Insect Repellents: Nothing is Working!
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Time to read 5 min
Introduction
If you’ve ever stood in a cloud of midges or been hunted by ravenous horseflies and still got bitten, you’re not alone. You've probably tried different brands and come to the conclusion nothing works, but there's an explanation to the struggle.
Finding the right insect repellent—for you or your horse—can be a frustrating process. Some products work brilliantly for some individuals and seem completely useless for others.
And that’s not surprising when you understand what makes someone (or some horse) attractive to biting insects. As we explained in our earlier post, "WHY ME?", there’s no single reason why insects prefer one target over another—it’s a perfect storm of biology, chemistry, and behaviour. Here's a breakdown of the main factors:
The 7 factors that determine 'attractiveness'
🦟 Body odour & skin chemistry
We each have a unique scent profile influenced by our sweat, hormones, and genetics. This includes horses, too. Certain compounds in body odour—like lactic acid or ammonia—are particularly attractive to mosquitoes and other biting flies (Verhulst et al., 2010).
🦟 Carbon dioxide output
Biting insects like mosquitoes, midges and to some extent, horseflies, use carbon dioxide (CO₂) to detect and locate their hosts. The more CO₂ you or your horse exhale (especially during or after physical activity), the more attractive you become (Dekker et al., 2005). That's why when you're out walking or riding, you're like a homing beacon for blood sucking insects.
🦟 Body heat & sweating
Insects are drawn to warmth. So, when you're working hard or your horse is sweating on a ride, you’re basically sending out an "open buffet" signal. There's a double whammy too, just as you become more attractive, sweat also dilutes repellents, reducing their effectiveness and how long they last.
RECOMMENDED PRODUCT
- An unscented, non-greasy insect repellent, suitable for sensitive skin
- Protects against biting flies, such as midges, horseflies and ticks
- Lasts for up to 8 hours in controlled efficacy trials (at rest).
🦟 Skin microbiome
Our skin hosts trillions of bacteria that play a major role in how we smell. These bacteria break down sweat into volatile organic compounds (VOCs)—some of which biting insects find irresistible (Zielke et al., 2021). The more VOCs, the greater the attraction.
🦟 Genetics
A fascinating 2015 study in PLOS One found that up to 67% of our attractiveness to mosquitoes is determined by genetics (Fernandez-Grandon et al., 2015). That means no matter what you do, some people and horses are just genetically more appealing. This means that a repellent may be under more pressure from the get-go and that particular active ingredient may just not be suitable for you.
🦟 Colour of coat or clothing
Yes, colour can matter—but it’s only one piece of the puzzle. Dark colours like black, navy, and deep reds are more visible to some insects. However, a grey, coloured or chestnut horse can still be a target if other cues—influenced by genetics, odour, heat etc—override colour preferences (Allan et al., 1987).
🦟 Movement
Moving bodies generate more heat and CO₂ and make you more noticeable. Horseflies and mosquitoes are excellent at spotting motion, which helps them hone in on a target even from a distance. Another reason why when you're out walking or riding, you may be pursued by horseflies or clegs.
🤷♀️ So, Why may a repellent not work as well as expected?
The performance of any insect repellent depends on how well it masks or blocks these attractive signals—and how persistent it is once applied. Since each individual gives off a slightly different "insect-attracting signature," it makes sense that what works perfectly for one person or horse may not work for another.
Stacking the odds in your favour
According to UK law, all products claiming insect repellent properties should be authorised by our regulatory authority (HSE). To achieve this status, they are required to undergo controlled efficacy testing. These tests are carried out on a variety of people under different conditions and are used to generate statistically significant data to validate the repellent’s label claims. But even with this rigorous testing, there are always outliers, so while it works well for most people it may not work for everyone as well.
In short, repellents are a tool, not a shield. Look for complete protection times when considering insect repellents to protect against diseases carried by insects, such as malaria and Lyme disease. In many circumstances, insect repellents will reduce bites over a period of time, not make you totally invincible.
How to get the best results
Reapply regularly, especially after sweating or swimming.
Ensure good coverage: application as a fine mist is essential to give ultimate protection.
Combine strategies: Use repellent along with fly sheets, fans, or stable management practices. Consider your clothing choices when out walking etc.
Note patterns: Keep track of when bites occur—time of day, activity level, weather—to understand what might be triggering them and time your applications accordingly.
Final thoughts
If nothing seems to be working and you're applying product well without overextending reapplication intervals, you're probably not doing anything wrong—you're just facing a more complex challenge than meets the eye.
Insect repellents work most of the time for most individuals, but personal biology and environmental factors still play a big role.
So don’t give up—try, test, and observe to find the product that suits you or your horse the best. We always here to help!
Regulatory update
Currently, only DEET, icaridin, IR3535, and citropel are authorised as insect repellents, and all are are having to be reassessed under much tighter regulatory standards introduced a decade or so ago.
Icaridin is the first of these ingredients to have gone through the new assessment process and now individual product formulations, including Hedgewitch icaridin for human and horse use, are undergoing further assessment by the UK authorities (expected by 2027). In the meantime, they remain authorised under the old national rules for insect repellents. As icaridin was not previously registered as a horse insect repellent, its current label is for human use.
Related readings
📚 Sources:
Verhulst, N. O., et al. (2010). "Chemical ecology of interactions between human skin microbiota and mosquitoes." FEMS Microbiology Ecology.
Dekker, T., et al. (2005). "Carbon dioxide detection in insects." Annual Review of Entomology.
Fernandez-Grandon, G. M., et al. (2015). "Heritability of attractiveness to mosquitoes." PLOS One.
Zielke, E., et al. (2021). "The skin microbiome and its impact on mosquito attraction." Nature Microbiology.
Allan, S. A., et al. (1987). "Effect of colour and pattern on mosquito response." Journal of Medical Entomology.
