Effect of combined applications of Metarhizium anisopliae (Metsch.) Sorokin (Deuteromycotina: Hyphomycetes) strain CIAT 224 and different dosages of imidacloprid on the subterranean burrower bug Cyrtomenus bergi Froeschner (Hemiptera: Cydnidae)

Abstract

Abstract The effects of combined applications of higher and lower dosages of imidacloprid (Confidor 350 SC) and Metarhizium anisopliae (Metsch.) Sorokin (strain CIAT 224) at different inoculum rates on the mortality of fourth instar nymphs of Cyrtomenus bergi Froeschner (Hem.: Cydnidae) were evaluated under laboratory and greenhouse conditions. Initially, the in vitro effects of three different concentrations of Confidor 350 SC or one concentration of its blank formulation on the vegetative growth of M. anisopliae were studied. In all tests on potato dextrose agar (PDA), significantly greater colony sizes than in the untreated control were recorded when M. anisopliae was cultured in the PDA medium amended with the insecticide and/or its blank formulation. Subsequently, C. bergi nymphal mortality was assessed in sand bioassays. A significantly higher C. bergi nymphal mortality was recorded in all experiments when M. anisopliae was applied in combination with imidacloprid compared to the fungus alone, and a synergistic interaction between the two agents was found throughout the entire experiments. Applications of M. anisopliae (1E+07 conidia/g sand) in combination with 1.05 μg imidacloprid/g sand resulted in 88.1% of C. bergi nymphal mortality compared to 39.5% when the fungus was applied alone at the same dosage, 20 days after application. The lower dosage of imidacloprid not only enhanced the efficacy of the pathogen but also lead to a reduced quantity of inoculum needed to cause high levels of nymphal mortality. 81.2% mortality was recorded 25 days after application of 1E+06 conidia/g sand and a lower dosage of imidacloprid, which was not significantly different to the mortality achieved using 1E+05 conidia/g sand and the same lower dosage. Interactions between M. anisopliae and imidacloprid were predominantly additive at low dosages of the fungus and synergistic at dosages greater than 1E+06 conidia/g sand. Combined applications of M. anisopliae and a lower dosage of imidacloprid also resulted in high mortality of C. bergi nymphs when the treatment was applied to a native Colombian soil under greenhouse conditions. Fifteen days after application, the interaction between M. anisopliae and imidacloprid was synergistic, and 30 days after the application of the treatments nymphal mortalities of 88.3 and 85% were recorded in sterile and non-sterile soil, respectively, compared to 70.8 and 41.7% in the two soil types following a sole application of M. anisopliae .