A boticide is a deworming product that kills bot fly larvae inside a horse’s digestive tract. You’ll see the word printed right on the label of common ivermectin pastes, where it appears alongside “anthelmintic” (the broader term for any anti-worm drug). While “boticide” sounds like a specialized treatment, it refers to the same oral paste dewormers most horse owners already use, specifically highlighting their effectiveness against bot flies.
What Bot Flies Do to Horses
Bot flies (Gasterophilus species) have a life cycle that plays out largely inside your horse. Adult flies lay tiny yellow eggs on a horse’s legs, chest, and jaw during late summer and fall. When the horse licks or nuzzles these areas, the eggs hatch and the larvae enter the mouth, burrowing into the gums and tongue for several weeks before migrating down to the stomach and upper intestine.
Once attached to the stomach lining, bot larvae create ulcers. Research examining the tissue damage found intense scarring beneath each ulcer site in the stomach. In the duodenum (the first section of the small intestine), a second species of bot larva causes even more concerning damage: the intestinal wall thins rather than thickens, with severe loss of the glands in the surrounding tissue. The body does mount a defense that generally prevents the larvae from perforating the gut wall entirely, but a heavy infestation can cause discomfort, poor condition, and digestive problems that persist until the larvae are eliminated.
Active Ingredients That Work as Boticides
Two drugs are commonly used against bot larvae in horses: ivermectin and moxidectin. Both are administered as oral paste or gel from a dial-a-dose syringe.
Ivermectin, dosed at 0.2 mg per kilogram of body weight, has high efficacy against both the oral-stage larvae (in the mouth and gums) and the gastric-stage larvae (attached to the stomach). It’s the more reliable of the two options for bots specifically. Moxidectin, dosed at 0.4 mg per kilogram, also kills gastric-stage larvae but with more variable and generally lower efficacy compared to ivermectin. Moxidectin is often chosen for its broader spectrum against other parasites, but if your primary goal is a bot cleanout, ivermectin is the stronger pick.
When to Give a Boticide
Timing matters more for bot treatment than for most other deworming. The standard recommendation is to administer ivermectin once yearly in late fall or early winter, after the first hard frost. The reasoning is straightforward: a hard frost kills adult bot flies and stops new eggs from being deposited. By waiting until that point, you ensure the larvae already inside your horse are the last generation of the season. Treating then eliminates them before they can mature, pass out in manure the following spring, and restart the cycle.
Treating too early, while adult flies are still active, means your horse can pick up new larvae the very next day. Treating too late isn’t as risky, but the larvae will have had more time to damage the stomach lining.
How to Administer the Paste
Boticide pastes come in syringes calibrated by body weight, typically in 50-pound increments up to 1,500 pounds. You’ll need an accurate weight before dosing, either from a livestock scale or a weight tape.
- Set the dose. Turn the dial ring on the syringe to match your horse’s weight.
- Clear the mouth. Make sure the horse has no feed or hay in its mouth.
- Place the syringe. Insert the tip into the gap between the front and back teeth (the bars of the mouth) and push the plunger to deposit the paste on the back of the tongue.
- Ensure swallowing. Raise the horse’s head slightly for a few seconds so it swallows the gel rather than spitting it out.
One syringe typically contains enough product for a single horse up to 1,500 pounds, or can be split between two lighter animals if you cap and save the remainder.
Colic Risk After Treatment
Any chemical dewormer can temporarily disrupt the microbial population in a horse’s hindgut, and killing a large load of parasites at once adds to that disruption. Research from Kentucky Equine Research found that horses dewormed within the previous week had a 2.4 times higher risk of colic compared to horses that had been dewormed at least eight weeks earlier.
This doesn’t mean you should skip deworming. It means you should minimize additional gut stress around treatment time. Avoid changing hay, grain, or turnout routines in the days before and after dosing. If you need to introduce a new feed or supplement, do it gradually and not during the same window. Keeping the rest of the horse’s routine stable gives the gut microbiome its best chance of recovering quickly from the dewormer’s effects.
Removing Bot Eggs From the Coat
Chemical treatment handles the larvae inside the horse, but you can reduce the number that get swallowed in the first place by physically removing eggs from the hair coat. During late summer and fall, check your horse’s legs, belly, and jaw for clusters of tiny yellow or dark specks glued to individual hairs. These are bot eggs, and they’re surprisingly sticky.
The most effective removal tools are a bot knife (a dull-bladed scraper designed for this purpose) or an abrasive grooming block. Scrape gently along the direction of the hair to dislodge the eggs. Avoid using razors or sharp blades, which can cut the skin. Regular grooming during bot season won’t eliminate the problem entirely, since horses can ingest eggs between sessions, but it meaningfully reduces the larval load your boticide treatment will need to handle later.