Evaluating the Interplay of Barrier Strategies and Monitoring Systems for Migratory Ochlerotatus caspius from Rice-Fields

Abstract Background Mosquito nuisance and arboviral risk are increasing in peri-urban Europe, where species such as Ochlerotatus caspius migrate from rice fields into settlements. Ultra-low volume (ULV) spraying remains a practical intervention, but its effectiveness depends on precise timing, meteorological alignment, and standardized evaluation. Methods A field trial was conducted in Alessandria Province, northern Italy (August 2025), at two peri-urban sites: Antenne and Montecastello. Vehicle-mounted ULV spraying with natural pyrethrum (ONLY PY®) was applied at 10 g a.i./ha along a fixed route at ~ 7 km/h, timed for the first 50 minutes after sunset. Mosquito abundance was monitored using CO₂-baited light traps in 20-minute intervals before and after spraying (24–48 h). Meteorological conditions were recorded, and reductions were analyzed using control-adjusted formulas and negative binomial GLMMs. Results Pre-treatment captures confirmed high evening abundance dominated by O. caspius . At 24 h, reductions ranged from 52.8% to 100% at Antenne and 46.7% to 81.5% at Montecastello. At 48 h, Antenne maintained complete suppression, whereas Montecastello exhibited partial rebound due to reinvasion from nearby habitats. Conclusion ULV pyrethrum spraying achieved rapid knockdown when aligned with vector activity and favorable weather. Persistence of suppression was site-specific, shaped by landscape connectivity. ULV should be integrated within broader vector management strategies.