Effect of Entomopathogenic Fungus Metarhizium (Nomuraea) rileyi (Farl.) Samson on the Third Instar Larvae of the Onion Armyworm, Spodoptera exigua Hübner (Lepidoptera: Noctuidae) under Laboratory Conditions

The armyworm, Spodoptera exigua Hübner (Lepidoptera: Noctuidae), caused severe destruction in the production of onion and other crops in the Philippines. This study examined the virulence of an entomopathogenic fungus against this insect pest. Metarhizium (Nomuraea) rileyi was successfully isolated from mummified larvae of S. exigua. In a laboratory bioassay, third larval instars were exposed to conidial concentrations of M. rileyi by mist spraying. The fungus was highly pathogenic to S. exigua, which caused infection starting 2 d after treatment. More than 50% of the larvae succumbed to infection 6 d after treatment. M. rileyi at 1 x 107 and 1 x 108 conidia mL-1 caused 100% mortality 7 d after treatment. Conidial concentrations caused 73–100% mortality 10 d after treatment with mean lethal time of 4.17–7.83 d. Calculated median lethal dose (LD50) from 7–10 d after treatment was 1.49 x 106 to 9.57 x 104 conidia mL-1. High epizootics due to M. rileyi resulted in low pupation rate (<20%) and adult emergence (3%). Larval duration, pupal size, and weight did not vary significantly. Our findings suggest the virulence of M. rileyi against S. exigua, which must be further evaluated to determine its potential as biocontrol agent and as an alternative management option to chemical pesticides.