In vitro biocontrol assays of Spodoptera frugiperda using indigenous entomophagous fungi from Ivory Coast

Abstract: Native to tropical and subtropical regions of America, the fall armyworm,
Spodoptera frugiperda, is an extremely polyphagous pest, which feeds on the leaves and stems of more than 353 plant species and is known for its very high reproductive rate. It causes significant damage to maize, tomato and other food crops in Africa and is also a potential major threat to central-southern Europe, where EPPO reports recently increased. Chemical control proved to be ineffective and led to the development of resistance, the reduction of natural enemy populations and environmental pollution. In this context, entomopathogenic fungi (EPFs) could be a valuable alternative. In this work, conidial suspensions of 5 isolates of Metarhizium spp. (T34, T132, T121, T313, T141), 2 of M. anisopliae (T331 and T35) and 3 of B. bassiana (A211, A214a and A214b) were applied on the 3rd larval stage of S. frugiperda. Larval mortality was observed from day 2 after application, with rates ranging from 10 % to 63.33 %, reaching 66 % to 100 % on day 9. All strains were sufficiently and quickly efficacious, with a LT50 < 4.98 days. B. bassiana strains demonstrated the fastest activity, and all tested strains revealed a high potential for controlling S. frugiperda. As EPFs may have a broad spectrum of action and can
infect many insect species, with a risk of secondary effects on non-target organisms, we
assessed the environmental safety of these entomopathogenic fungal strains. At this end, the
best performing strains against S. frugiperda (M. anisopliae T331, Metarhizium sp. T34, B.
bassiana A211, A214a, A214b) were applied in vitro to adult individuals of Apis mellifera. To
test chronic and acute intoxication in in vivo conditions, the EPFs were applied in increasing
concentrations to adult bees. In both oral and cuticular applications, only strains T331 (M.
anisopliae) and T34 (Metarhizium sp.) showed increasing mortality with higher concentrations. The strains A214a and A214b caused no harm to honeybees and could be retained to enter the registration process for the development of a commercial insecticide to control S. frugiperda, which would offer promising perspectives for the sustainable management of the fall armyworm.