Pathogenic fungi as a source of insecticides
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The potential of fungal pathogens for the control of insect pests has been recognized since the letter part of the 19th century. Lethal infection of insect host involves following steps : (i) attachment of the pathogen to the host surfaces, (ii) penetration of the host cuticle by infection hyphae or germinating conidia, (iii) invasion of host haemocoel and internal organs by fungal hyphae. Host death results from the cumulative effect of mechanical damage and enzymatic digestion of insect tissues, production of mycotoxins and/or nutrient exhaustion by the parasite. Penetration of host cuticles is achieved through a combination of enzymatic and mechanical methods. A number of cuticle degrading enzymes, including proteases, lipases and chitinases, are produced during penetration. Since proteins may account for up to 70% of the cuticle, proteases play a major role in the penetration process. The correlation between pathogenicity and enzyme activity remains unclear. Many entomopathogenic fungi produce toxic metabolites: (1)nonpeptide toxins, (2) linear and cyclic peptide toxins and (3) protein toxins. Some of these compounds have been identified in mycosed insects , but their role in disease development remains unclear. Destruxins, the best known mycotoxins produced by entomopathogenic fungi, inhibit mitochondrial ATPase, secretion of fluid by Malpighian tubules, disturb function of calcium channels what leads to tetanic paralysis as well as suppress host defense. However, the knowledge of the mechanisms of fungal pathogenesis in insects is still fragmentary.
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M. I. Bogus, Instytut Parazytologii PAN, ul. Twarda 51/55, 00-818 Warszawa, Poland