When it comes to 25(b) exempt pesticides, it’s common to see products with a long list of essential oils on the label, typically 4-10 different actives in the same formula. These formulas often include oils at high concentrations, with total essential oil content reaching 10%–12% or higher.
At first glance, this seems like a strength. To many consumers, more essential oils and stronger fragrance equals more effectiveness. The logic is simple: 1) “It smells strong, so it must work.” 2) if consumers like Citronella and Cedar, put both in the formula, 3) if my competitors use Lemongrass and another uses Peppermint, use both in my formula.
But does that assumption actually hold up under scientific scrutiny? What it tells us is that someone didn’t do their homework.
What the Research Shows
Pesticide Technology has conducted extensive testing to confirm how combining multiple essential oils together in the same formula actually impacts the performance (kill and/or repel) of such products. Our results were eye-opening. Our team tested every essential oil on the EPA’s 25(b) exempt list in basic insect assays (both kill and repel against 3 species each), evaluating how oils performed when combined. We classified the relationships into three categories:
- Neutral – 2 Oils that didn’t affect each other’s performance
- Synergistic – Performance became more effective when 2 oils were combined
- Antagonistic – Oils that reduce each other’s performance when mixed.
Contrary to popular belief, most oil combinations fell into the last category. Instead of working better together, many oils actually lose efficacy when blended. Neutral performance levels were documented in some cases and synergism was a rare occurrence. What this means is that the more actives a formula has, the higher the probability is that the actives are hurting each other’s performance. In essence what we found is that if a formula has 3% total active blended from 3 different essential oils, in nearly all cases, using 3% of just one of those actives would provide better results than the blend of 3 different actives.
A Surprising Example: Cedarwood Oil
One of the most striking findings for our extensive research into essential oil combinations involved cedarwood oil. When tested on its own, cedarwood showed limited insecticidal properties. But when combined with any other essential oil, it consistently reduced overall performance. In every case, cedarwood created an antagonistic effect, weakening the mixture compared to the single oil alone.
This means that many multi-oil formulas that rely on cedarwood as a base may actually underperform compared to simpler, more focused formulations.
What This Means for the Industry
These findings highlight a critical point: more essential oils do not always mean a better product. In fact, piling on oils at high concentrations can backfire resulting in a less effective pesticide that only “smells strong.”
For brands, this underscores the importance of data-driven formulation instead of following marketing trends. For consumers, it’s a reminder that scent and strength don’t always correlate with efficacy.
The future of natural pesticides lies in precision: choosing the right oils, at the right concentrations, supported by real science.
Pesticide Technology LLC 