It was a hot day in early June. I was in the paddock behind the barn at Mary’s farm. She had asked me to check “Doc”, a Quarter Horse with an allergy problem. She was holding a lead that was snapped onto Doc’s halter. Mary has competed Doc for years. He’s a seasoned horse and is always cooperative. That’s why we were both surprised when he suddenly got fidgety, and we saw the whites of his eyes. He started stomping with his left fore and throwing his head. Finally Mary and I heard a loud buzzing and realized what he was upset about. Doc was the target of a bot fly. These insects are extremely aggressive. The females live only one week and have just one goal. Lay as many eggs as possible on the hair of any horse in the area. They are very determined and very fast.
The bot fly darted in, laid a few eggs on a front leg, flew away a short distance and then attacked from a different direction, her wings making that buzzing noise. These “super flies” may lay up to a hundred eggs on one horse. The fact is that she will lay those eggs unless you kill her. You have to slap her hard with an open hand against the horse the second she lands. Hesitate and, too late, she’s off again. It’s next to impossible to catch her in the air. Horses hate the noise of bot flies and their unpredictable actions. Equines at pasture will often take off running, totally out of control when the bot fly is at work.
The female bot fly is unique to the insect world. She looks like a scrawny honey bee. Her body is bent almost double in the middle so that her egg laying hind end is tucked under and faces her head. After she lays all her eggs, she dies. The male bot fly’s life is even shorter. His life is over just after mating with her. Neither the male nor female has mouth parts. They can’t eat. Their only function as flies is to reproduce. The entire life cycle of a horse bot is one year, and at least 10 months of that time is spent not in the air as an insect, tormenting horses, but actually inside your horse.
Each egg that the fly lays is cemented to a single hair. They look like tiny yellow grains of rice. The eggs will stay glued to the horse’s hair coat until you either physically remove them, or they hatch. From egg deposit to hatch is about 5 days. The hatch begins when the horse brings his mouth down to his leg or side where the eggs are. It is thought that the carbon dioxide from the horse’s breath stimulates the hatching. The tiny released larvae get into the horse’s mouth and burrow into the tongue or the gums. They stay there for a month. Then they burrow out, pass down to the stomach, and attach to its lining with their sharp mouth parts. In the stomach they grow to look like a grub that you might find in the woods under a log. They are fat, ugly, and about an inch long. Their stay in the stomach is like you on a 10 month Caribbean cruise. You have access to unlimited food and are a world away from the cold winter.
Somehow those bots, deep in the horse and far away from daylight or weather, know when the world gets warm. They release their hold and pass through the GI tract to go out with the manure. Once on the ground they burrow beneath the surface and stay there for a month. Just like a butterfly coming out of a cocoon they metamorphose into the next stage, which is the flying insect, the bot fly. Males and females find each other and within hours the female with her fertilized eggs flies off to find a horse to lay them on, and the yearly cycle starts again.
I don’t see as many bot flies as I used to. I think it’s because ivermectin and moxidectin (Quest) at the proper dose kills all the bots in the stomach, and most horses receive one of those wormers at least once a year. That doesn’t mean that you won’t ever see the bot fly or her eggs, it’s just that right now they aren’t as common as they were 25 years ago. Perhaps they will develop a resistance to those medications over time. For sure they will always be around. If you buy a horse with an uncertain history, and it’s winter, it would be prudent to worm with one of the wormers mentioned. Incidentally, because the bots attached to the stomach don’t lay eggs, there is no way to check for their presence in a fecal exam.
So, what are the implications for your horse? Actually, unless there are huge numbers in the stomach, horses tolerate them OK. The bot is one of those parasites that have it pretty well figured out. Attach to the host and get nutrition, but don’t drain the horse too much, or you both lose. There are reports of bots penetrating the stomach and causing peritonitis, but it’s rare. Given enough bots in the stomach you can expect some general unthriftiness. There are definitely problems caused by the super fly female herself. Horses get spooked and sometimes crash into fences or do something equally silly to get away from them. If you are riding or driving a horse and hear that buzz, hang on! I have a theory about this. Horse Flies (different species) also buzz, and they bite. They are large, triangle shaped, and have big eyes. The females bite to get a blood meal from any mammal (like us at the beach}. The blood is necessary for the development of her eggs. Perhaps horses get frantic around bot flies mistaking them for horse flies?
The best time to get rid of bots in the stomach is in the fall, one month after the first killing frost. By that time all of the bots have arrived and are implanted there. Personally I like getting rid of these parasites before they get to the mouth and stomach. If you see the tiny eggs attached to a horse’s legs or flanks, get them off. They won’t brush off because of the glue secreted with the eggs that binds them to the hair. Some people use a bot knife or a pumice stone, each made for that purpose. For me a hoof knife works well. One of my early mentors always had a piece of glass from a broken bottle handy to scrape them off. Every egg that you remove will be one less bot inside your horse.
When summer starts the bot flies are out, and sometimes it’s your horse’s behavior that will alert you. If you can’t kill the fly as she pesters your horse, watch for those eggs on the legs or trunk. They are found in clusters, and usually on the front legs. Occasionally you will see them on a shoulder or flank, but never very far back. There is one species of bot that lays its eggs under the chin, and another that prefers the throat. Both of these are unusual in New England. Scrape off the eggs wherever you find them, and you have stopped the life cycle of the bot before it ever gets going.