Metarhizium (=Nomuraea) rileyi (Farlow) Samson from Spodoptera exigua (Hübner) Cross Infects Fall Armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) Larvae

Abstract

Mycobiocontrol is a promising management strategy in mitigating the fall armyworm Spodoptera frugiperda (J.E. Smith) infestation in the Philippines. An isolate of Metarhizium (=Nomuraea) rileyi (Farlow) Samson from onion or beet armyworm, S. exigua, which induced high mortality to this pest, was assessed against different larval instars of S. frugiperda. Surface-sterilized corn leaves were treated with different conidial concentrations and fed to S. frugiperda larvae. Cross infection of this entomopathogenic fungus to S. frugiperda was confirmed with a fungal infection that was initiated at 1–2 d post-treatment depending on the age of the larvae. Larval mortality significantly increased at 4–5 d post-treatment. Up to 100% larval mortality was recorded at 7 d post-treatment. Early larval instars (1st–3rd) were more susceptible than late larval instars (4th–6th). Higher conidial concentrations caused a higher and faster rate of larval mortality than lower conidial concentrations. The inflicted mycoses due to M. rileyi resulted in a slightly lower lethal dose (LD50) (1.44 x 105 to 9.36 x 108 conidia ∙ mL–1) and shorter mean time to death (4.51–8.89 d). Mummification of the cadaver confirmed fungal infection with white fungal growth that later changed to green during sporulation. Our findings suggest the infectivity of M. rileyi and can be explored as a biological control agent against S. frugiperda.