risen. The eggs in the river bank become moistened and the larvae burst forth in their millions. The newly hatched larvae are only about 0.5mm long.

The fly larvae are first observed in the river in March and pupate after about five weeks. Streams and rivers can support vast numbers of blackfly larvae (perhaps 10,000 per square metre of river bed) which filter and clear the water. They can only feed in rapid flowing water and are unable to survive in the still or slow moving waters of lakes and garden ponds.

The River Stour at Blandford is effectively a series of lakes separated by weirs and it does have steep banks. In addition, it contains a high density of weed and high levels of suspended solids, organic matter and algae, all of which seem to suit the requirements of the fly. Adults hatch in May and are on the wing throughout May, June and early July. The flies range up to 1km from water and fly close (less than 0.5m) to the ground.

The females require a blood meal before they can lay their eggs and, while they can and do bite all domestic and wild animals, humans and dogs seem to be particularly favoured. Having had a blood meal the female must wait for its 200-300 eggs to mature before returning to the river to lay them in the desiccation cracks of steep, shady river banks, well above the summer water levels. The eggs adhere to soil particles in moist, humid conditions.


Blandford Fly bites tend to be confined to the lower limbs and women tend to be bitten more frequently than men, possibly because skirts leave the lower limbs uncovered. The effects of the bites on humans range from small blisters to large (up to 22cm diameter) haemorrhagic lesions, which can produce intensely painful stabbing sensations. The saliva of the fly, which passes into the wound, often causes severe irritation, pain, swelling and blistering. Secondary infection of the lesions can also occur.

Prevention of Bites

In order to reduce the chances of being bitten it is recommended to wear trousers, to apply insect repellent and to avoid open areas of garden, parkland or river bank particularly in fine weather in the middle of the day. Unlike mosquitoes, the Blandford fly tends to bite least in the early morning and late evening.

Control of the Fly

Control of the fly is difficult and early attempts were limited to habitat modification by cutting or dragging out river weed. However, this was both labour intensive and ineffective. In the late 1980s, scientists from the Centre for Ecology and Hydrology (formerly the Freshwater Biological Association) suggested using a biological insecticide, Bacillus thuringiensis israelensis (or Bti for short). Bti is a naturally occurring bacterium which produces spores which, when they are eaten by blackfly larvae, rapidly kill the insects. The spores carry a toxin which destroys the gut walls of Simuliidae (blackflies) and Chironomidae (midges).

It was, of course, necessary to assess the environmental impact of such a treatment and in 1989 permission was obtained for the trial treatment of a side-channel of the River Stour at Blandford. Detailed monitoring of this showed that Bti successfully killed larvae of the Blandford Fly while having no other effect on other river life. By careful application of the treatment, it was possible to target only the larvae of S. posticatum, leaving even the larvae of other blackfly species (which are vital to the ecology of the river) unharmed.

In 1991 the first large scale treatment of the river was carried out and annual applications of Bti have continued ever since; resulting in the destruction of 80-90% of the Blandford Fly larvae and a corresponding reduction in the numbers of people bitten. Indeed, it is reported that the number of people bitten has dropped to less than one hundredth of those affected in 1989.

It is considered that this use of Bacillus thuringiensis israelensis in Blandford is probably the best example of the use of an eco-friendly biological pesticide, anywhere in the world.

Blandford Fly Trivia

  • According to the Badger Brewery, based in Blandford St Mary, the local folklore is that zingibain, a proteolytic1enzyme present in ginger, can help reduce the fever and swelling that accompanies the bite of the Blandford Fly2. This has inspired them to incorporate it into an ale called Blandford Fly.

  • The Blandford Fly appeared as an eco-terrorist weapon in a little-known mystery novel by John Sherwood called Creeping Jenny (1993). The plotters were planning on releasing swarms of the fly during the Queen's visit to the Chelsea Flower Show.

1Proteolysis: the breakdown of proteins or peptides into amino acids by the action of enzymes.2Research has shown that proteolytic enzymes such as zingibain, bromelain (present in pineapple) and papain (present in papaya) have anti-inflammatory properties.

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