Effect of Pesticides on Germination and Growth of Three Fungi of Rice Insects

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International Rice Research Notes





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Populations of rice brown planthopper (BPH) Nilaparvata lugens (Stal) in the field and in insect-rearing cages are commonly infected with specific fungi (Deuteromycotine: Hyphomycetes). Conidia of the fungi germinate on the insect cuticle, and penetration follows. The mycelium multiples in the homocoel by yeast-like budding. Conidia produced on the outside of the cadaver can infect healthy BPH. Infect fungus epizootics can completely control BPH populations. Several chemical pesticides that suppress germination and growth of the insect fungi can prevent the occurrence of epizootics. Three BPH pathogens - Beauveria bassiana (Bals.) Vuill., Metarhizium anisopliae (Metsch) Sorokin, and Hirsutella citriformis Speare - were treated with technical formulations of the fungicides benomyl and edifenphos and the insecticide carbaryl. Germination rates were determined by incubating 10 to the sixth power conidia/ml of a 1% dextrose/0.5% yeast extract broth on a shaken machine. Pesticides were applied at 0.1, 1, 10, 100, and 1,000 ppm in 5 replications. After 24 h, at least 250 conidia of each replication were examined under the microscope for the presence of germination tubes. Germination rates were computed. Mycelium growth rates were determined in a 3% dextrose/1.5% yeast extract broth in shaker flasks. The flasks were inoculated with 1 ml conidia suspension (5 x 10 to the sixth power conidia/ml). Mycelium dry weight was determined after 3 d of growth. All concentrations of the pesticides tested inhibited conidia germination. H. citriformis was significantly more susceptible to benomyl at 10 and 100 ppm concentrations.

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