Non-maize hosts outperform maize in sustaining fall armyworm population during off-season irrigation in the tropics

The fall armyworm (FAW), Spodoptera frugiperda, has emerged as a major global threat to maize production due to its highly invasive nature, absence of diapause, and capacity for continuous reproduction in tropical environments. While off-season irrigated maize is generally recognized as a "green-bridge" for FAW population continuity between seasons, yet its actual contribution relative to alternative hosts remains unclear. To quantify the roles of maize and non-maize hosts in sustaining FAW populations across seasons under climatic continuity and cropping system structures typical of tropical agroecosystems, we assessed FAW seasonal persistence by integrating controlled cage experiments with intensive pest surveys across 123 fields during both irrigated and rain-fed seasons in the Koga Irrigation Scheme, northwestern Ethiopia. Host preference assays revealed that FAW exhibited little host discrimination, feeding readily on finger millet and barley and causing 100% plant mortality, while wheat exhibited 75% severity by 30 days after larval hatching (DAH). Tef sustained significantly slower damage progression (10%, P < 0.001), yet suffered total destruction in the absence of maize. Field survey exhibited near-ubiquitous FAW presence (99.2% prevalence), with 4.3-fold increased FAW incidence and 2.9-fold increased severity (P < 0.001) during the irrigation season. Structural equation modeling further showed that potato and finger millet exert the strongest positive effects on FAW incidence (β = 0.73 and β = 0.65, respectively; p < 0.001), followed by maize (β = 0.49, p < 0.01). Elevated infestations after crop rotations, combined with the minimal host preference, demonstrated that FAW persists throughout the irrigation season regardless of crop type. This makes off-season irrigated fields a critical "green bridge", with some alternative hosts contributing better than maize in sustaining FAW populations. Integrating intensified off-season FAW management strategies into a coordinated, landscape-level framework would be essential for lowering population carry-over and mitigating pest pressure sustainably in the subsequent cropping seasons.