Endophytic Beauveria bassiana modifies flowering phenology, floral volatile profile and pollinator behaviour in melon

Entomopathogenic fungi have a high potential as microbial control agents of insect pests, both through direct application and through endophytic colonisation and the subsequent effects on the phytophagous insects. Recently, such symbiotic relationships with their host plants and compatibility with other natural enemies of insect pests have been characterized, including the modification of plant volatile emissions and the consequent effect on the insects. The present research aimed at investigating whether systemic Beauveria bassiana (Balsamo) Vuillemin colonisation of melon plants reached the floral tissues and affected flowering, pollen production and pollinator behaviour. Melon plants sprayed with B. bassiana conidial suspensions were endophytically colonised throughout the entire plant growth cycle and reached the floral tissues. In addition, the endophytically-colonized plants produced a significantly higher number of pollen grains, and flowered earlier than control plants. A foraging preference of honeybees and bumblebees for the control plants in choice assays was detected, likely due to the advanced maturation stage of colonised plants flowers. Noteworthy, changes in the emitted blend of volatiles were observed in the endophytically-colonised plants, which emitted more of the pollinator attractants D-limonene and benzeneacetaldehyde than control plants. Future research should consider the mechanisms for endophyte-mediated early flowering and variations in volatile profiles throughout the blooming process, together with their impact on pollination under greenhouse conditions.