The yellowfever mosquito (Aedes aegypti) typically prefers humid climates, but it has been able to establish itself in the arid southwestern United States by utilizing manmade containers in residential areas for breeding sites. A new study conducted near Tucson, Arizona, reports that, in particular, flower pots and the saucers underneath them hosted disproportionately more larval mosquitoes than other types of containers. (Photo credit: Flickr/U.S. Department of Agriculture)

By John P. Roche

The yellowfever mosquito (Aedes aegypti), is a severe health threat, spreading dengue virus, Zika virus, and chikungunya virus. Dengue fever alone infects 400 million people each year, and all of these diseases are expanding their range. The mosquito is native to Africa, but it is it has colonized many subtropical and tropical regions globally as an invasive species.

Aedes aegypti is prevalent in humid areas. But, because mosquitoes often develop in water standing in containers, the presence of human containers has allowed the species to spread through desert regions of the southwestern United States. There is a paucity of information on how many water containers are present in residential properties in the southwestern U.S. and about the knowledge and attitudes southwestern U.S. residents have about transmission of mosquito-borne diseases. Kathleen Walker, Ph.D., assistant professor at the University of Arizona, and colleagues addressed this problem by conducting an extensive survey of residents and their properties in Tucson, Arizona. Their results are reported in a paper published this week in the Journal of Medical Entomology.

Walker and colleagues, who included scientists from the University of Arizona, El Colegio de Sonora in Mexico, and the National Center for Atmospheric Research in Colorado, combined two categories of data. One category detailed the containers found in residential properties: They gathered data on types of containers, numbers of containers, the proportion of containers that held water, and the proportion of containers that held immature Ae. aegypti mosquitoes. The second data category consisted of interview information from residents, assessing people’s knowledge of arboviral transmission factors. The rationale for combining these two types of data was to better inform educational strategies and improve mosquito-control methods.

The investigators chose 20 locations in Tucson, Arizona, and then randomly sampled 20 houses in each location, for a total of 400 houses. Of these 400 houses, data from both a container survey and an interview survey were available from 355 houses; this subset of houses made up the study’s data set. Sampling was performed in August 2012.

For their container survey, the investigators surveyed the outdoor areas of properties. For their resident knowledge survey, they interviewed residents of houses. Walker and colleagues used a list of about 80 interview questions called a Knowledge, Attitude, and Practices (KAP) survey. “The exact number of questions for the KAP survey varied,” Walker says, “because certain responses could trigger additional questions. For example, if a respondent had heard of dengue virus, we would ask related follow-up questions on dengue; if not, we would skip those questions.”

The study found a total of 3,948 containers. Twenty percent of containers held water, 1.8 percent of containers held yellowfever mosquito larvae, and 0.7 percent of containers held mosquito pupae. Ninety one percent of houses had at least one container, and 13 percent of houses had at least one container with larvae or pupae.

Flower pots were the most abundant type of container. Flower pots and plastic containers were the container types most likely to contain water, and flower pots contained disproportionately more larval mosquitoes than other types of containers. “The most important finding of the study,” Walker says, “was the importance of flower pots and associated saucers underneath them as preferred Aedes aegypti larval development habitat.”

Interestingly, 400 pieces of trash were found in the survey, but only three pieces of trash had immature mosquitoes. Similarly, discarded tires, which are often viewed as primary breeding grounds for mosquitoes, were not a problem: of 78 tires found, only two had immature mosquitoes. Also, unmaintained pools were not a problem: of 11 unmaintained pools discovered, none contained mosquitoes.

In their cross-comparison of container-survey data and interview data, Walker and colleagues found that homeowners had more containers and more immature mosquitoes than renters, and households with more people had more containers and more immature mosquitoes than houses with fewer people. The observation about renters versus homeowners makes sense: Renters stay at properties for shorter periods of time, and containers would tend to be removed after each renter left, resulting in fewer containers on average.

Another important finding of the study was that the investigators found significantly fewer mosquito larvae at houses with residents who reported checking for standing water more than once per week.

The investigators concluded that the Ae. aegypti mosquito is present throughout the Tucson metropolitan area, that flower pots are a prime habitat for mosquito larval development, and that checking containers more than once per week reduces the presence of larvae. Applications of this study include educating residents that frequently checking for standing water can help reduce presence of the yellowfever mosquito, with a particular focus on flower pots.

Walker is building on these research findings with studies of other communities and investigations of larval density. “We are currently sampling Aedes aegypti in suburban communities outside Phoenix,” Walker said. “We want to determine whether the findings from Tucson are consistent in other Arizona communities. We also want to examine how well larval density measures match adult trap counts using specialized mosquito traps called BG-Sentinel traps.”

This ongoing research is valuable in helping to inform education efforts aimed at efficient and effective reduction of immature Ae. aegytpi mosquitoes, which in turn will help contain the spread of dengue virus and Zika virus.

John P. Roche, Ph.D., is an author, biologist, and science writer dedicated to making rigorous science clear and accessible. He has published more than 185 articles, and has written and taught extensively about science. For more information, visit https://authorjohnproche.com.