Water deficiency and endophytic Beauveria bassiana enhance the repellent activity of Allium cepa L. (onion) extracts against grapevine mealybug (Planococcus ficus)

This study aimed to determine how water deficiency and Beauveria bassiana inoculum affect the volatile organic compounds and bioactivity of Allium cepa (onion) extracts against grapevine mealybug. During the study, A. cepa seedlings were exposed to B. bassiana or a no-fungus treatment and one of three watering interval treatments (3-day, 5-day, or 7-day). After ten weeks, A. cepa bulbs were harvested, crushed, and extracted with dichloromethane (DCM) at room temperature for 24 hours. The DCM extract was filtered, evaporated to dryness, and then reformulated in DCM to achieve concentrations of 20% w/v, 10% w/v, and 5% w/v, which were subsequently evaluated in a disc-repellent bioassay. The bioassay results showed that insect repellence varied significantly among the treatments (fungus, no-fungus, positive and negative controls) irrespective of the watering interval. B. bassiana inoculation of A. cepa plants significantly enhanced the repellent activity against grapevine mealybugs when extracts were tested at 20% w/v. The best insect repellent activity was recorded on plant extracts from the fungus treatments. The GC-MS analysis of the DCM extracts showed that bulbs of plants inoculated with B. bassiana produced significantly more volatile organic compounds than the no-fungus plants only under the highest water stress treatment (7-day watering interval). In conclusion, B. bassiana inoculum can enhance the volatile organic constituents and the repellent activity of onion extracts against mealybugs.