Oct 4, 2022
Trio of publications explain regional mosquito patterns
Like most residents of the Midwest, St. Louisans are no strangers to mosquitoes. As the weather gets warm, it becomes common practice to swat at exposed skin where the insects land to feed, from the Arch all the way out to the Ozark foothills. Most do not stop to get a closer look at those mosquitoes as they close in; the telltale high-pitched buzz of flight is often enough to set people’s palms at the ready to strike.
For Tyson Research Center scientists Dr. Katie Westby and Dr. Kim Medley, however, “getting a closer look at mosquitoes” is in the job description. As leaders of Tyson’s mosquito lab, they set out to explore the abundance of these insects across the St. Louis region, paying specific attention to the nonnative Aedes albopictus. Also known as the tiger mosquito, Ae. albopictus is one of the most globally invasive species, recorded on every continent except Antarctica and recognized as an important disease vector. In St. Louis particularly, Ae. albopictus has risen to become the region’s prevailing mosquito species.
Katie Westby
Kim Medley
Yet, to become a dominant species is not a simple task. A trio of recent publications from Tyson’s mosquito lab study the complex interactions between Ae. albopictus and its competitor Ae. triseriatus to determine how Ae. albopictus has come to rule - and to find the areas where it has fallen short.
WHERE HUMANS DOMINATE, ALBOPICTUS DOES TOO
These days, one cannot go far at Tyson Research Center without hearing the words “urbanization gradient.” This term, referring to the gradual transition of urban to rural landscapes branching westward from downtown St. Louis, features in studies ranging from wildlife monitoring to plant pathogen tracking. Mosquito research, too, embraced this concept with a 2021 study eyeing its subjects through the lens of urban ecology.
Map of mosquito breeding cups placed along the St. Louis urbanization gradient. (Rural: light gray, Suburban: medium gray, Urban: dark gray)
Tyson undergraduate fellow Tejiri Agbigbe measuring water level in an urban mosquito breeding cup during summer 2018.
Westby et al., in this Insects publication, ask whether Ae. albopictus abundance and phenological patterns differ based on land use type. That is, do we see the same number of tiger mosquitoes in urban areas versus rural areas? Are their life cycles progressing at the same rates in each habitat? The research team placed 48-56 mosquito breeding cups across the St. Louis urbanization gradient and sampled larvae weekly during the summer months of 2017 and 2018. At each cup they also captured data on water chemistry, water evaporation rates, and microclimate - the temperature and humidity in the immediate surroundings of each cup. Notably, larval abundance of Ae. albopictus was significantly higher among the urban and suburban sites compared to the rural sites. Additionally, these more forested areas saw a significant delay in larval detection, and rural Ae. albopictus populations did not grow over the course of the summer the way populations closer to the city did.
Thus, here in St. Louis Ae. albopictus and humans appear to thrive in the same areas - those characterized by roads and buildings, by asphalt and artificiality. The abiotic factors measured at each cup were a start at asking why, but a fuller understanding required researchers to look at biotic factors as well. For mosquitoes, that meant zooming in to interactions in the larval habitat, the place where it all begins.
PRIORITY EFFECTS A CONTRIBUTOR TO REGIONAL MOSQUITO PATTERNS
Dotted across the woods at Tyson are a collection of large black buckets, filled with water, plant matter, and, if the time is right, mosquito larvae. For artificial container-breeding mosquitoes such as Ae. albopictus, which often lay eggs in debris such as old car tires or flower pots, these buckets make for ideal places for the insects to start their life cycle…at least, when nobody else is around.
Aedes triseriatus, or the Eastern treehole mosquito, is another species found in St. Louis. As the name implies, Ae. triseriatus prefers to breed in natural spaces such as treeholes, but they can also be found breeding in artificial containers. Much is still to be learned about the mechanisms of coexistence between Ae. albopictus and Ae. triseriatus, but generally fortune seems to favor the former. As one of the world’s most successful invaders, Ae. albopictus often causes local declines of resident mosquito populations in the areas where it becomes established. Prior research has posited that while Ae. albopictus is unlikely to take over treeholes as a larval habitat, it shows promise in outcompeting Ae. triseriatus in artificial breeding containers.
Kim Medley checks on a set of mosquito breeding buckets at Tyson Research Center.
This was the prediction Westby, Juliano, and Medley tested in their Journal of Medical Entomology publication. Using repurposed data collected at Tyson’s artificial containers during 2007-2017, they found instead that Ae. albopictus failed to dominate Ae. triseriatus in these larval habitats throughout the decade of study.
Why? This answer lies at least partially in the final publication of the trio, appearing in Biological Invasions. Here, Westby et al. brought the concept of priority effect into focus as a mechanism dictating larval habitat function. Priority effect (PE) operates like a race, wherein one species gets the advantage of a head start over another. Among St. Louis mosquitoes, Ae. triseriatus eggs naturally hatch earlier in the season than do Ae. albopictus eggs. When Ae. albopictus does finally emerge, the larvae face the challenge of not only larger and more developed competitors but a denser population of them as well - this is PE at work.
Westby et al. used both laboratory and field experiments to ultimately conclude that PE reduced Ae. albopictus survival and therefore its rate of population increase. Such a result, when considered on the macro scale, could help explain the regional pattern of mosquito inhabitation introduced in Insects. In urban and suburban areas, where artificial containers dominate the landscape, Ae. albopictus thrives virtually alone. In rural, forested areas, where both treeholes and artificial containers are found, priority effects are brought on by otherwise “less competitive” species like Ae. triseriatus help keep the invader Ae. albopictus in check.
THE POPULATION PICTURE
While mosquitoes are fascinating critters on their own, as humans we are probably most interested (and concerned) by their interactions with us, rather than interactions between two of their species. We focus on the most localized of interfaces, the tiny landing pad where mosquito meets skin, where a hand comes down to swat it away. However, urbanization, public health - these are all forces much bigger than the individual. The tiger mosquito is a vector of several diseases, demonstrated to prefer urban landscapes in a world where more than 50% of the global human population now resides in cities. Protecting ourselves means understanding the movements of Ae. albopictus - where it is, and why it’s there.
“There is no mosquito-transmitted disease where there are no mosquitoes,” Westby says, “so it is crucial to understand how the landscape contributes to abundance patterns of important vector species.”
About the author
Colleen McDermott is an undergraduate at Washington University in St. Louis studying environmental analysis and writing. They created Placemaking Tyson during summer 2022 and currently serve Tyson as Communications Associate and TCC Student Coordinator.