The relationship between terpenes and insects is ancient and bidirectional. Plants produce terpenes partly to repel insects that would eat them; insects evolved to use terpene-related molecules as pheromones to communicate, navigate, and reproduce. Human applications of terpene-based insect control exploit the repellent side of this relationship, using plant-derived terpene compounds to interfere with insect sensory and physiological systems in ways that deter or eliminate them without the toxicological burden of synthetic pesticides.
How terpenes repel insects
The repellency mechanism varies by terpene and target insect, but the dominant pathways involve olfactory receptor interference (the insect simply avoids high concentrations of certain volatiles), central nervous system disruption in insects at higher concentrations, and disruption of insect pheromone signaling that makes finding mates and navigating more difficult. Geraniol and citronellal mask the carbon dioxide and lactic acid signals that attract mosquitoes; alpha-pinene is toxic to certain bark beetles; pulegone disrupts the nervous system of many soft-bodied insects. These mechanisms are specific to insect physiology and largely non-toxic to mammals at typical application concentrations.
Citronella, the most widely-used natural insect repellent, is a terpene blend dominated by citronellal, geraniol, and linalool, all botanically derived monoterpenes. The same class of compounds that makes cannabis aromatic and lavender calming is the backbone of the world's most familiar botanical insect control agent.
Key terpenes for pest management
Geraniol is among the most effective mosquito repellents in the botanical category, with studies showing duration of protection competitive with DEET in some formulations. Linalool is effective against fleas, fruit flies, and cockroaches at higher concentrations. Thymol (from thyme) and carvacrol are highly effective against mites and some fungal plant pathogens, making them useful in organic agricultural applications. Neem oil, which contains high concentrations of azadirachtin and multiple terpenoid compounds, is one of the most studied and regulated organic pest management inputs globally.
The repellent recipes and working concentrations, by target insect, application surface, and carrier system, with skin-safety thresholds, are in Chapter 12 of the printed book.