Repellence and attractiveness: The hormetic effect of aromatic plant essential oils on insect behavior

Aromatic plant essential oils (EOs) represent a promising alternative to synthetic compounds for humans, food and crop protection from insect pests. However, although many studies have demonstrated the toxic and repellent effect of EOs against harmful insects, some studies indicate that EOs can act, on the contrary, as attractants for harmful insects. In the practical use of EOs as active ingredients in insecticidal/repellent formulations, these contrasting effects may generate unexpected responses from insects. In this work, we tested the hypothesis that EOs may act in a hormesis biphasic concentration-response effect characterized by a lowconcentration stimulation (attraction) and a high-concentration inhibition (repulsion) of the insect chemotaxis. For this purpose, the EOs obtained from twelve aromatic plant species belonging to eight botanical families were chemically characterized and tested for their toxicity (by fumigation) and attractiveness or repellence (by two-choice olfactometer bioassays) against the stored cereals pest Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae). EO compositions were variable both qualitatively and quantitatively: while phenylpropanoids were the most represented chemical class in the EOs of Cinnamomum verum, Myristica fragrans, Ocimum basilicum, Piper aduncum, and Syzygium aromaticum, monoterpene hydrocarbons dominated the Citrus aurantium and Citrus reticulata EOs compositions, and they represented over 60% of the Schinus molle EO; the Clinopodium nubigenum and Clinopodium tomentosum EOs, as well as the Elettaria cardamomum EO, were mainly rich in oxygenated monoterpenes, while Piper nigrum was the only species among those studied whose EO composition was mainly represented by sesquiterpene hydrocarbons. Toxicity tests showed a wide range of activity among the tested EOs, with LC50 values ranging from 2.19 to 218.32 mu L L-1 for C. nubigenum and C. verum EOs, respectively. Behavioral tests showed that C. aurantium, C. nubigenum, C. tomentosum, and O. basilicum EOs exerted an overall repellent effect, which was significant at concentrations ranging from 1.19 to 23.87 mu L EO L-1 air. Conversely, C. verum and S. aromaticum EOs were predominantly attractive, with a significant attractiveness in the range of concentrations from 2.39 to 11.94 and at 23.87 mu L EO L-1 air, respectively. However, half of the tested EOs (C. reticulata, E. cardamomum, M. fragrans, P. aduncum, P. nigrum, and S. molle) showed both negative and positive chemotaxis depending on the EO concentration. Overall, the results indicated that EOs can exert attractive and repellent activity depending on their chemical composition and concentration. Since EOs contain volatile substances whose concentration varies over time after the application, the implication of these results will be discussed in relation to their possible use as insect repellents for active packaging or as attractive lures for trapping.