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Blandford fly treatment on the River Stour completed for 2024

The Blandford fly, Simulium posticatum, is a species of black fly found in Europe and is prevalent in the Stour valley in Dorset. It spends its larval stage filter feeding in the weedbeds of slow flowing rivers (attaching itself to ranunculus, in particular) and when the fly emerges, the female seeks a blood meal before mating. It usually bites the lower legs causing pain, itching and swelling.

Larval stage of the Blandford fly

Blandford fly bites can be very serious, and often result in medical treatment being required. A control programme has, therefore, been in place for the past thirty years to keep populations under control in the River Stour.

Blandford fly larvae filter feed whilst attaching themselves to weed (particularly ranunculus) in slow flowing rivers

Control is effected by targeting the filter feeding larval stage with a specific larvicide Vectobac AS, which is an aqueous solution of Bacillus thuringiensis israelensis (Bti) - a group of bacteria used as biological control agents for larval stages of certain dipterans. Bti produces toxins which are effective in killing various species of mosquitoes, fungus gnats, and blackflies, while having almost no effect on other organisms.

BUG operative spraying an aqueous solution of Bti at one of the sites on the River Stour

BUG has been commissioned by Dorset County Council to undertake the annual control programme, which involves treating specific areas of river with extensive ranunculus beds. Surveys are timed for a critical two week period of development where only larvae of Simulium posticatum are targeted.

BUG’s annual control programme is effective at killing >95% of Simulium posticatum larvae, greatly reducing the number of bites during the summer months and, subsequently, alleviating pressure on the health service.


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