and this from another site:
G. intestinalis – G. nasalis
G. intestinalis lays up to 1,000 pale-yellow eggs on the horse's forelegs and shoulders. Moisture and friction from the horse's licking itself cause the eggs to hatch in about seven days. After hatching, G. intestinalis larvae are licked into the mouth.
G. nasalis lays about 500 yellow eggs around the chin and throat of the horse. These eggs are not dependent on the horse's licking them to hatch. G. nasalis burrows under the skin to the mouth, wandering through it for about a month before migrating to the stomach for overwintering. Then the cycle begins again.
Signs of Bot Infestation
Horses that show no outward signs of illness can be severely infested, giving no clue to damage occurring inside. However, some horses do show signs of infestation, including an inflamed mouth area and stomach irritation. Infestation with bot larvae may cause ulcers in the stomach lining. If the infestation is severe, the opening from the stomach to the intestines may be blocked, which can cause irritation, ulcers and even colic. The burrowing larvae can cause small tears in the skin, which can become infected.
Treatment for Bots
Traditionally, horses are treated for bots in the fall, after a frost that kills the adult flies, and again in the spring, to rid the stomach of all the larvae. In the past, the treatment was worse than the disease, with extremely toxic chemicals given via stomach tube to the horse. Modern anthelmintics like ivermectin are extremely effective and safe in the treatment of bots and have had an impact on lowering the number of bot flies in areas where good anthelmintic treatment is practiced.
I'm wondering if a product like the tri tec would kill the larvae before they get to the horse's mouth and nose... even if it doesn't get the adult fly, I'd be happy knowing the larvae are dying in vast numbers before they ever get beyond my horses' legs.
"We are here to laugh at the odds and live our lives so well that death will tremble to take us."