Terpenoid Mixtures as Repellents Against the American Cockroach: Their Synergy and Low Toxicity Against Non-Target Species

The repellent activities (<i>Periplaneta americana</i>) of lone and binary mixtures of terpenoids-geranial, <i>trans</i>-anethole, and <i>trans</i>-cinnamaldehyde-against adult American cockroaches were evaluated in this study. The respective efficacies of these mixtures were compared with that of 12% (<i>w</i>/<i>w</i>) DEET. Safety bioassays for all formulations on non-target species, namely, earthworms (<i>Eudrilus eugeniae</i>) and guppy fish (<i>Poecilia reticulata</i>), were conducted to identify natural compounds with repellent efficacy equal to or surpassing that of DEET while ensuring ecological safety for non-target organisms such as fish and earthworms. All mixtures (RC₅₀ of 0.3 to 1.6 µL/cm³) were more effective than all lone terpenoids (RC₅₀ of 6.2 to 9.1 µL/cm³) and DEET (RC₅₀ of 3.0 µL/cm³), demonstrating strong synergy, with an increased repellent value (IV) of 79 to 96%. The strongest repellency, 98.0% at 72 h and an RC₅₀ of 0.3 µL/cm³, was achieved using geranial + <i>trans</i>-cinnamaldehyde (1:1). This mixture was 5.9 to 10 times more effective as a repellent than DEET. The toxicity of every lone terpenoid and terpenoid mixture to non-target species was low; hence, these mixtures can be considered safe, whereas DEET was highly toxic to non-target species (100% mortality). The terpenoid mixture geranial + <i>trans</i>-cinnamaldehyde showed strong repellency against <i>P. americana.</i>