Evaluation of VectoMax FG application frequency for the control of Aedes albopictus and Culex species in urban catch basins: evidence from a randomised controlled trial.

BACKGROUND: VectoMax FG (Valent BioSciences, Libertyville, IL, USA) is a biological mosquito larvicide, combining Bacillus thuringiensis var. israelensis and Lysinibacillus sphaericus. Bacillus thuringiensis var. israelensis demonstrates a low propensity for resistance development, whereas L. sphaericus exhibits prolonged residual effectiveness in organically polluted aquatic environments. The manufacturer recommends treatments at least every 4 weeks; however, recent evidence suggests that less frequent applications may achieve comparable effectiveness, which is important for reducing operational costs related to larvicide volume and labour as well as reduced environmental exposure. METHODS: To provide data-driven guidance for vector control programmes targeting Aedes albopictus in catch basins, we conducted a randomised controlled trial in Basel, Switzerland, from May to October 2024. A total of 180 catch basins, randomly selected from 768 basins in an urban area infested with Ae. albopictus, were assigned to treatment intervals of 2, 4, 6, 8 or 10 weeks, alongside untreated controls. Emergence traps were used to capture adult mosquitoes developing from larvae within the basins, allowing comparison of mosquito abundance reductions across treatment frequencies. Generalised additive and linear mixed effects models were applied to quantify the effects of larvicide application frequency, temperature, precipitation and time since treatment on mosquito and non-target dipteran populations. RESULTS: Suppression of all taxa peaked within 20-30 days post-treatment. Over 50% reductions in mosquito abundance were sustained for up to 10 weeks following treatment, with Culex spp. exhibiting persistent suppression exceeding 90% for up to 6 weeks, and Ae. albopictus maintaining comparably high levels of suppression for up to 4 weeks. CONCLUSIONS: While Culex spp. responded well even at longer intervals, Ae. albopictus required more frequent treatment to avoid rebound. Our findings show effective (> 90%) suppression in both Ae. albopictus and Culex spp. when VectoMax FG was reapplied at 4-week intervals. Increased application frequency not only enhanced overall effectiveness but also reduced variability in mosquito abundance, contributing to more stable vector control.