Assessing Near-Infrared Spectroscopy (NIRS) for Evaluation of Aedes aegypti Population Age Structure

SIMPLE SUMMARY: Mosquito-borne pathogens require the obligate mosquito vector to shuttle the pathogen between vertebrate hosts. This typically requires the mosquito to acquire the pathogen from an initial bloodmeal, have the pathogen mature and reach the mosquito salivary glands and be transmitted to another vertebrate host in the saliva during subsequent blood feedings. Depending on the pathogen, this incubation period can be up to two weeks. Considering the short lifespan of adult mosquitoes, this means that the oldest mosquitoes are responsible for a disproportionate amount of pathogen transmission. Knowing the age structure of mosquito populations in the field could provide important insights in the likelihood of pathogen transmission occurring. Unfortunately, the current methods of age grading mosquitoes in the field are limited by accuracy, technical challenges and cost. Near-infrared spectroscopy (NIRS) has been shown to be capable of age grading large numbers of mosquitoes cost effectively, although accurate age predictions are still a challenge. In this work, we compared the ability of NIRS to age grade field-collected mosquitoes with two other methods, parity and SCP1 transcript expression. While we did not find NIRS to be suitable for predicting the precise age of individual field-collected Aedes aegypti mosquitoes, we believe that this technique has the potential to monitor changes in the age structure of Ae. aegypti populations over time. ABSTRACT: Given that older Aedes aegypti (L.) mosquitoes typically pose the greatest risk of pathogen transmission, the capacity to age grade wild Ae. aegypti mosquito populations would be a valuable tool in monitoring the potential risk of arboviral transmission. Here, we compared the effectiveness of near-infrared spectroscopy (NIRS) to age grade field-collected Ae. aegypti with two alternative techniques—parity analysis and transcript abundance of the age-associated gene SCP1. Using lab-reared mosquitoes of known ages from three distinct populations maintained as adults under laboratory or semi-field conditions, we developed and validated four NIRS models for predicting the age of field-collected Ae. aegypti. To assess the accuracy of these models, female Ae. aegypti mosquitoes were collected from Maricopa County, AZ, during the 2017 and 2018 monsoon season, and a subset were age graded using the three different age-grading techniques. For both years, each of the four NIRS models consistently graded parous mosquitoes as significantly older than nulliparous mosquitoes. Furthermore, a significant positive linear association occurred between SCP1 and NIRS age predictions in seven of the eight year/model combinations, although considerable variation in the predicted age of individual mosquitoes was observed. Our results suggest that although the NIRS models were not adequate in determining the age of individual field-collected mosquitoes, they have the potential to quickly and cost effectively track changes in the age structure of Ae. aegypti populations across locations and over time.