The scanning electron microscopic study of the infection and conidial development of Aspergillus tamarii Kita on its host, the silkworm, Bombyx mori Linn.

Development of Aspergillosis on the integument of the silkworm, Bombyx mori Linn., was examined by scanning electron microscopy. Aspergillosis is a fungal disease caused by an insect mycopathogen Aspergillus tamarii Kita, which infects the silkworms in countries where sericulture (the rearing of silkworms)is prevalent. The present study showed the course of infection and the conidial development of A. tamarii on the integument of B. mori. Five different strains (KA, NB18, NB4 D2, NB7 and PM) of B. mori were inoculated on their body surface with ca. 1 × 106 conidia/ml. Among the five breeds tested, the conidial germination was greatest on the larval surface of KA breed, and least on PM. Most of the conidia germinated on the cuticle approximately 8–12 hours after inoculation, forming a suctorial appressorium within 24 hours. The hyphae reached the hemocoel, where they grew and multiplied extensively, forming a mycelial complex and causing death of the host larva in about 5–6 days. The death of the host was followed by growth of the fungus through mesodermal and epidermal tissues, leading to larval mummification about 6–7 days post-inoculation. Extensive aerial outgrowths of the fungus followed, mostly through the intersegmental regions of larvae. Abundant branched conidiophores developed, forming a confluent yellow brown mat over the entire host body 7 days after inoculation. Each conidiophore had an apical vesicle bearing numerous phialides from which conidia were developed in long chains.

Comparative analysis of the in vivo and in vitro metabolites produced by the entomopathogen Beauveria bassiana

Beauveria bassiana, like other insect mycopathogens, has evolved mechanisms to penetrate the insect exoskeleton via germ tubes and to replicate in the host hemocoel. Our initial studies have shown that biologically active metabolites released in the hemolymph during the vegetative growth phase of B. bassiana disrupt the host immune response and metamorphosis. These components cause an immediate reduction in filopodial-producing hemocytes and an increase in the level of serum phenoloxidase. Radiolabeling of tissues explanted from healthy versus infected larvae has demonstrated both the induction and repression of polypeptides in B. bassiana infected hemolymph. None of the polypeptides detected with 35S pulse labeling were responsible for the cytotoxic and insecticidal activities detected in infected hemolymph. Western blots of SDS gels containing chromatographic fractions from healthy and infected sera probed with both antibodies against B. bassiana cell homogenates and culture filtrates contained a complex of antigens. The results of lectin labeling and sodium periodate treatments suggested that carbohydrates were the major epitopes being recognized by both monoclonal and polyclonal probes. Key words: insect mycopathogen, Beauveria bassiana, fungal metabolites, entomopathogen, Spodoptera exigua.