Repellency of anethole- and estragole-type fennel essential oils against stored grain pests: the different twins

Aromatic plants essential oils (EOs) are promising alternatives to chemical insecticides and insect repellent for the post-harvest protection of crops. Fennel (Foeniculum vulgare Mill.) is a highly aromatic plant, cultivated worldwide of which several chemotypes can be distinguished on the basis of the relative content of its main compounds (E)-anethole and estragole. Fennel is well known for its pharmacological, antioxidant antimicrobial and acaricidal activities, and several studies showed its effectiveness as insecticidal and repellent against insects. In this study the repellency of the EOs extracted from two chemotypes, anethole- and estragole-type of F. vulgare fruits, against Rhyzopertha dominica (F.), Sitophilus zeamais Motschulsky and Tribolium confusum Jaqcquelin du Val, three of the major worldwide post-harvest grains insects was assessed by in vitro bioassays. Along with the EOs, we also tested the repellency of their major chemical components (E)-anethole, estragole, limonene and, fenchone and we evaluated the co-repellency effect of (E)-anethole and estragole. Finally, the repellence of the fennel EOs in the presence of maize was tested by a two-choice pitfall bioassay. (E)-anethole and estragole content in the anethole-type was 78.4 and 8.0%, respectively while in the estragole-type fennel EO anethole and estragole content was 0.9 and 85.5%, respectively. RD50 values showed that the estragole-type EO was the most effective repellent against the three insect species with values of 0.007, 0.051 and 1.124 mg cm-2 for R. dominica, T. confusum and S. zeamais, respectively. Consistently, relative median potency analysis showed that estragole was significantly more repellent to the three pest insect species than (E)-anethole. Interestingly, in the EOs, (E)-anethole and estragole showed a synergistic co-repellent effect. The strongest synergy was observed against R. dominica (CRC = 634.51). The repellency of fennel EOs also in the presence of maize was confirmed by a two-choice bioassay. Also in this case, the most overall effective EO resulted to be the estragole-type. The results highlight the importance of a chemical standardization based on the bioactivity of the fennel EOs and indicate the estragole-type fennel EO as suitable for the development of eco-friendly repellents for the post-harvest protection of grain crops.