In vitro evaluation of the antifungal activity of plant essential oils against entomopathogenic fungi

Abstract

Entomopathogenic fungi, particularly Beauveria bassiana, are promising biological control agents, whereas plant-derived essential oils are receiving increasing attention because of their insecticidal and repellent properties. However, interactions between these biological agents remain insufficiently understood. In this study, vapor-phase and contact effects of thyme, rosemary, and eucalyptus oils applied at 1% and 3% were evaluated against B. bassiana BMAUM M6-4 and BbEMRKZAkar-1. Antifungal activity was assessed based on mycelial growth inhibition and conidial germination. Conidia produced after essential-oil exposure were also tested against Tenebrio molitor larvae. The 3% thyme-oil treatment generally produced the strongest inhibition of mycelial growth, reaching 83.8% under contact exposure. Effects on conidial germination depended on isolate and exposure method. In the bioassay, terminal mortality ranged from 50.0% to 76.7% after contact exposure and from 66.7% to 96.7% after vapor-phase exposure. However, treatment effects were not significant within either exposure method (P>0.05). The 3% thyme-contact bioassay was not performed because conidial production was insufficient to prepare the required suspension concentration. Overall, the effects of the essential oils on B. bassiana varied according to oil type, concentration, fungal isolate, and exposure method. Although essential-oil exposure affected fungal growth and conidial germination under some conditions, it did not significantly alter the subsequent pathogenicity of BbEMRKZAkar-1 against T. molitor. These findings provide a preliminary assessment of the compatibility between the tested essential oils and B. bassiana and support further laboratory investigation before their potential combined use is evaluated under semi-field or field conditions.