Route of invasion and histopathology of Metarhizium anisopliae in Culex quinquefasciatus.

Abstract The fungus Metarhizium anisopliae , known to be a pathogen of terrestrial insects, has been shown to be a potentially successful biocontrol agent of the Culicidae. Two methods of inoculation of Culex quinquefasciatus were examined and compared as to rate of kill, route of invasion, histopathology and pathogenesis, and presence or absence of toxin-related mortality. One method of inoculation involves covering the surface of the water containing the larvae with dry, hydrophobic conidia. Larvae contact the fungus when they break the water tension with their perispiracular valves for air intake. Histopathological studies reveal complete or near complete blockage of trachea with mycelial growth, beginning at the siphon tip and extending only one-third of the way into the trachea. Extensive hyphal growth and formation of appressoria with a subsequent melanization reaction by host tissue were clearly observed in the portion of the siphon invaded, but no hyphal bodies were evident, as occur in terrestrial insects. The waxy-coated conidia adhere to the inside surface of the valves, germinate, and invade the siphon tip tissue, much like the first stages of invasion seen in terrestrial insects. Death did not begin in the treated population until 48 hr post-treatment and appeared to be due primarily to suffocation. An alternate route of exposure examined is ingestion of detergent-wetted conidia that sink to the bottom of the container of water. When ingested, the conidia completely filled the larval gut, causing mortality to begin within 6 to 24 hr after ingestion. Toxin is apparently released during digestion; partially digested conidia are evident in electron micrographs of midguts of moribund larvae. Some conidia were shown to be at the begining stages of germination in the midguts of moribund larvae but no invasion of tissue was apparent. In both treatments, conidia must be viable in order to cause mortality in mosquito larvae.