Fly spray can be a challenge for some horses, and their aversion to it is often due to various factors, including the smell, the sensation, and indeed, the noise it makes when being sprayed.
Fly sprays typically contain strong-smelling chemicals designed to repel insects. Some horses may be sensitive to these odors and find them unpleasant, which can make the application of fly spray a stressful experience for them.

The sensation of the liquid mist landing on their coat can be unsettling for horses, especially if they are not accustomed to the feeling. Some horses may react by twitching, moving away, or showing signs of discomfort.
As you mentioned, the noise of the spray bottle can also be alarming for some horses, particularly if they are already wary of unfamiliar sounds. The hissing or spraying sound of the bottle can startle or frighten them.
To help horses become more comfortable with fly spray, gradual desensitization is essential. Here are some tips for introducing fly spray to a horse:
Start by allowing the horse to sniff and investigate the spray bottle without spraying anything. Let them become familiar with the object and its smell.
Reward the horse with treats or praise when they show calm behavior around the fly spray bottle. This positive association can help them feel more at ease.
Before applying fly spray directly to the horse, practice spraying the product away from them to reduce the noise and let them hear and see that it is harmless.
Begin by spraying a small amount of fly spray on a cloth or sponge and then gently rub it on the horse’s body. Gradually increase the amount of spray and the areas you cover.
Pay close attention to the horse’s body language during the process. If they appear anxious or stressed, take a step back and give them time to relax before trying again.
Some horses may be more receptive to natural fly repellents that have milder scents. There are various essential oil-based or herbal fly sprays available that might be less intimidating for sensitive horses.
Consistency and patience are key. It may take time for a horse to become comfortable with fly spray, so be patient and work with them regularly to build trust.
Remember that each horse is an individual, and what works for one may not work for another. If a horse’s aversion to fly spray persists or seems extreme, it may be helpful to seek guidance from a professional trainer or behaviorist who can provide specialized assistance. The goal is to make fly spray application a stress-free experience for both the horse and the handler.