In France and Italy, where silk production was important in the 16th and 17th centuries, heavy losses of larval silkworms were experienced every year from "muscardine." In 1835, the Italian scientist Agostino Bassi de Lodi (the "Father of Insect Pathology") showed that the problem affecting the silkworms was actually caused by a fungus that multiplied in and on the body of the insect. This was the first microorganism to be recognized as a contagious agent of animal disease. Yes indeed, the first animal pathogen to be understood was of insects, not humans! The fungus was later named Beauveria bassiana in honor of its discoverer. The very distinctive and noticeable white mummies of affected caterpillars gave rise to the name muscardine, which is derived from the French word for the bonbons which the mummified specimens resembled. Today the term muscardine refers to an insect fungus or disease caused by a fungus.
Beauveria bassiana is a common soilborne fungus that occurs worldwide. It attacks a wide range of both immature and adult insects. Besides silkworm, the extensive list of hosts includes such important pests as whiteflies, aphids, grasshoppers, termites, Colorado potato beetle, Mexican bean beetle, Japanese beetle, boll weevil, cereal leaf beetle, bark beetles, lygus bugs, chinch bug, fire ants, European corn borer, codling moth, and Douglas fir tussock moth. Natural enemies, such as lady beetles, are susceptible too, and it has even been found infecting the lungs of wild rodents, and the nasal passages of humans. There are many different strains of the fungus that exhibit considerable variation in virulence, pathogenicity and host range. It occurs in the soil as a saprophyte.
As with all insect-pathogenic fungi, Beauveria produces spores that are resistant to environmental extremes and are the infective stage of the fungal life cycle. The spores (called conidia in this case) infect directly through the outside of the insect's skin. Under favorable temperature and moisture conditions, a conidium (singular of "conidia") adhering to the host cuticle will germinate. The fungal hypha growing from the spore secretes enzymes which attack and dissolve the cuticle, allowing it to penetrate the skin and grow into the insect body. Once inside the insect it produces a toxin called Beauvericin that weakens the host's immune system. After the insect dies, an antibiotic (oosporein) is produced that enables the fungus to outcompete intestinal bacteria. Eventually the entire body cavity is filled with fungal mass. When conditions are favorable the fungus will grow through the softer parts of the insect's body, producing the characteristic "white bloom" appearance. Relative humidity must be 92% or more for B. bassiana to grow outside the insect. These external hyphae produce conidia that ripen and are released into the environment, completing the cycle.
In addition to infecting insects, B. bassiana can colonize corn plants, having the capability of living in the vascular tissue of certain corn cultivars as an endophyte. European corn borer tunnelling is reduced in corn plants with the fungus. In studies in Iowa, the fungus colonized the plant when applied as a granular formulation of conidia on foliage at whorl stage, moved internally in the plant, and persisted throughout the season to provide significant suppression of corn borers.
B. bassiana is available commercially as a microbial insecticide since Beauveria can now be mass produced by a fermentation process and formulated to enable the fungus to withstand ultraviolet light, and temperature and humidity extremes commonly encountered in the field. There are several products that contain B. bassiana, including Naturalis® and Mycotrol®; it is likely additional products will be registered with the EPA. Because it takes 3-7 days to kill an insect with B. bassiana, it will take some time to suppress the pest population when using these products. Thorough spray coverage is essential because fungal spores must contact the insect for infection to occur.
- Susan Mahr, University of Wisconsin- Madison
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