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

## TL;DR

A natural mixture of geranial and trans-cinnamaldehyde is more effective at repelling American cockroaches than DEET and is safe for non-target species like earthworms and guppies.

## Contribution

The study identifies a synergistic terpenoid mixture that outperforms DEET in cockroach repellency while being non-toxic to non-target organisms.

## Key findings

- The geranial + trans-cinnamaldehyde (1:1) mixture was the most effective cockroach repellent, surpassing DEET in efficacy.
- All terpenoid mixtures showed strong synergy, with repellency values increasing by 79 to 96%.
- The terpenoid mixtures were non-toxic to non-target species, unlike DEET, which caused 100% mortality in some cases.

## Abstract

The American cockroach (Periplaneta americana (L.)) is a serious vector of human diseases and allergens, especially in children. Safe and effective natural insecticides and repellents are needed to safeguard human health and environmental safety, especially in sensitive areas where American cockroaches reside, such as the sewers and drains of hospitals and nursing homes, residential homes, and cafeterias or other food-handling places. Natural repellents, namely, terpenoids from plant essential oils, are understood to be strong candidates for cockroach management. In this study, the repellent efficacy of lone terpenoids and terpenoid mixtures—geranial, trans-anethole, and trans-cinnamaldehyde—as repellents against adult cockroaches was investigated and compared with those of DEET (N,N-diethyl-meta-toluamide) and distilled water. The mixture of geranial + trans-cinnamaldehyde (1:1) was found to be the most effective cockroach repellent and was more effective than all lone terpenoids and DEET, with a long repellent time and low repellent concentration. All binary mixtures showed a high increase in repellency. All treatments except DEET were safe for the two non-target species considered: earthworms (Eudrilus eugeniae) and guppies (Poecilia reticulata). Our findings demonstrate the potential of a binary mixture of geranial + trans-cinnamaldehyde (1:1) to be developed into valuable natural repellents for American cockroach control.

The repellent activities (Periplaneta americana) of lone and binary mixtures of terpenoids—geranial, trans-anethole, and trans-cinnamaldehyde—against adult American cockroaches were evaluated in this study. The respective efficacies of these mixtures were compared with that of 12% (w/w) DEET. Safety bioassays for all formulations on non-target species, namely, earthworms (Eudrilus eugeniae) and guppy fish (Poecilia reticulata), 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 + trans-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 + trans-cinnamaldehyde showed strong repellency against P. americana.