Ascomycota

The Ascomycota fungal division contains some of the most important entomopathogens currently being studied as microbial control agents. These fungi were formerly divided into two groups, Ascomycota and Deuteromycota, the latter of which were known as Fungi Imperfecti, species of fungi for which no sexual stage was known. It is now known that these “imperfect fungi”, formerly Class Hyphomycetes in the Deuteromycota, are anamorphs (asexual forms) of the Ascomycetes. Molecular techniques are being used to match these anamorphs with the telemorphs (sexual forms). Some of the more important entomopathogenic genera are Beauveria, Metarhizium, Nomurea, Verticillium, and Paecilomyces.

Taxonomic identifications of entomopathogenic ascomycetes are made based on the structure of the conidiophore (spore bearing “stalks”), the conidiogenous cells (on which spores are produced), and the color and morphology of the conidia. Strain differences are being elucidated using molecular techniques. Note the white color of the ant infected with Beauveria sp. and the green color of the looper infected with Nomurea rileyi.

The asexual life cycle of these fungi is relatively simple. An infectious conidium attaches to the cuticle of a susceptible insect, germinates and sends a germ tube through the cuticle of the host and into the hemocoel. Inside the hemocoel the fungus replicates as budding hyphal bodies until the hemocoel is filled with hyphal bodies and the insect dies. In the early stages of the infection, the hyphal bodies circulate in the blood throughout the hemocoel. A photomicrograph through a light microscope shows these early stages in a caterpillar infected with Metarhizium. After the caterpillar dies and the fungus has completely filled the body by saprophytic growth, fruiting structures emerge from the cadaver and produce infectious spores on the body surface of the caterpillar. If these spores encounter a susceptible host insect, the infection process is repeated.

Most species of the ascomycetes fungi can be grown on artificial media. Nomurea rileyi growing on artificial media. for example, produces green conidia, but the mycelial growth preceding formation of the conidia is white in color. Because these fungi can be grown on artificial media and are often relatively pathogenic, they have excellent potential as microbial insecticides.

Biological control: Entomopathogenic ascomycetes are increasingly being used for biological control, and research continues to produce information on efficacy of strains and to improve fermentative culture and formulation. More than 30 products have been registered for control of insect and nematode pests; most are strains of B. bassiana and M. anisopliae. Benefits of use may be important in sustainable crop production, in premium produce, and in other niche markets where chemicals are deemed unacceptable. Problems are similar to those of other microbials- cost of production, lower efficacy in the field than chemicals, registration issues, and technology transfer (Butt et al., 2001). .