Controlling a low-flying Dorset pest

Blandford Forum, a historic town on the river Stour in Dorset, has some lovely Georgian architecture, a connection with Thomas Hardy and a local independent brewery. It may sound idyllic but one of the locally brewed ales, called until recently the Blandford Fly (but now the Blandford Flyer), has overtones of ginger for the reason — albeit folkloric — that zingibain, a proteolytic enzyme in ginger, can help reduce the fever and swelling that accompanies the bite of a notorious local pest.

The Blandford fly (Simulium posticatum) is a biting blackfly distributed in an arc running from East Anglia through Oxfordshire into Dorset. It was so named when it came to the attention of public health officials near Blandford Forum 50 years ago. Unfortunately S posticatum seems to be spreading. NHS Herefordshire recorded 500 cases involving the small black insect between April and June in 2011 and says it has received a similar number of calls this year.

The adult females, which hatch in their millions in early summer, lay their eggs in steep banks just above the river water level. But they first require a blood meal. Although they also bite many other animals, humans seem to be particularly favoured.

The bites tend to appear on the lower limbs, since the insects fly only just above ground level. The effects can range from small blisters to large intensely painful haemorrhagic lesions. Severe swelling and secondary infection can also occur.

Control of the fly is difficult but has been fairly successful in Dorset where aggressive treatment of rivers dramatically reduced the incidence of bites.

In the late 1980s, a biological insecticide, Bacillus thuringiensis israelensis (Bti), was introduced. Its spores carry a toxin that destroys the gut wall of blackflies and midges. Its employment in Dorset is considered one of the best examples of the use of an eco-friendly biological pesticide anywhere in the world.