The effect of three entomopathogenic endophytes of the sweet sorghum on the growth and feeding performance of its pest, Sesamia nonagrioides larvae, and their efficacy under field conditions

Sesamia nonagrioides, commonly known as the Mediterranean corn stalk borer, is one of the most important pests of the sweet sorghum in the circum Mediterranean countries. In the present study, the feeding performance of larvae, post treatment with the entomopathogenic fungi Beauveria bassiana, Metarhizium robertsii and Isaria fumosorosea, was first evaluated in laboratory experiments. The efficacy of the three entomopathogens in protecting the sorghum from its pest was then examined in a tritrophic field experiment in a farm, under natural conditions. The fungi, efficiently induced the mortality of larvae and reduced their relative growth rate (RGR), with the virulence of I. fumosorosea being relatively higher (90-93%). The feeding performance of larvae was significantly affected by the three entomopathogens. Reduced food consumption (39-64%), relative food consumption rate (15-38%) and the feces produced (48-81%) compared to untreated larvae, resulted in slight changes of digestibility. The weight of treated larvae remained almost unchanged while ECD and ECI increased in infected larvae compared to control. The infected larvae were also sluggish and static, and unwieldy on the food surface. The impact of infection should be rather attributed to their altered behaviour than to disturbed digestion. The tritrophic field experiment confirmed the efficacy of entomopathogens which were experimentally introduced into the plants as endophytes, against larvae of S. nonagrioides under natural conditions. In fact, they reduced infestation by 20-30% and the tunnelling length by 19-43%, and increased larval mortality by 57-82%, without affecting sorghum growth. Consequently, the three fungi could be used for the control of S. nonagrioides in sorghum crops, at least in areas with a Mediterranean-like climate.