How old is that mosquito? Age grading Aedes aegypti in the desert Southwest

The ability to quickly and accurately assess the age structure of mosquito populations in the field would be a valuable tool in assessing vector control strategies, understanding vector ecology and developing predictive models. Using a variety of age grading techniques, including parity analysis, expression of the age associated gene SCP1, and near-infrared spectrophotometry, we assessed the feasibility of age-grading individual female Aedes aegypti collected from a suburban research site in Phoenix, AZ. During 2017 through 2019 monsoon season we collected over 5500 female Ae. aegypti. Approximately 15% of these mosquitoes were assayed for parity status and all parous mosquitoes were age grading based on the expression of the age associated SCP1 gene. For near-infrared spectrophotometry we generated four models based on lab reared UGAL strain mosquitoes, semi-field reared UGAL, a local Tucson colony of Ae. aegypti, and a colony established form mosquitoes collected at our study site. All collected female Ae. aegypti were age graded using NIRS. Using both SCP1 and NIRS we were observed a strong correlation to parity status, with nulliparous mosquitoes aged significantly younger than parous mosquitoes across all years. NIRS had a high degree of variation that limited its usefulness of age grading individual mosquitoes. However, the rapid, nondestructive and inexpensive NIRS analysis has promise for exploring population level changes in age structure, especially when utilized in concert with other age grading techniques.