# Plant-based insect repellents

How mosquitoes find a host, why repellents act in the vapour phase, the volatility trade-off that fixes protection time, and the difference between oil of lemon eucalyptus and lemon eucalyptus essential oil.

A repellent has to evaporate to be smelled, and evaporating is how it runs out. Protection duration is set by vapour pressure — which is why the essential-oil actives are short-lived by chemistry, not by formulation failure.

Source: https://en.bioecon.ru/docs/biochem-industrial/specialty-fine-chem/bio-insect-repellents/
Updated: 2026-08-25



Topical repellents do not kill and mostly do not touch the insect. They interfere with how it finds you, which makes them a problem in olfaction and in physical chemistry.

## What the mosquito is following

Host-seeking is a staged behaviour, each stage cued differently. **Carbon dioxide** in an exhaled plume acts at long range and switches the insect into an active search. Closer in, **skin volatiles** provide the specific signal — lactic acid, ammonia, short-chain carboxylic acids and 1-octen-3-ol, in ratios that differ between people and are partly determined by skin bacteria. At short range, **body heat and water vapour** guide the landing.

A repellent has to disrupt one of these stages before contact. Two mechanisms are described: **interference with olfactory receptor signalling**, so the host cues are masked or the signal is confused, and **direct avoidance**, in which the insect detects the repellent itself as aversive. For DEET both have been argued and the balance remains debated; for the plant terpenoids, receptor-level interference is the usual proposal.

## Volatility is the whole trade-off

The consequence is that **the active must be in the vapour phase above the skin to work**. It is not a barrier and it does not need to be touched.

That requirement is also the limit. A molecule volatile enough to maintain a vapour layer is volatile enough to be lost from the skin, and the loss is accelerated by warmth, wind and sweating — exactly the conditions under which repellent is used. **Protection duration is therefore governed by vapour pressure, and cannot be separated from efficacy.** An active made less volatile lasts longer and repels less well.

Monoterpenes such as citronellal, geraniol and limonene sit at the high-volatility end. Their vapour pressures are far above DEET's, so they establish a vapour layer quickly and lose it quickly, which is the physical reason typical essential-oil repellents give roughly twenty minutes to an hour rather than several hours. Formulation can slow the loss — microencapsulation, polymer film formers, higher-boiling co-solvents — but it works against the same trade-off, since anything that slows release lowers the headspace concentration.

## The PMD distinction is the one to get right

**Oil of lemon eucalyptus, as a registered repellent active, is not lemon eucalyptus essential oil.** The essential oil of *Corymbia citriodora* is dominated by citronellal. Treating that oil with acid cyclises the citronellal into **para-menthane-3,8-diol (PMD)**, a heavier, much less volatile diol, and it is PMD — enriched to a defined level — that carries the activity.

The difference is exactly the volatility trade-off made deliberately. PMD's lower vapour pressure gives it the several-hour protection that puts it in the same duration band as lower-concentration DEET, and it is the only plant-derived active registered as a skin-applied repellent by the US EPA and recommended alongside the synthetics by the CDC. **The unrefined essential oil does not carry that recommendation**, and the two appear under similar names on labels.

## The claims that do not follow

**Wearable patches and area devices** face a physical objection rather than a chemical one: a vapour source not on the skin protects the volume around it, and any air movement displaces that volume. Field evaluations of citronella-based wristbands and patches have generally found little to no protection at the skin surface.

**Duration claims deserve the most scepticism**, because the standard measurement — complete protection time, the interval to the first confirmed bite in a controlled arm-in-cage or field test — is well defined, so a product not quoting it is choosing not to.