While there are more than 60 species of mosquitoes in Louisiana, temperature and precipitation are most often the primary drivers of annual changes in presence and abundance. On the cold extreme of temperature, the entering and exiting of winter hibernation, called diapause, can be fatal to a mosquito if it’s not prepared. In cold environments in the United States, most adult mosquitoes prepare for winter by increasing sugar reserves and finding a sheltered environment. Other species of mosquitoes will survive by spending the winter in the egg stage of development.

In 2023, researchers in the Healy lab published a study that compared the impact of high temperatures on mosquito survival, biology and behavior. It was found that mosquitoes that typically spend the winter as adults freeze at colder temperatures than mosquitoes that spend the winter as eggs. Even when trying to trick mosquitoes into thinking they were engorging on sugars by feeding them artificial sweeteners instead, researchers did not see a drastic reduction in populations in freezing temperatures. While these data are not surprising, it is an excellent example of how different mosquitoes adapt to cold environments.

On the other extreme, high summer temperatures can also limit a mosquito’s ability to survive unless it has already adapted to this type of environment. While we think of Louisiana summer temperatures as being ideal for mosquitoes, very high temperatures can greatly affect the simple biological systems that are occurring within a mosquito, such as digestion and metabolism. Unless a mosquito has been well adapted to these changes, it is unlikely it can survive those extremes.

Not only does temperature affect mosquito survival, but it also affects other aspects of mosquito biology and behavior. In general, there is a relationship between temperature and how fast a mosquito develops from egg to adult. The interesting thing about this relationship is that it is so consistent that we can use it to predict when and where we might see mosquito activity. In 2019, researchers in the Healy lab published a paper comparing different populations of Asian tiger mosquitoes from New Jersey to Louisiana. Researchers evaluated how quickly mosquitoes developed from egg to adult at different temperatures. Regardless of population, it was observed that mosquitoes consistently develop at certain rates depending on temperature. This information helped to establish a “seven-day rule” for mosquito development, since at most temperatures, it takes about seven days to go from egg to adult mosquito. This allowed the AgCenter to encourage residents to check their yards for standing water once a week to help adequately reduce Asian tiger mosquitoes near their homes.

By further looking at temperature-based development rates, it was also possible to predict when mosquitoes were likely to exit winter diapause. These dates were predicted using simple formulas that took into account the environmental temperature in relation to lab-based results. Because of this, it is consistently expected that the first Asian tiger mosquitoes will emerge in the spring around February or March. Interestingly enough, these dates are similar whether you are in the cooler, northern limit of the mosquito in New Jersey or if you are in its range in Louisiana.

Recently, an AgCenter study evaluated 10 years of Baton Rouge temperature data to see if there is a trend in the emergence rates in early spring. As shown by the data, there continues to be annual fluctuations of when these mosquitoes emerge in the spring. These fluctuations in emergence and abundance will likely vary every year, depending on temperature and rainfall.

In addition to presence, researchers also evaluated if early emergence of this mosquito predicted a higher-than-normal level of activity in West Nile virus. While this mosquito is not the primary driver of West Nile virus in Louisiana, it can be used as a proxy for temperature-based development rates. It is important to recognize that temperature impacts a mosquito’s ability to transmit pathogens by modifying how quickly a virus can replicate within the body of a mosquito. There is likely complex physiology within a mosquito that balances temperature, biology and viral infection. Interestingly enough, the research did not show a direct correlation between the week in the spring in which these mosquitoes first appeared and the number of West Nile virus human neuroinvasive cases. It was noted, however, that high case numbers do seem to occur in years where mosquitoes emerge in February as opposed to March, but understanding if this relationship is real or an artifact of the methodology is still yet to be understood.

Temperature also plays many other unique roles in a mosquito’s biology and behavior. It also can affect how quickly the small, standing bodies of water that mosquitoes sometimes use as habitats dry up in an environment. In 2011, LSU researchers modeled rates in which an Asian tiger mosquito habitat would evaporate in certain temperatures as it relates to how quickly the mosquito develops. At higher temperatures, it is a challenge for mosquitoes to develop in small container habitats before the water dries up. This is interesting, because in 2023, Louisiana experienced an unusual drought. As a result, there were in fact fewer mosquitoes in collections overall. Therefore, it is important to note that too high of a temperature isn’t always going to improve a mosquito’s ability to develop quickly enough. In 2018, AgCenter research showed that a mosquito’s metabolism does decrease tremendously under higher temperature extremes. While some mosquito species might be adapted to this type of temperature and environment, it can present a challenge for others to survive.

Because immature mosquitoes, called larvae, require water to survive, adult mosquitoes lay eggs in areas that are either flooded with water or will eventually become flooded. Mosquito larval habitats range from small, standing water habitats, such as cups, tree holes or rock pools, to much larger permanent habitats, such as swamps and swimming pools. Preference for the type of habitat will be different depending on which mosquito species is present. For example, the Asian tiger mosquito loves backyard habitats near people’s homes. While we find larvae in abundance in backyard tires and buckets, we would not expect to see this species laying eggs in swamps. As a result, precipitation is more likely to affect this species, as it depends on those small containers to get flooded during rain events.

While these are only a handful of the many ways that temperature affects mosquitoes, these relationships become more complex as we also understand the role precipitation plays in their presence as well.

Kristen Healy is an associate professor of medical entomology and public health entomology. Healy leads the AgCenter’s research into mosquito biology, behavior, surveillance and control through her own studies and as director of the Healy Lab.

This article appeared in the spring 2024 edition of Louisiana Agriculture.

Asian tiger mosquito. Photo courtesy of Bugwood.com