Efficacy of the entomopathogenic fungi Beauveria bassiana and Metarhizium anisopliae against Uvarovistia zebra (Orthoptera: Tettigoniidae) and Eurygaster integriceps (Heteroptera: Scutellaridae)

Abstract

The potential for the control of insect pests by entomopathogenic fungi has been studied for decades, and Metarhizium anisopliae and Beauveria bassiana are the best known of these entomopathogenic fungi. In this study the pathogenicities of B. bassiana DEBI 001 and M. anisopliae 715C obtained from the Iranian Research Institute of Plant Protection were evaluated for the first time against a long-horned (tettigoniid) grasshopper Uvarovistia zebra. Uvarovistia zebra is a univoltine grasshopper, distributed on the southern slopes of the Alborz Mountains in the north of Qazvin province in Iran. In some years, following good conditions for population growth, they can invade and damage field crops, and rangeland grass. Initially the efficacy of the two fungal isolates was assessed using topical application and ingestion. Experiments were carried out in the laboratory with field collected U. zebra to determine the effect of the fungi on food consumption by and mortality of the insect. Both fungi caused significant mortality of U. zebra by both contact and ingestion and both caused a significant reduction of food consumption and faeces production by the insects. The effect of fungal spore formulations in oil or water on lettuce discs was evaluated. Both oil and water formulations had lethal effects on U. zebra nymphs, but the spores formulated in oil on lettuce discs were more effective than those in water. Pathogenicity of the fungi was also evaluated against the Sunn pest Eurygaster integriceps. Experiments were conducted in the laboratory with different nymphal instars of E. integriceps to determine the relative susceptibility of nymphal instars and adults to the fungal isolates. Results showed that the fifth-instar nymph was the most susceptible with 60% and 46% mortality caused by B. bassiana and M. anisopliae respectively. The viability, mycelial growth and conidia production of the B. bassiana and M. anisopliae isolates preserved in different storage media were assessed. Assessments were done before and after storage to determine the extent of change and degeneration of the fungi caused by storage in PDA, distilled water, or sand. A significant decrease in germination of both fungal isolates was observed after 6 months storage in all media. Colony growth of the fungi preserved in sand and water media was significantly reduced after 6 month storage. There were no significant differences in conidia production before and after storage for B. bassiana preserved in PDA and sand and for M. anisopliae preserved in PDA. Virulence of the B. bassiana and M. anisopliae isolates was evaluated when they were passed through artificial media and through an insect host U. zebra. The fungi were subcultured in vitro subcultures and passaged in vivo to determine the possibility of changes in their virulence. The virulence of the fungi was measured using mortality monitored before subculturing in vitro, after subculturing in vitro and after passage in vivo. The virulence of both fungi reduced after the fourth subcultures in vitro, but this reduction was not quite significant for B. bassiana. Although there was no significant enhancement in virulence of the fungi passaged through the insect, the virulence of fungi was increased. The pathogenicities of the isolates of B. bassiana and M. anisopliae were not unexpected, but the demonstration that tettigoniid species are susceptible to entomopathogenic fungi is novel. The isolates tested have potential for use in management programmes against Uvarovistia zebra and other pests, such as Eurygaster integriceps. Further work is now required to identify more virulent strains of the fungi, examine methods for mass production and finding the best formulation for application of these entomopathogenic fungi.