Piperitenone oxide-rich Mentha longifolia essential oil and its nanoemulsion to manage different developmental stages of insect and mite pests attacking stored wheat

Horsemint, Mentha longifolia (L.) Huds. (Lamiales: Lamiaceae), is a member of the genus Mentha, widely used for the repellent and insecticidal effectiveness of its essential oil (EO). Several chemotypes have been reported for this species. Here, the efficacy of a M. longifolia EO, belonging to the piperitenone oxide-type, and a 10% (w/w) M. longifolia EO-based nanoemulsion (NE) were examined as wheat protectants, at 500 and 1000 ppm (mu L/kg). The two products were tested against four insect and a mite species, i.e., the red flour beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae), the confused flour beetle, Tribolium confusum Jacquelin du Val (Coleoptera: Tenebrionidae), the yellow mealworm beetle, Tenebrio molitor (L.) (Coleoptera: Tenebrionidae), the saw-toothed grain beetle, Oryzaephilus surinamensis (L.) (Coleoptera: Silvanidae), and the flour mite, Acarus siro L. (Sarcoptiformes: Acaridae). The NE showed a mean hydrodynamic diameter below 200 nm after preparation and no relevant changes in particle size distribution at least up to 4 months. The highest concentration of M. longifolia EO was the most effective grain protectant, causing 84.4% and 100% mortality to T. castaneum adults and larvae respectively, 42.2% and 100% mortality to T. confusum adults and larvae respectively, 100% and 34.4% mortality to T. molitor adults and larvae respectively, 100% mortality to both adults and larvae of O. surinamensis, and 87.8% and 67.8% mortality to A. siro adults and nymphs, respectively. Tribolium castaneum larvae were controlled by 500 ppm of the EO and 1000 ppm of the NE, which caused 100% and 93.3% mortality, respectively. Similarly, 1000 ppm NE caused 96.7% mortality against T. confusum larvae, while 500 ppm EO and 1000 ppm NE killed 90.0% and 91.1% of T. molitor adults, respectively. Complete mortality (100%) of O. surinamensis adults was achieved by 500 ppm EO. The findings of the current study suggest that the M. longifolia EO and the M. longifolia EO-based NE may be considered as promising alternative wheat protectants against different insect and mite storage pests, depending on their developmental stage.