Simulation of infection in the overwinter refuge of the South American rice water weevil Oryzophagus oryzae (Costa Lima) by the entomopathogenic fungi Beauveria bassiana (Bals.) Vuill.

Abstract

Oryzophagus oryzae (Costa Lima) (Coleoptera: Curculionidae) is a key pest of irrigated rice in the south region of Brazil. Although biological control of the species with entomopathogenic fungi is promising, there is still a lack of studies on pathogen-host ecology for field implementation. The objective of this work was to simulate infection of O. oryzae adults by Beauveria bassiana (Bals.) Vuill. in overwinter refuge. In the first simulation, the efficiency of the fungi present in insect carcasses with externalized mycelia collected 20 months ago was verified, while in the second simulation, the infected carcasses were obtained 2 days earlier. The first simulation was set up in gerboxes with the bottom lined with moistened paper towels, with 10, 50 and 100 live insects placed with 0, 1, 5 and 10 infected carcasses. In the second simulation, 4, 8, 21 and 42 live insects were placed with 0, 2 and 4 carcasses in acrylic boxes (9,5 x 4,5 x 3cm). In both trials, the design was completely randomized with 4 replicates. The mortality rate and the number of insects that had externalized sporulations caused by the fungi were evaluated. Infected carcasses obtained 20 months ago did not result in increased mortality for any of the inoculum amounts and insect densities. In the second simulation, there was significant mortality and externalization of mycelia in the insects for the number of carcasses, regardless of the density of insects in the boxes. It follows that the biological control of O. oryzae in overwinter refuge by B. bassiana depends on how long-lived the fungal inoculum is.