Plant Repellents for Biting Insects

I live in Western North Carolina and I work with horses. We - the horses and I - are quite distressed during the Spring and Summer with biting insects. I wish to know about plants that might be grown near barns or arenas to repel biting insects.

There are no non-controversial solutions to your problem. Any of the lemon scented geraniums are supposed to repel insects, but they do not stand up well to experimental testing for that repelling property. It might be because the repelling oils are only released when the plants are disturbed. There are other lemon scented plants, but their scent oils are even less readily released.
Using green larvicides to get rid of mosquitoes and black flies in the larval form might be a better option. They all need to be reapplied to all bodies of still (for mosquitoes) or flowing (for black flies) water repeatedly over the course of the summer and the sooner in the Spring you start the less chance the pests get to multiply. I have used the bacterial disease Bacillus thuringensis variety israelensis with good results. It is available under various trade names in a dormant form. It needs to be applied once a week.
To combat biting flies: (they look like house flies but have their wings spread out just a bit further)- they breed in manure and only careful manure free conditions and composting of manure in closed containers keeps them in check.