Studies on the Pathogenesis of the Local Isolates of Nomuraea rileyi against Spodoptera litura

Abstract

The mode of infection of Nomuraea rileyi in Spodoptera litura larvae was examined using local strains isolated from infected larvae of S. litura collected from the field in Mae Chaem district, Chiang Mai province, Thailand. Spore suspensions were topically applied to larvae followed by examination using standard histological techniques. Germ tubes penetrated through the cuticle surface within 48 hr after inoculation and then along the cuticle. Lysis of the endocuticle was found before hyphae penetrated and invaded the epidermis ca. 2.5 days after inoculation. The invasive hyphal stage then transformed into hyphal bodies that replicated within the hemocoel. These non-invasive hyphal bodies filled the hemocoel and then converted to invasive mycelia ca. 5-6 days after inoculation. At the end of the infection cycle, mycelium emerged from the cuticle and produced condiophores ca. 7 days after inoculation. The larvae became completely covered by white mycelium before turning green 1-2 days later as a result of spore production. The complete developmental cycle of N. rileyi in S. litura lasted approximately 8 to 9 days.