Infectivity of Metarhizium flavoviride Gams & Roszypal to the Gregarious Nymphs of Schistocerca gregaria (Forskal) and the Enhancing Effect of Phenylacetonitrile on Mortality

Abstract

Conidia of M. flavoviride Gams & Roszypal (Mfl 5) were formulated in cotton seed oil at different concentrations of 102 to 108 spores/ml, and applied topically to the desert locust gregarious nymphs [Schistocerca gregaria (Forskal)], under the laboratory conditions at 27± 0.30°C and 74.1±2.11% RH. After infection of the nymphs, food consumption was reduced as well as insect responsiveness to mechanical stimulation, followed by change in colour and finally death clinging to the wire mesh of the cages by their legs. Mortality rate was dose dependent, where 100% mortality was observed at high concentrations of 107 and 108 spores/ml, five days post-inoculation, while only 36.6% mortality was recorded at lower concentrations of 102 and 103spores/ml, eleven days post-inoculation. The LD50 was calculated as 1.5x105spores/ml five days after application of the inoculum. However, mediation of the atmosphere in the cages with phenylacetonitrile, adult aggregation pheromone major component, four days prior to inoculation, predisposed the nymphs severely to the lower doses of 102 to 104 spores/ml previously used. When nymphs were exposed to 0.01% concentration of phenylacetonitrile, a significant increase in insect mortality ranging between 37% and 55% was recorded over the nymphs treated with the fungus only (control).