by Karen Briggs
Most commercial fly repellents are based on compounds called pyrethroids: synthetic versions of a natural fly repellent substance found in certain types of chrysanthemums.
Because the supply of natural pyrethrins can vary widely from year to year, and because a little chemical manipulation can increase their effectiveness and stability, manufacturers now prefer the synthetic alternatives, and boost their usefulness with additives like piperonyl butoxide (commonly called PBO).
However, an increasing number of horse owners are getting worried about the implications of putting these synthetic chemicals on their horses’ skins. Manufacturers are responding with fly sprays based not on pyrethroids, but on other “natural” products, usually herbs.
There are a number of herbs reputed to have fly repellent action; sometimes the reputation is deserved, sometimes not. But for the consumer looking for a natural alternative, such products are immediately attractive. After all, doesn’t “essential oil of pennyroyal” sound a whole lot more inviting than “piperonyl butoxide”?
The big question, of course, is efficacy. Do herbal preparations really do the job? Karen Norkaitis, director of scientific affairs for W.F. Young Inc. (makers of the Absorbine line of fly sprays) notes of her company’s pyrethroid-free, PBO-free SuperShield Green fly spray, which was launched in 1999, “We haven’t done any direct comparisons with our SuperShield formulations, but we have compared it to other herbal formulas, and have found it to be very effective.”
SuperShield Green contains six herbal extracts and essential oils with repellent properties, says Norkaitis: citronella, lavender, eucalyptus, pine needle, tea tree, and pennyroyal (which contributes to the scent of the product). Aloe, witch hazel, and lanolin are added to help soothe insect bites and other minor skin irritations.
Norkaitis does note that consumers shouldn’t expect herbal fly sprays to last as long as formulations with PBO in them. “We’ve shown efficacy for four to six hours, though,” she says.
In his examinations of what really repels flies, Jerry Butler, Ph.D., a medical and veterinary entomologist with the Entomology and Nematology Department at the University of Florida, has looked at an estimated 4000 different herbal formulas, and tested them against various species such as horn flies, stable flies, house flies, mosquitoes, ticks, and no-see-ums (“one of the hardest bugs to repel”, he says).
“Some of them are pretty effective,” he notes. “Pennyroyal, lavender, and wormwood come to mind – though we tried pure wormwood oil on horses and quickly learned they don’t like it much!” (Wormwood has a particularly pungent odour, which is masked in some preparations by including other, sweeter-smelling herbs.)
As for the various commercial herbal formulas available, Butler notes that most companies rely more on folklore than on double-blind studies. “They put it out on the market and see if it works. If the customers come back, there’s at least a perception that it’s effective.”
The same goes for the dozens of “homemade” fly spray recipes now so readily available on the Internet. Most of these contain essential oils of the herbs already named, with the possible addition of basil, bay leaf, cloves, tansy, cedar, black pepper, rue, rosemary, southernwood (related to wormwood), santolina, spearmint, neem (a tree that grows in India), sassafras, or thyme.
As a rule, these are mixed up with olive oil or another emollient, plus an emulsifier (to spread the oil droplets) such as a commercial dishwashing liquid. Most of these recipes will have some mild repellent action, though how long it will last, and against what species of bugs it will be most effective, is hard to say.
What about the legendary Skin-So-Soft, the Avon bath oil that has gained such a reputation for fly repellency? Despite its manufacturer being prevented by law from making any claims about Skin-So-Soft’s bug repelling action, a great many horse people swear by it, attributing both efficacy and gentle action to the oil. But does it really work?
“It’s not completely bogus,” says Butler. “It works very well, especially against no-see-ums and black flies – for about 10 minutes. It’s got very short-term activity.
“There are about five essential oils in Skin-So-Soft, three of which are probably good repellents. The problem is in getting it to stick around! On top of that, the other two essential oils in the product work as attractants – so you really get a mixed effect.
“The product’s reputation is based on a little activity and a lot of imagination,” Butler concludes.
How about apple cider vinegar? Historically, it has been used both as a topical fly repellent (wiped on the coat), and as a feed additive that is supposed to make the horse’s flesh less appealing to biting flies.
There’s little evidence that the latter approach is effective. Most nutritionists suspect you would have to feed many cups of vinegar per day to effect any change in the horse’s overall flavour. As for the topical application, it’s possible it would have some effect, says Butler. “Acetic acid, or vinegar, is a repellent at high enough concentrations, as is lactic acid, but at low doses, they act as attractants.”
If you’re really concerned about the toxicity of fly spray preparations, you might be relieved to hear that pyrethroids, whether extracted naturally from chrysanthemums or mixed up in the lab, break down rapidly in sunlight and leave almost no toxic traces for mammals. They’ve been closely monitored by the Environmental Protection Agency since the 1970s, which notes there have been remarkably few reports of adverse health effects in either horses or humans from dermal (skin) exposure or inhalation.
However, pyrethroids can be toxic to some forms of aquatic life, so you should avoid contaminating any water source with it … and of course you do want to keep your horses from ingesting it if possible, so try not to spray it near your hay bales or grain supplies.
If you’d rather try an herbal preparation, you may have to experiment to find the one that works best against your local flying scourges.
“The best suggestion I can make,” says Butler, “is to try (a herbal preparation) side by side with a pyrethrin-based formula, on two horses in the same conditions. That will tell you better than anything.”
