Synergistic repellency of Citrus reticulata and Coffea arabica extracts mediated by R-(+)-limonene and hexadecanoic acid against the pea aphid Acyrthosiphon pisum

The pea aphid Acyrthosiphon pisum Harris is a sap-feeding insect pest that causes significant yield losses on Fabaceae crops. Its sustainable management remains complex, as the use of conventional insecticides is increasingly restricted worldwide due to growing concerns about their impact on human health and the environment. In this context, the use of plant extracts (PEs) as biorepellents represents a promising alternative due to their favourable toxicological and ecotoxicological profiles. Using dual-choice bioassays, we evaluated the repellent activity of seven commercially available PEs against A. pisum and tested whether combining the most active extracts could enhance efficacy through synergistic interactions. We identified the essential oil of Citrus reticulata Blanco peel and the extract of roasted Coffea arabica L. beans as strongly repellent against A. pisum , and demonstrated that their blend exhibits a synergistic interaction resulting in a 3.5- and 9.4-fold increase in repellent potency compared to individual extracts. To further investigate the chemical basis of this phenomenon, we tested the binary interactions between compounds derived from these PEs, which were identified through chemical composition analysis. The results revealed that hexadecanoic acid and R-(+)-limonene, the main compounds of each extract, interact positively and contribute substantially to the blend’s activity. These findings highlight that the repellent activity of PE blends may result from interactions among their constituents and support the rational design of more effective and sustainable pest management strategies based on biorepellents.