Ridecamp: [RC] mane and tail rubbing

My daughter's 12 year old Morgan marehas recently started rubbing out parts of her mane and tail.  We've had her for 2 years, and this is a new problem over the past 6 weeks or so.  No changes in food, herd or lefestyle.  The horses are fed in their stalls, but otherwise are out to graze all day and night.  She does seem to be worse if she stays in her stall any longer than absolutely necessary to finish eating.  I've tried everything I can think of to get her to quit.  The vet checked her and didn't see any medical problems, so I'm thinking it's from bugs, but no bug spray I've tried has made any difference.  As of today, she's rubbed out at least half of her mane, and the whole top half of her tail.  
My daughter is 16, and this disfigurement of her beautiful mare is wreaking havoc with my daughter.  Anyone have suggestions?  It won't cool off here anytime soon, as we live in central Florida, so waiting for frost is going to be tough.  Please help.  Thanks

Maris Ramsay