Germination, penetration, and invasion ofBeauveria bassianaon silkworm,Bombyx mori, causing white muscardine

Abstract

White muscardine is a common disease of the silkworm Bombyx mori caused by an insect mycopathogen Beauveria bassiana. The present study revealed the germination, penetration, and invasion of B. bassiana on the larval integument of B. mori. The NB 18 strain of B. mori was surface inoculated with concentrations of 1×10 6 conidia/ml. Each conidium, germinated approximately 8 h after inoculation, forming an appressorium within 24 h and the hypha entered the cuticle by forming a clear circular hole. The hyphae reached the haemocoel, where they grew and multiplied extensively, forming a mycelial complex, causing death of the host larva in about five-six days. This occurred with minimal breakdown of the internal tissues. Death of the host was followed by ramification of the fungus through the mesodermal and epidermal tissues, leading to larval mummification about six days post-inoculation. Extensive fungal outgrowths followed, mostly through the intersegmental regions of the larva, and formed aerial hyphae, which gave rise to branched conidiophores forming a confluent white fungal mat over the entire larval body in six-seven days. The life-cycle of the fungus on B. mori was completed in seven-eight days.KEY WORDS: Bombyx mori -Beauveria bassiana -White muscardine -Silkworm disease. ACKNOWLEDGEMENTS

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

“…Kumar et al [15] described the extensive fungal growth of A. flavus in the hemocoel of infected B. mori larvae 3 − 4 days p.i. Similar to the previous observations on B. bassiana [2] and on A. flavus [15], the hyphae of A. tamarii multiplied in the hemocoel by the budding of hyphal bodies. The detached hyphal bodies elongated to form vegetative hyphae.…”

“…5 days after inoculation, a large number of hyphae of A. tamarii emerged through the intersegmental regions of the integument, and developed extensive aerial hyphae that covered the entire larval surface by 7 days after the inoculation. These observations coincide with the observations in the larvae of B. bassiana and A. flavus, respectively [2,15]. Zacharuk [29] described that when the larval cuticle is highly sclerotized, the hyphae tend to grow between lamellae, and in such cases penetration to the hemocoel may take several days.…”

“…As the intersegmental membrane affords less resistance to penetration, that was the preferred site for maximum re-emergence of the fungus [15,28]. Similar to the observations on B. bassiana and on A. flavus [2,15], a large number of multi-layered crystals of different shapes and sizes were observed on the integument and in the fringes of spiracles of the infected larvae of A. tamarii (Figure 2e). However, these crystals were not observed on the integument of uninfected larvae.…”

“…and continued to occur up to 12 hours p.i. However, another study [2] suggested that the conidial germination period varied according to the texture of the substrate and the presence or absence of nutrients in Beauveria bassiana spores on B. mori. It has also been reported that the hydrocarbons in the cuticle of insects are the major source for conidial nutrition, and they influence the germination period of the conidia [16].…”

“…The diseases of B. mori have been the subject of intensive research because of their commercial importance for silk producing countries [1]. It is prone to several diseases caused by fungi, bacteria, protozoans and viruses, leading to cocoon crop loss and affecting the entire sericulture industry [2]. Among the fungal diseases, aspergillosis caused by the pathogenic species of the genus Aspergillus, is very common and spreads especially during rainy and winter seasons.…”