38th Annual Meeting of the Society for Invertebrate Pathology

How does transmission of microsporidia occur in the field? The known portion of the life cycle of microsporidia in the genus Nosema (for example) is as follows: after ingestion by the host, infective spores germinate in the midgut lumen of the host larva. The polar filament pierces the midgut epithelial and muscle cells of the host and the sporoplasm enters the cell. After a cycle of vegetative reproduction, primary spores are produced, which immediately extrude their polar filaments. Filaments allow spores to infect target tissues such as salivary glands, fat body, and Malpighian tubules. The release of spores from some of these targeted tissues into the environment, in silk and/or feces, is a key step of transmission, but details of this process are poorly understood. For the past two years (2004 and 2005) we have attempted to quantify transmission of a microsporidian infection in forest lepidopteran larvae: as far as we are aware this is the first time such an experiment has been performed under semi-field conditions. The microsporidium we used for this work was a Nosema sp., isolated from a L. dispar population near Levishte, Bulgaria. L. dispar larvae (1st day in 3rd instar) were inoculated and marked by clipping the first left proleg. A study plot was established on a young oak plantation with trees of about 2 to 3 m height near Cifare, Slovakia. Cages (1x1x2 m) were installed around these trees, and infected and uninfected L. dispar were placed in the cages. Larvae were removed after 21 d of exposure. Initially-infected and test larvae were separated according to proleg markings. Initially-infected larvae were frozen and diagnosed for microsporidia under phase contrast microscopy (400X). Susceptible larvae were reared individually for another 11 d before microscopic examination.