Towards scalable age-grading of Aedes albopictus mosquito using mid-infrared spectroscopy and machine learning

The age structure and dynamics of mosquito populations are crucial for understanding their ability to spread diseases and assessing the effectiveness of anti-mosquito control measures. However, available methods to age-grade mosquito populations are labour-intensive and imprecise, particularly for Aedes species. We investigated the potential of Mid-Infrared Spectroscopy (MIRS) combined with Supervised Machine Learning (ML) to rapidly and accurately predict the age of adult females and males of the arbovirus vector, Aedes albopictus. First, we demonstrated the ability of MIRS-ML to age male and female mosquitoes reared under laboratory conditions. Second, we optimised the model with adults emerged from wild collected eggs reared under natural conditions in a semi-field facility, to expose them to more realistic ambient conditions. For each sex we developed three ML models based on the resolution of the predicted adult age class: low (grouping of the mosquitoes by age in 9-day interval), medium (6 days) and high resolution (3 days) from 1 to 15 or 33 days for males and females, respectively. The prediction accuracy decreased as the resolution increased. In males, the accuracy dropped from 99% (low resolution model) to 93% (medium resolution model) and 85.8% (high resolution model); in females the low and medium resolution models showed 89.4% and 78.5% accuracy, which decreased to 72.6% for the high resolution. In a simulated vector control intervention, the high-resolution models allowed to detect shifts in the age-structure of Ae. Albopictus populations with minimal sampling effort (< 100 specimens). Finally, we validated MIRS-ML on two unseen data and reconstructed plausible age structures in (1) laboratory-reared and (2) field-collected Ae. albopictus males and females. Overall, the results represent a first step towards the development of a sound and reproducible MIRS-ML approach for age-grading of Ae. albopictus populations in the wild.