Entomopathogenic fungi and plant essential oils are not compatible in controlling Tribolium castaneum (Herbst)

Entomopathogenic fungi (EPF) and essential oils (EOs) can show either positive or negative interactions when used for controlling insect pests. First, the insecticidal efficacy of EPF including Beauveria bassiana isolates Z1 and IRAN1395C, Lecanicillium lecanii, and Paecilomyces lilacinus was tested against adults of Tribolium castaneum using two methods (standard insect dip and wheat diet incorporation). Additionally, the toxicity of EOs from Trachyspermum ammi, Foeniculum vulgare, Eucalyptus globulus, Salvia mirzayanii, Majorana hortensis, and Thymus vulgaris was evaluated against adult T. castaneum. Thereafter, the effect of an LC25 concentration of F. vulgare (86.13 mu l L-1), T. ammi (235.2 mu l L-1), and E. globulus (111.33 mu l L-1) EOs on mycelial growth, spore germination, and sporulation of the EPF was determined. In standard dip bioassay, the lowest LT50 of 10.4 days was induced by L. lecanii, while the wheat diet incorporation method resulted in LT50 values ranging between 13.1 and 15.2 days. The LC50 values for E. globulus, F. vulgare, and T. ammi were 162.3, 140.3, and 310 mu l L-1 air against adults, respectively. The EOs examined showed strong inhibition of mycelial growth, conidial germination, and sporulation at sublethal concentrations. EOs of F. vulgare and T. ammi completely inhibited mycelial growth and sporulation of the tested EPF. Germination inhibition ranged from 100% in L. lecanii exposed to EO from F. vulgare to 52.3% in B. bassiana Z1 exposed to EO from T. ammi. Based on the results, although EOs and EPF are successful agents to control adults T. castaneum when used separately, it cannot be applied in combination because of the conflicting effect.