Large-scale performance assessment of the BG-Counter 2 used with two different mosquito traps

Background: Mosquitoes are important vectors of pathogens. They are usually collected with CO ₂ -baited traps and subsequently identified by morphology. This procedure is very time-consuming. Automatic counting traps could facilitate timely evaluation of the local risk for mosquito-borne pathogen transmission or decision-making on vector control measures, but the counting accuracy of such devices has rarely been validated in the field. Methods: The Biogents (BG)-Counter 2 automatically counts mosquitoes by discriminating the size of captured objects directly in the field and transmits the data to a cloud server. To assess the accuracy of this counting device, 27 traps were placed at 19 sampling sites across Germany and used in daily, weekly or bimonthly intervals from April until October in 2021. The BG Counter 2 was attached to a CO ₂ -trap (BG-Pro trap = CO ₂ -Pro) and the same trap converted to also attract gravid mosquitoes (upside down BG-Pro trap with a water container beneath, CO ₂ -Pro-gravid). All captured mosquitoes were identified by morphology. The number of females (unfed and gravid), mosquito diversity and the number of identified specimens in relation to the counting data of the BG-Counter were compared between both trapping devices to evaluate sampling success and counting accuracy. Results: In total 26,714 mosquitoes were collected during 854 trap days. The CO ₂ -Pro-gravid trap captured significantly more mosquitoes per trap day for all specimens, gravid females and non-gravid females, while there was no difference in the mosquito diversity. The linear model with the captured mosquitoes as response and the counted specimens as a predictor only explained little of the variation within the data (R² = 0.16), but per individual trap the value could reach up to 0.62 (mean R² = 0.23). The counting accuracy for the daily samples had a significant positive correlation with sample size, resulting in a higher accuracy for the CO ₂ -Pro-gravid trap and higher accuracy for sites and sampling months with high mosquito abundance. Conclusion: While the accuracy of the BG-Counter 2 is quite low, the device is able to depict the mosquito phenology and inform about local population dynamics.