Written by Buyung Hadi, former SDSU Extension Pesticide Applicator Training & Urban Entomology Specialist.
Population Observations
Cool springs tend to delay the buildup of mosquito populations. Yet some sporadic rainfall events of late will provide optimum conditions for some mosquito species. Mosquito trapping efforts across the state in the last seven years showed that there are over 20 species of mosquitoes occurring in South Dakota, yet only two species dominate the surveillance data: Aedes vexans and Culex tarsalis (Figure 1).
In the trap data, A. vexans populations typically climb up in the beginning of the season, reaching hundreds per trap in June or July and steadily decrease afterward. While C. tarsalis ranks second in the numbers of mosquito specimen trapped, their populations do not normally reach the magnitude of A. vexans population levels. Culex tarsalis populations characteristically start to increase in July, staying high in August before tapering off in the fall.
West Nile Virus Risk
While both mosquitoes are capable to transmit the virus, C. tarsalis has a much higher WNV transmission rate than A. vexans. Birds are the primary vertebrate hosts for WNV where virus amplification occurs. Thus, mosquito species that alternate between birds and human, such as C. tarsalis, has a high potential to act as a ‘bridge vector’ for the virus. In contrast, A. vexans is reported to prefer human hosts, feeding on birds only occasionally. In the light of their WNV transmission capabilities and feeding preferences, A. vexans may be seen as a nuisance mosquito that do not contribute much in WNV spread while C. tarsalis should be considered an important vector mosquito with a higher risk on public health.
In the last four weeks, the overall number of mosquitoes trapped by municipal vector control agencies and university collaborators across the state has increased sharply. Aedes vexans still dominates the trap data but the number of C. tarsalis has steadily increased. South Dakota Department of Health collaborates with city/municipal vector control agencies to test trapped mosquitoes for WNV. West Nile Virus detection is conducted by testing pools of trapped mosquitoes (1-50 individuals per pool depending on the trap result) for the virus. In a recent press release, SD DOH reported that so far this year three pools of C. tarsalis mosquitoes have been tested positive for WNV, two pools from Brookings county and one from Hughes county. In the same press release, SD DOH reported the first human case of WNV for 2013. The human case was reported from Buffalo county.
Management Efforts
In South Dakota, multiple natural habitats can serve as breeding grounds for either C. tarsalis or A. vexans. Additionally, both mosquito species have extensive flight ranges. Aedes vexans was reported to have over 15 miles flight range while C. tarsalis can travel up to 3.7 miles to search for a host. In this context, the role of city or municipal mosquito control agencies in making area-wide impacts in suppressing mosquito populations, either by source reduction, larviciding or adulticiding, can never be overstated. While perimeter application around a relatively small area may protect the area temporarily, the effect can’t be expected to last. Thus, it is important to support the work conducted by your local mosquito control agencies.
Safety Considerations
There are steps that can be taken to personally protect oneself against WNV-carrying mosquitoes. While A. vexans, the nuisance mosquito, is known to search for hosts both during daylight and in darkness (with the bulk of host seeking activities conducted at night), C. tarsalis, the vector mosquito, seek bloodmeal when between dusk and dawn. Minimizing outdoor activities between dusk and dawn can consequently reduce risks of exposure against the vector mosquitoes. When conducting outdoor activities at night, wear long sleeve shirts and pants to limit mosquito exposure. Mosquito repellents are available commercially and known to provide some protection against mosquitoes. Examples of active ingredients in commonly available repellents include DEET, IR3535 and PMD (synthetic counterpart of lemon-eucalyptus oil). Typically the higher the concentration of active ingredient in a repellent product, the longer protection it provides. Environmental Protection Agency (EPA) offers an online tool to choose a mosquito repellent product based on how much protection time is needed.
