Optimising the application frequency of VectoMax® FG for the control of Ae. albopictus and Culex spp. in the urban environment: findings 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 B. sphaericus exhibits prolonged residual efficacy 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 efficacy, 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, 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 Aedes 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. Moderate (> 50%) reductions in mosquito abundance were sustained for up to 10 weeks following treatment, with Culex spp. exhibiting persistent suppression exceeding 80% for up to 8 weeks, and Ae. albopictus maintaining comparably high levels of suppression for up to 5 weeks. Conclusion Our findings show that reapplication of VectoMax® FG at 5-week intervals is necessary for sustained suppression of Ae. albopictus and Culex spp. While Culex responded well even at longer intervals, Ae. albopictus required more frequent treatment to avoid rebound. Optimised application frequency not only enhanced control but also reduced variability in mosquito abundance, highlighting the importance of locally tailored treatment schedules in integrated vector management.