Laboratory and field trials reveal the potential of a gel formulation of entomopathogenic nematodes for the biological control of fall armyworm caterpillars (Spodoptera frugiperda)

The fall armyworm (FAW), Spodoptera frugiperda Smith (Lepidoptera: Noctuidae) can cause tremendous yield losses in maize. Its invasion into Africa and Asia has dramatically increased the use of insecticides in maize agro-ecosystems. Safe, effective and readily available alternatives are urgently needed. Entomopathogenic nematodes (EPN) represent a promising and sustainable option to control fall armyworm caterpillars on maize. Commonly used against soil insect pests, EPN can also be applied to control above-ground pests if formulated appropriately. We explored the possibility to control FAW by incorporating the EPN species Steinernema carpocapsae into protective formulations that can be easily applied into the whorl of maize plants, where the caterpillars mostly feed. We tested this approach in laboratory cage experiments as well as in field trials. In the laboratory, treating maize plants with a low dose of S. carpocapsae (3000 infective juveniles per plant) formulated in a carboxymethyl cellulose (CMC) gel caused 100% mortality of FAW caterpillars and substantially reduced plant damage, whereas EPN applied in water or a surfactant-polymer-formulation (SPF) caused 72% and 94% mortality, respectively. Under field conditions, one-time treatments with S. carpocapsae applied in water, SPF or CMC decreased plant damage, but only the EPN-gel formulation significantly reduced FAW infestation. As compared to control, about 40% fewer caterpillars were found on plants treated with EPN formulated in the gel. Notably, the EPN-gel formulation was as effective as a standard dose of cypermethrin, a pyrethroid insecticide commonly used against FAW, in reducing FAW infestation. Repeated applications may be needed to reduce re-infestations by FAW across a whole cropping season depending on the local maize phenology and pest dynamics. These findings demonstrate that EPN, when properly formulated, are excellent candidates for the biological control of FAW, and can be a safe and sustainable alternative to synthetic insecticides.