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An invasion of the llanera locust Rhammatocerus schistocercoides (Rehn) occurred in 1994 in the Oriental Plains of Colombia. This insect implies a potential risk for cultures like rice, sorghum, and sugar cane. According to the potential that biological control represents, the objective of the present work was to select under laboratory and field conditions, natives strains of Metarhizium anisopliae (Metschnikov) to control the llanera locust. In the laboratory (28°C and relative humidity 70%) three natives strains of M. anisopliae (M3, M5, and M11) were evaluated over the eighth instar nymphs of R. schistocercoides. The M5 strain produced at the eleventh day a 100% of mortality in locust, while the M3 and the MI 1 strains produced 100% of mortality at the thirteen-day. The median lethal doses (LD50) was determined for the three strains, using the statistical program PROBIT. An LD50 of 5 conidia/insect was found for the M5 strain and 7 conidia/ insect for the strains M3 and M11. These results did not present significant differences between them (f=0.78; g1=4; p<0.05). In order to evaluate biological control of strains under field conditions (36°C and R.H 56%), strains were reactivated, massively produced, and formulated by means of conidia encapsulation with ultraviolet protectors. The assay was carried out in cages containing adult locusts separated into two groups. The first group was treated with formulated fungus suspended in an oil-water emulsion, the other group was treated with non formulated fungus (naked conidia). Strains of M. anisopliae were applied in two concentrations (1x107and lx108 conidia/ml) using an ultralow volume sprayer. After seventeen days, M. anisopliae produced 67.8%, 64.1%, and 56.5% mortality when applied the formulated strains M5, M11 and M3 respectively. These results were significantly different (f=4.66; g1=12; p<0.05) to the obtained when applied non formulated conidia (naked conidia). In the latter case, were found mortality percentages of 9.5%, 26.4%, and 28.2% for the strains M5, M 11, and M3 respectively. We could demonstrate that the formulation permits a good entomopathogenic activity of the fungus under field conditions because of confere tolerance to the conidia towards adverse conditions.

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