HISTOPATHOLOGICAL EFFECT OF SPODOPTERA LITTORALIS NUCLEOPOLYHEDROSIS VIRUS (SlNPV), BACILLUS THURINGIENSIS KURSTAKI (BTK), AND THEIR COMBINATION ON CULEX PIPIENS L. LARVAE (CULICIDAE: DIPTERA)

Abstract

The aim of the present work is to seek the histopathological alterations in Culex pipiens larvae midgut that may explain to some extent, the synergistic effect of SlNPV-Btk combination treatment which was noticed in our previous work. Third larval instar of Culex pipens were exposed to LC50 of SlNPV, Btk and SlNPV-Btk (1.3X 103 PIB/ml, 2.5 X105 spores/ml and 3.6 X 106 mixed polyhedra and spores/ml) respectively, and alterations inmidgut regionwere recorded 6, 12 and 24 h post-infection. Data showed increasing damages of Cx. pipiens larval midgut epithelium for all three treatments but acceleration in midgut alterations was more evident in case of SlNPV-Btk combination sections. The SlNPVhistopathological results revealed a serious damage on epithelial cells, Brush boarder, nuclei and basement membrane. The peritrophic membrane (pm) was not completely lysed untill 24h post-infection. Btk treatment of Cx. pipiens larvae promoted structural alteration changes in midgut and by 24h post infection, swelling and dislodging were observed in epithelial cells but no lyses. SlNPV-Btk combination had proven to bemore effective against Cx. pipiens larval midgut cells than using either SlNPV or Btk solitary. Serious alterations were recorded by 6h post-treatment.Moreover, the abnormal aggregation of mitochondria, the bulbous-shaped microvilli, and the nuclear tubules that originated from inner membrane of nuclear envelope, were all noticed only in SlNPV-Btk combination sections. By 24h, only combination sections exhibited the highly necrotic lyses midgut appearance. Biocontrol agents and their combinations may provide an excellent alternative to conventional insecticides or at least minimize their use.