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Demystifying Fly Spray Active Ingredients

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Fly Spray for Horses

According to the calendar, summer is winding down, but at least here in the midwest, the flies don't seem to have gotten the message.  Just sitting on the porch can be a workout as you swat at them, nevermind heading into the barn or pasture, or onto the trails, where flies have lots of moist environments to breed and congregate. So while you may have hoped you purchased your last fly control product in July, late August and September may find you once again in the fly spray aisle trying to figure out which product works best.

One of the things horse owners find most confusing about fly sprays is the ingredients themselves.  The two primary ingredients used in commercial fly sprays (pyrethrin and permethrin) even sound similar, so no wonder we're all confused!  Here's a short primer on the differences between the two.


Pyrethrin is derived from the parent chemical pyrethrum, which you'll sometimes hear people refer to when they're talking about pyrethrin, and is produced by a certain species of the chrysanthemWhere does pyrethrin come fromum plant. Pyrethrins are contact poisons that work by quickly penetrating the shell or skin of the insect, paralyzing its nervous system.  It’s important to understand that while pyrethrins are powerful “knockdown” agents, they aren’t actually killing the insect; a fly’s enzymatic system degrades or detoxifies the pyrethrin allowing some of them to recover.  For this reason, many fly spray manufacturers add synergists (more on that in a minute) to their formulas to help delay the insect’s enzyme action, assuring a lethal dose.


Permethrin is a synthetically produced insecticide that, like pyrethrin, works as a neurotoxin, overstimulating the nervous system on contact.  Similarly, permethrin’s knockdown capabilities are often combined with synergists in order to actually kill insects.  Permethrin is more stable than pyrethrin, so formulas containing permethrin typically can provide longer protection.

A note about repellents and synergists.  Repellants, unlike the compounds listed above, discourage flies and help to keep them away from your horse, but don’t actually knock them down or kill them.  The most common repellent found in fly sprays is Butoxypolypropylene glycol.  As mentioned above, synergists are used in fly sprays to increase the potency of the pyrethrin and/or permethrin in the formula by inhibiting the enzymatic activity within the insect that breaks down the insecticide and allows the fly to get back up again.  The synergists you’ll see most often in fly sprays for horses are Piperonyl butoxide and Bicycloheptene Dicarboxamide.

So, are these chemicals safe? Pyrethrins and Permethrin are both classified as “Restricted Use Pesticides” by the EPA, which means that their use in products is restricted to certified manufacturers.  That said, they are considered relatively non-toxic to mammals (including humans).  Because pyrethrin is derived of a natural source (the chrysanthemum flower) it is readily biodegradable when exposed to oxygen, moisture and sunlight, so are not considered particularly dangerous to the environment.  Permethrin also breaks down fairly easily and is not associated with any significant environmental issues. 

It should be noted, however, that pyrethrin is highly toxic to fish and moderately toxic to birds and bees, and permethrin is fatal to cats and fish, and highly toxic bees and other beneficial insects, so take care when using fly sprays containing these compounds if you have any of these other species on your property.

The active ingredient for Pro-Force is Permethrin.  ProForce-1.pngProven to repel  and kill over 70 species of pest, the spray is highly versatile and can even be use don dogs! 

You can learn more about Pro-Force here!



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