Effect of Nosema on Mortality and Histopathology of Nosema-infected Plutella Xylostella (Lepidoptera: Plutellidae)

Biological control, using pathogenic microsporidia, may have the potential to be an alternative to chemical control against the diamondback moth (DBM), Plutella xylostella (Lepidoptera: Plutellidae). The microsporidium, Nosema bombycis (NB), is one of many disease agents that can be used in the Integrated Pest Management (IPM) of DBM. However, its pathogenicity or effectiveness can be influenced by many factors, especially temperature. As such, this study was conducted to investigate the effect of temperature on the NB infection of DBM larvae. Infection was performed on second instar larvae at different dose (spore number/concentration) levels (0, 102,103 104,105), at temperatures of 15°, 20°, 25°, 30° and 35 °C, and at 65% RH, 12:12 (L: D) and larval mortality were recorded. The spore concentration had negatively affected larval mortality at all temperatures, although this effect was more pronounced (92%) at 35 °C than at 20° and 30 °C (≃50%) and 25 °C (26%). Histological observations showed that Nosema preferentially infected adipose tissue and the epithelial cells of the midgut resulting in marked vacuolization of the cytoplasm. These findings suggest that Nosema exert their negative effect by damaging the midgut epithelial cells.