Abstract: Tree endophytic fungi play an important role in reducing insect herbivory, either by repelling them or killing them directly. Identifying which fungi show such activity could lead to new environmentally friendly pesticides. In this study, the Mediterranean basin climate conditions are projected to harshen in the next decades, will increase vulnerability of tree species to pest invasions. Endophytic fungi were isolated from wood and leaves of Quercus pyrenaica, Q. ilex and Q. suber and tested for virulence against adults of the mealworm beetle, Tenebrio molitor L. using a direct contact method. Only 3 of 111 sporulating isolates had entomopathogenic activity, all identified as Lecanicillium lecanii. The pathogenicity of L. lecanii on T. molitor resulted in a median lethal time (TL50) of 14–16 d. Compared with commercial products, L. lecanii caused faster insect death than the nematode Steinernema carpocapsae and nuclear polyhedrosis virus (no effect on T. molitor survival), and slower than Beauveria bassiana (TL50 = 5), Beauveria pseudobassiana (TL50 = 8d) and Bacillus thuriengensis (80% mortality first day after inoculation). Mortality was also accelerated under water stress, reducing TL50 by an additional 33%. Remarkably, water stress alone had a comparable effect on mortality to that of L. lecanii isolates. This study confirms T. molitor as a good model insect for pathogenicity testing and agrees with management policies proposed in the EU Green Deal.

Key words: Biocontrol, Pesticide, Tree pest, Native fungi, Climate change