Pathogenicity of Metarhizium anisopliae on the migratory locust, Rhammatocerus schistocercoides Relin, in Colombia
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Abstract
The migratory locusts are a group of insects that cause large economic losses to agriculture. When these migratory insects arrive at a place, they can eat all the plant material available especially grasses; they appear in large clouds which can weigh up to 100 tons, and are capable of consuming daily the equivalent to their own weight. The duration of an outbreak can last 4- 5 years. In Colombia in Los LLanos Orientales (Arauca, Casanare, Guaviare, Meta, Vichada), a severe outbreak of Rhammatocerus schistocercoides Rehn (Orthoptera: Acrididae) has been registered since 1993 in pastures over an area estimated at 4 million hectares. An alternative to control this insect is fungal entornopathogens, specially Meturhizium anisopliae (Metschnikoff) Sorokin. This study was planned to develop a bioassay to select highly virulent isolates for further studies to control R. schistocercoides under field conditions. The isolates evaluated were Ma9218 from Teleugryllus comodus (Orthoptera: Gryllidae) isolated in Australia and Ma9236 from an unknown host. A completed randomized design was used to test treatment effects. The experiment had three treatments, the two isolates of M. anisopliae and a control. Isolates were cultivated in Sabouraud Dextrose Agar plus lactic acid at 0.37%. Nymphs of 4th instar collected from Carimagua, were infected by submerging them for 30 seconds in a M. anisopliae suspension containing 1 x10' spores/ml. The control treatment was exposed to sterile distilled water. Then the treated nymphs were placed in square plastic cages (17.5x12x7cm) provided with lateral holes covered with nylon cloth for ventilation. Five repetitions were set up for each treatment and five nymphs were deposited in each cage. The experimental units were provided daily with fresh food. The effect of M. anisopliae on nymphs of R. schistocercoides was evident from the second day after inoculation. On the fifth day. independent of isolate evaluated, the highest daily mortality occurred: 20% for Ma 9218 and 28% for Ma 9236. Significant differences were found (P= 0.05) between treatments at the end of evaluation reaching mortalities of 68.0 ± 4.9% for Ma 9218 and 84.0 ± 7.5% for Ma 9236; mortality due to fungus was not observed in the control. Isolate Ma 9236 can be considered a candidate to be evaluated in a control program against this insect under field conditions. Bioassay methodology developed in this study can be used to test further isolates.
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