Laboratory and field evaluation of the fungus Chrysosporium lobatum against the larvae of the mosquito Culex quinquefasciatus

Eleven fungal species in three genera were isolated from the soil at Agra, India by the feather-baiting technique. Out of the 11 species, Chrysosporium lobatum Scharapov, a deuteromycetous (Moniliales: Moniliaceae) fungus, caused high mortality of Culex quinquefasciatus Say (Diptera: Culicidae) larvae in the laboratory. Laboratory and field trials were carried out to study the efficacy of C. lobatum against various instars of C. quinquefasciatus. In the laboratory bioassays, first, second and third instar larvae were assessed separately. Conidia of C. lobatum that were 15 days old were used both in laboratory and field bioassays. Six different concentrations were used in the laboratory bioassays (10, 10³, 10⁴, 5 × 10⁴, 10⁵ and 10⁶ conidia/mL), and one concentration was tested in the field trials (10⁶ conidia/mL). The LC₅₀ values with 95% fiducial limits and probit equations were calculated by the probit analysis. Significant (analysis o.f variance (ANOVA): P < 0.0001) mortality difference between the instars of C. quinquefasciatus was observed after 72 h of exposure to conidia of C. lobatum. The third instar C. quinquefasciatus were 319- and 25-fold more susceptible to C. lobatum (0.47 × 10³ conidia/mL) than the first and second instars, respectively. The cuticle of the abdominal region and anal papillae were densely covered by the C. lobatum conidia. In the field trials, populations had declined significantly (t test, P = 0.003) 15 days after inoculation in the four test pools. Significantly (t test, P = 0.03) higher mortality was observed in the pools with water quality that was lower in total dissolved solids, hardness, chemical oxygen demand and conductivity. Based on the performance and survival in the trial, C. lobatum may be considered as a bio-control agent of mosquitoes.