209 - Battle of the Basin: Efficacy of Spinosad Against Culex Larvae in Urban Catch Basins

Abstract: In response to documented resistance to S-Methoprene across the eastern two-thirds of the Northwest Mosquito Abatement District (NWMAD), we conducted a large-scale field evaluation of Natular® XRT (Spinosad) for larval control of Culex spp. mosquitoes in urban stormwater catch basins. The study design involved stratified treatment of basins as follows:

• Natular XRT: 1/3 of basins


• Altosid® XR (S-Methoprene): 1/3 of basins, based on continued susceptibility


• Sumilarv® WSP (Pyriproxyfen): 1/3 of basins

One-month post-application, 153 basins treated with Natular XRT were randomly sampled. Each basin was assessed using three standard dips to detect larval presence. Basins were considered positive only if late instar larvae (3rd or 4th instar) or pupae were observed. Results indicated that 63% of Natular-treated basins contained advanced-stage larvae or pupae, suggesting suboptimal control.

Two primary hypotheses may explain the reduced efficacy of Spinosad in this context:

1. Physical sequestration: High organic loads in many basins likely caused the briquets to become embedded in detritus, limiting diffusion of the active ingredient into the water column.


2. Metabolic resistance: Chronic exposure to pesticide runoff from residential sources may have selected resistance mechanisms in local Culex populations.

These findings underscore the importance of considering both environmental conditions and potential resistance pathways when deploying larvicides in urban infrastructure. Further investigation into formulation behavior in high-detritus environments and resistance profiling of local Culex populations is warranted.