Toxicity and physiological interruptions of a proteinaceous toxin from Metarhizium anisopliae against the olive fruit pest, Bacterocera oleae (Diptera: Tephritidae)

Abstract

The use of natural bioactive compounds mainly proteinaceous secondary metabolites of fungi is one of the promising pest control methods because of their lethal effects on insects in low concentration, limited persistence in environment and easily decomposition into environmentally safe compounds. The olive fruit fly, Bactrocera oleae (Rossi) (Diptera: Tephritidae), is a destructive pest of olive fruits around the world. In the current study, the proteinaceous compounds were extracted from the two isolates (MASA and MAAI) of Metarhizium anisopliae and their effects were evaluated on toxicity, feeding performance and antioxidant system of the adult's olive flies. Both extracts from MASA and MAAI showed entomotoxicity against the adults by 2.47 and 2.38 mg/mL as LC50 concentrations. Also, LT50 values were recorded 1.15 and 1.31 days for MASA and MAAI, respectively. No statistical differences were recorded in the consumption rate of the adults on control and secondary metabolite contained protein hydrolysate. In contrast, the adults fed on LC30 and LC50 concentrations of MASA and MAAI demonstrated significant reduction in the activities of digestive alpha-amylase, glucosidases, lipase, trypsin, chymotrypsin, elastase, amino- and carboxypeptidases. Activity of antioxidant enzymes changed in the adults of B. oleae fed on the fungal secondary metabolites. Catalase, Peroxidase and Superoxide dismutase elevated in the treated adults with the highest amounts of MAAI. Similar results were found in activity of ascorbate peroxidase and glucose-6-phosphate dehydrogenase except for malondialdehyde amount in which no statistical differences were recorded between treatments and control. Relative gene expression of caspase enzymes revealed the higher expression in the treated B. oleae compared to control with the highest level of caspase 8 for MASA and caspases 1 and 8 for MAAI. Results of our study showed that the secondary metabolites extracted from the two isolates of M. anisopliae caused mortality, interrupted digestion and induced oxidative stress in the adults of B. oleae.