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Bot Fly Eggs

Bot fly eggs on a horse's pastern.

Image: 2012 K. Blocksdorf

Bots and Bot Fly Eggs:

By the time mid-December rolls around, most areas of the continent that will get a killing frost, will have had one. This is significant to horse owners because it means it's time to administer de-worming medication to get rid of any parasite load the horse has acquired during the late summer or autumn. Some people use a daily parasite control program. Others will be on an eight or twelve week program. These strategies can be effective if the right products are used at the right intervals. Regardless of what program you normally use, it's wise to pay special attention to those little little yellowish specks on your horse's legs, mane, shoulders and flanks. Bot flies do more than make unsightly little dots on on your horse, they can be ingested and turn into a damaging internal parasite.

What are Bot Flies?:

Bot flies look a bit like skinny honey bees. One or several flies will hover around the horse's legs and body and dive in to attach their eggs to the hair strands. Bot flies don't sting like a bee, or bite like a horse fly. They are basically little flying egg laying machines and they dart at the horse repeatedly, attaching each egg to a single hair. The horses stomp and swish at the irritation. I grew up understanding that as the insect attaches the egg, it pulls on the hair. Whether the horse is irritated because its hair is actually being pulled, or it thinks it's a biting horse fly is difficult to say.

Preventing Bot Flies from Landing:

Protect your horse from bot flies just as you would any other pesky biting insect. Bot flies don't normally lay eggs around the head and ears, so a fly bonnet won't be much help. One species of bot lays eggs around the horse' s nose. It's hard to protect your horse from these ones. Bots also lay eggs on the horse's mane, legs, shoulders and flanks, so insect repellents and fly sheets may help.

Don't worry that you or the horse will get bit if you swat this fly. More likely you'll be disgusted by a smear of yellowish eggs on your hand or horse. If this happens, wash the eggs off your hands or horse, but don't let them fall where the horse may accidentally ingest them. Hopefully if you squish one, you can clean up the mess with a tissue and throw it in the trash or toilet. (Just one more use for baby wipes in the barn!)

Removing Bot Eggs:

There are lots of ideas out there about the best way to remove bot eggs. If there's just a few eggs, you can pick them off with your fingernails. Wiping mineral oil or petroleum jelly over the eggs is supposed to suffocate them, killing them before they hatch. This leaves a greasy mess, but is harmless to the horse—and possibly the bot eggs too. Kerosene and diesel are not good for your horse, so don't use these. Sponging the eggs with warm water is supposed to pull the eggs off as if the horse is licking it's coat, but I've never seen this to be effective. Pulling a block of foamed glass, pumice or grill cleaning stone over the areas where eggs have been laid helps pull them off the hair.

I've found the easiest way to pull the eggs off is with a knife blade, special bot knife or scissor blade. I just open my scissors wide and use one blade to scrape the eggs off. Be careful to keep the blade almost flat against the horse's skin. You don't want to accidentally cut your horse or yourself.Pull the blade down in the direction of the hair growth and the eggs come off easily. Sweep up any dropped eggs and put them in the garbage where horse's won't eat them.

Worming Medication for Bots:

You will need to use a de-worming medication that contains ivermectin or moxidectin to kill internal bot larvae. These medications also kill other internal parasites, so can be used in place of your regular product. Give your horse this medication after the first hard frost that kills the insects in the autumn. Administer it again in early spring to kill any remaining bot larvae in your horse's system before they are expelled with the manure and develop into mature flies that will begin the bot's life cycle again.

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