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There are many species of midge, a tiny 1.4mm wingspan fly, which inhabit the UK. The female midge, is the cause of the problem. "After fertilisation the females require a meal of blood for the eggs to develop fully." says Alan Watson Featherstone in Trees for Life.

Living in a wet, humid environment in undergrowth, long grass and trees, or near water, the female midge can only fly relatively short distances in still or calm air. The midge hates wind; also hot dry conditions.

With a crepuscular habit, preferring dusk, dawn, and overcast days - hating bright sunshine - horses are at greatest risk of being bitten at these times.

For more about the Highland Biting Midge read this article in Trees for Life.

Dr Doug Wilson a vet from Bristol describes how midges bite horses and set off the allergic reaction:

"After alighting on a horse, midges crawl down the hair shafts to the skin surface. Their mouth parts are too short to probe for a blood vessel like their larger cousins, the mosquitoes, so they have to chew their way through the tough outer layers of skin.

To assist their efforts, they secrete saliva containing a mixture of enzymes that digest and soften the skin tissue as well as agents to encourage extra blood to flow to the site of the bite and several factors that will prevent the blood clotting. A small pool of blood forms just under the skin surface and is then sucked up by the midges.

The whole process takes about 15-20 minutes and, over the course of an evening, a horse may be bitten by hundreds or even thousands of midges, each one injecting a small amount of saliva containing foreign proteins into the horse."

To read the rest of the Dr Doug Wilson's article please go here.

Midges that trouble horses are split into two groups (a) Dorsal Feeders and (b) Ventral Feeders.
Dorsal feeders attack the dock, rump, withers, mane, poll and ears whereas ventral feeders - less common - go for the face, chest and belly of the horse.

Information supplied by Aberdeen University on midges and humans. Report Monday 15th September 2008 by Professor Jenny Mordue.

"The Gory Facts
·        A swarm of midges can deliver approximately 3,000 bites an hour
·        Researchers have estimated that in an hour, up to 40,000 midges can land on an unprotected person
·        A female midge can detect people from a range of up to 100 metres.  Midges are attracted to the carbon dioxide vapours and other chemicals released from human breath and the skin
·        It is estimated that in some parts of Scotland, one single hectare of land may host up to five biting midges for every man, woman and child in Scotland – that’s 25 million biting midges per hectare

The Rest…
·        Midges have been around in Scotland for some 8,000 years  
·        There are around 35 species of midges in Scotland
·        Only the females bite. It gives them protein and energy to produce their eggs
·        A female will feed on the skin for up to four minutes taking 0.1 microlitres of blood
·        Male’s mouthparts are not strong enough to pierce skin and they feed on liquids such as nectar from flowers.
·        Midges are very small – they only have a wingspan of 1.4mm
·        Midges do not like the wind, low temperature or very dry conditions
·        A certain species of midge cause sweet-itch, a debilitating incurable problem which affects up to one in twenty of the UK’s horses and ponies"

James Logan of Aberdeen University says this about research into an insect repellent for farmyard midges "PhD students are currently working on farmyard midges in the UK and a large project in India. We are hoping these projects will allow us to develop a useful product which can be used to control midges around livestock".

Read more about midges behaviour and insect repellents here. Also please refer to Sweet Itch News & Research