Additional to the standard grooming process you will need to make sure your horse is not harassed by bugs all day also.  In hotter climates and seasons flies and mosquitoes etc, can mean your horse is literally tormented day and night without the ability to escape.  

It is up to horse owners to manage this.  Horses love fly masks.  If you have a horse with a pink nose it pays to get one with a flap to cover the nose (make sure it doesn't cover the nostrils).  This will keep your horses nose from becoming badly sunburnt.

Flies harass horses biting the soft skin around their eyes to the point it becomes raw open flesh.  Fly masks in Australia are necessary during Spring and Summer at least.  Fly masks that are rigid and thick are best so they sit of their own accord off the eye.

Horses definitely prefer having mesh fly combo's on also, even on sweltering hot days they will beg you to put them back on if you remove them because of the flies.

You can also buy fly boots and leave these on during the day.

Shelters and stables can be sprayed with a good repellent / insecticide for horses.  Don't spray around water, feed, or hay or grass they pick at though. 

I also spray horses legs daily with repellent / insecticide for horses.  Most horses you will have to halter for this and they will hate it.  But be happier for it daily ;)  If it's easier you can spray a cloth and wipe the legs down with it instead, it helps to establish language they understand regularly like saying 'I'll get the flies' while wiping them off their eyes with your hand often, then when applying the repellent say the same thing so they start to understand it's relationship to flies and be more happy about it.  I also like to spray the combo on the outside for extra protection.

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