Activity of entomopathogenic fungi against fall armyworm, Spodoptera frugiperda: comparison of conidia produced on artificial media and insect hosts

Corn (Zea mays) is grown over millions of hectares worldwide. Fall armyworm, Spodoptera frugiperda, is an ubiquituous pest over most of the corn-growing areas of the Americas. Entomopathogenic fungi (EF) are insecticidal against larvae of this pest. One concern is possible changes in virulence when EF are cultivated on artificial media. We compared the activity of conidia of one strain each of Beauveria sp., Isaria fumosorosea, and Metarhizium brunneum (Ascomycota: Hypocreales), produced either on infected insect hosts (harvester ants, Pogonomyrmex barbatus), or on artificial medium (potato dextrose agar). Fungi were passaged successively 3-4 times on either insects or agar. Conidia were harvested and suspended in water + 0.05% surfactant (Bionex ^TM, Arysta). Conidial concentrations were similar for the two substrates/fungus. Alfalfa (Medicago sativa) leaflets were dipped in fungal suspensions and placed individually with one 2nd-instar S. frugiperda larva in 50 ml cups (n=60 for each fungal treatment). After five days, larval mortality of control was 8%; the fungi killed 47% (Beauveria insects, 5.2 x 10⁶), 45% (Beauveria medium, 8.7 x 10⁶), 65% (Metarhizium insects, 2.1 x 10⁷), 53% (Metarhizium medium, 2.1 x 10⁷) and 100% (Isaria medium, 1.3 x 10⁷) (results from Isaria from insects pending). Quantities are spores/ml. Regression analysis for Beauveria- and Metarhizium- induced mortality lines indicated that mortality progression was not different between species and (more importantly)  for each fungus, mortality was not different for conidia produced either in successive passage on medium, or on insects. Succesive passage on artificial substrate or natural hosts did not seem to affect the virulence of these Metarhizium and Beauveria strains.