Entomopathogenic Fungi

While the focus on the practical use of entomopathogenic fungi has been on biocontrol using whole organisms, either as inoculative or inundative agents (Eilenberg et al. 2001), these fungi are known to produce a number of toxins and enzymes. Some of these extracellular metabolites have been studied with the aim of using them as bioactives against insect pests. This biotechnological approach to utilizing entomo-pathogenic fungi can be demonstrated by the discovery and formulation of spinosyns, insecticidal toxins produced by an actinomycete. From the discovery of a strain of Saccaropolyspora spinosa in the Carribbean, Dow Agrow Sciences have successfully developed a number of "green chemistry" insecticide products, such as Success™ and Naturalyte™. It may be possible to utilize active components from entomopathogenic fungi in a similar or novel fashion.

It is not surprising that entomopathogenic fungi produce extracellular enzymes and toxins. These compounds are required to both assist penetration of the host cuticle and overcome other host defenses, while excluding competing microbes. Proteases produced by entomopathogenic fungi to degrade cuticle and assist entry into the host are similar to proteases used by insects to degrade their own cuticle during molting (Samuels and Paterson 1995). A number of enzymes are known from entomopathogenic fungi, such as the proteases, lipases, and chitinases that assist in cuticular breakdown. These enzymes can be thought of as bioactives and there has been increasing interest in use of these enzymes in pest control. Screen and St Leger (2000) have reported on the occurrence of typsins and chymotrypsins in M. anisopliae. The novel chymotrypsin (CHY1) is similar to bacterial chymotrypsins. Because paralogous genes for the chymo-trypsins are not found in genome sequences for yeast, gram eubacteria, archaebacteria, and mitochondria they hypothesis that chy1 arose from horizontal gene transfer.

Entomopathogenic fungi also produce insecticidal toxins. The early literature on toxins from entomopathogenic fungi was reviewed by Roberts (1981) and more recently by Strasser et al. (2000). Several metabolites from entomopatho-genic Deuteromycetes are well known and described. For example, Beauveria spp. are known to produce beauvericin, a depsipeptide metabolite which has shown toxicity to a number of invertebrates (Roberts 1981). Not all Beauveria can produce beauvericin, but it has been isolated from Paecilomyces fumosoroseus mycelium. B. bassiana is also reported to produce beauverolides, isarolides, and bassiano-lides, all cyclotetradepsipetides. Metarhizium strains are also well known for producing toxic metabolites, the best described of these are the destruxins. These cyclodepsi-peptides are toxic to a number of insects, but susceptibility varies considerably, ranging up to 30 times between silkworm larvae and Galleria (Roberts 1981). Hirsutellin A is produced by Hirsutella thompsonii and is not proteolytic, but was toxic to a range of insects (Mazet and Vey 1995). Aspergillus species are occasionally insect pathogens and are known to produce many insecticidal metabolites. However, the occurrence of aflatoxin production in many Aspergillus that infect insects has restricted interest in this group, although it is by no means necessary that insecticidal strains produce aflatoxins in any appreciable amount (Roberts 1981). Not all entomopathogenic fungi produce toxins in the disease process. In some cases, toxins are suspected, but not conclusively demonstrated. Injection of culture filtrates of some entomopathogenic Entomophthorales into Galleria sp. resulted in blackening similar to that found in fully infected larvae [e.g., Roberts (1981)]. Some of the lower fungi, such as Coelomomyces and the Entomophthorales, may possess only weak toxins, if any at all. It is more likely they overcome hosts by utilizing the nutrients and invading vital tissue (Roberts 1981).

Some entomogenous fungi produce antibiotics. As entomopathogenic fungi must compete for utilization of cadavers with numerous resident and environmental bacteria, it is not surprising that a number of antibiotics are produced by the various strains and species. Hirsutella and the allied genus Cordyceps also produce a number of metabolites that may be weak toxins or antibiotics. Krasnoff and Gupta (1994) described an antibiotic, phomalactone, from the H. thompsonii var. synnematosa that was also toxic to apple maggots, Rhagoletispomonella (Dipt., Tephritidae). Phomalactone was inhibitory to other entomopathogenic fungi (Beauveria, Tolypocladium, and Metarhizium). Cordyceps-infected caterpillars are a traditional medicine in parts of Asia. This may be partly based on the production by Cordyceps of a weak antibiotic, cordycepin. Zabra et al. (1996) reported that metabolites from Z. neoaphidis had antibacterial activity. In the future, bioactives from entomopathogenic fungi may have a role in pest insect control, either formulated as pesticides, or through transgenic expression. Direct toxicity may not be the only aim, as some toxins or metabolites have antifeedant type activities (e.g., http://www.item.ba.cnr.it/biopesti.htm).

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