Surveillance of South Dakota Mosquito Abundance, Infection Rate, and Insecticide Susceptibility

The aim of this dissertation was to survey and evaluate the abundance, infection rate, and insecticide susceptibility of mosquito fauna present in South Dakota. Mosquito surveillance has been conducted across South Dakota to record and track potential West Nile virus (WNV) vectors from 2004 to 2017. The nuisance mosquito Aedes vexans was found to be the most abundant species overall in the state and most abundant in many of the regions. The WNV vector, Culex tarsalis, was found to be the second most abundant mosquito and the most abundant vector mosquito across the state. However, geospatial variation did exist between both the vector and nuisance, as well as between different WNV vectors latitudinally across the state. A total of 22 mosquito species were identified, and 6 were found each year. Positive relationships were found between average Cx. tarsalis weekly abundance and average weekly human cases of WNV at a two to three-week lag. Weaker relationships existed between Ae. vexans and human cases of WNV and between the ratio of Cx. tarsalis to total mosquito abundance and human cases of WNV. We were not able to identify any relationship between yearly vector abundance and human cases. Logistic regression modeling using mean daily temperatures and total daily precipitation determined the best weekly collection period for collections to have the highest proportion of Cx. tarsalis. Infection rates of potential mosquito vectors were calculated using the minimum infection rate method. Though Cx. tarsalis infection rate was lower than other present vector species, due to its large relative abundance to other highly susceptible species it is still considered to be the primary method for human WNV infection. In testing susceptibility to the insecticide permethrin, we determined the diagnostic dose for multiple time periods and ranged from 27.0 μg/ml at 60 min to 38.4 μg/ml at 30 min. There was no significant difference detected in mortality rates between Cx. tarsalis and Ae. vexans for any diagnostic time and dose. For practical purposes, mosquitoes in 2017 were tested at 38 μg/ml for 30 min; expected mortality rates were 93.38% for Cx. tarsalis and 94.93% for Ae. vexans. Actual 2017 mortality rates were 92.68% for Cx. tarsalis and 96.12% for Ae. vexans, validating the usefulness of this baseline at an additional location and year. These findings suggest that mosquito control efforts are not selectively diminishing nuisance mosquitoes that may contribute to human avoidance behaviors that may limit exposure to WNV. These base line time and diagnostic values can allow for future studies to monitor these two important South Dakota mosquito species for resistance to insecticides