It rains a lot in South Louisiana. Intuitively, rain creates standing water where most mosquitoes lay their eggs and rear their offspring as larvae and pupae. Most mosquito species’ populations demonstrate a positive association with increasing amount or frequency of rainfall. Counterintuitively, the southern house mosquito bucks this trend — this West Nile vector is associated with the lack of rainfall. Additionally, since rain dilutes nutrient pollution from OWTS, the impact of rainfall on sewage discharge is also critical to understanding sewage-mediated determinants of mosquitoes. Research studies launched this year aim to understand the relationship between rainfall and the southern house mosquito and are yielding some exciting insights into population triggers.
Beginning in early May, STPMAD Field Biologists Briana Hornsby, David Giron, Sydney Ferguson, Eric Martin, and Haley Marquette began observing the presence of mosquito larvae and mosquitofish weekly at 60 precise locations. Locations were chosen throughout St. Tammany as transects of 20 sites within three residential blocks for each biologist (a total of 15 block transects across all five biologists-300 observations weekly). Individual observation sites were established every 18 ft. and marked on the pavement for weekly visits (see images below). This systematic approach allows us to control static environmental circumstances (such as ditch elevation) and to evaluate changes in the distribution of mosquitoes and mosquitofish that are the result of time, such as rainfall.
We analyzed the effect of the total rainfall (provided by the NWS) from the seven days prior to these observations and found that fish and mosquito larvae are more likely to be found together when sites experience greater rain totals. Further, we evaluated how the number of days since the last appreciable rain event (>0.25 in. of rain) effects the distribution of these organisms. This analysis sensitively predicts the presence of all three conditions – mosquito only, fish only, and both fish and mosquitoes. The greater number of days without appreciable rainfall prior to an observation, the more likely fish will be absent and mosquitoes will be present. This model is further improved when controlling for the static characteristics of the ditch being observed.
Why is it important to understand both the natural rain-mediated triggers and man-made sewage-mediated triggers of the southern house mosquito? The answer is simply prediction. The more we know about the precursors of mosquito production, the more likely we will be able to predict where and when problems may exist. Figure 8 demonstrates a map of the “where”–drier conditions that may be conducive to mosquito production. The “where” and “when” are necessary for proactive larviciding and environmental management, so that we may minimize vector populations and thereby reduce human risk from arboviruses.