Wings of female Asian tiger mosquitoes sampled and measured in LaDeau et al.’s study. Wing length was used as a convenient measure of body size in the study. Photo credit: Dr. Shannon LaDeau.

By John P. Roche

In mosquitoes, females bite hosts such as humans, take a bloodmeal, and use the nutrition from the bloodmeal to develop their eggs. With more female mosquitoes in a habitat, there will tend to me be more mosquitoes biting people, and when mosquitoes carry viruses dangerous to humans, that will tend to increase the risk of disease. The number of female mosquitoes is a result of the interplay of several life history traits, such as lifespan, number of offspring, and size of offspring. To explore one aspect of the life history patterns of Asian tiger mosquitoes, Aedes albopictus, Grace Katz, Paul Leisnham, and Shannon LaDeau examined if the body size of female mosquitoes differed with the proportion of abandoned houses among different neighborhoods in Baltimore. The study was recently published in the Journal of Medical Entomology.

Asian tiger mosquitoes are invasive throughout much of the globe. They arrived in the U.S. from Japan in the 1980s and are of concern because they can spread West Nile Virus in the U.S. They have also been documented to spread dengue virus and chikungunya virus in temperate habitats in Europe and Asia, so they might be able to spread those diseases in North America.

Unlike Culex and Culiseta mosquitoes that bite in the nighttime, Asian tiger mosquitoes bite humans during the daytime. This poses an inconvenience, but it also poses increased risk of disease transmission because many more people are outside during the day. The eggs of the mosquito are resistant to drought, so they can remain alive for a long time under dry conditions. But they have a limited capacity for dispersal, tending to remain within 100 meters of their juvenile habitat.

Life history traits can affect mosquitoes’ ability to transmit disease. One life history characteristic that is often examined in mosquito studies is body size. Larger body size tends to correlate with increased survival and increased fecundity, both factors that would likely increase vector ability by contributing to population growth and an increased number of biting females. The increase in the number of biting females is in turn likely to cause an increase in disease transmission if disease viruses are present in the mosquito population pools.

Better Habitat, Bigger Mosquitoes

A member of Shannon LaDeau’s research team, sampling Asian tiger mosquitoes in a neighborhood in Baltimore. LaDeau and colleagues found that in neighborhoods with more abandoned buildings—which correlates with more discarded containers that provide juvenile mosquito habitat—the size of female mosquitoes was larger. This larger size was significant because it could correlate with higher survival and greater population growth, which could create greater risk of transmission of mosquito-borne diseases. Photo credit: Dr. Shannon LaDeau.

Two factors that influence population size in mosquitoes are the amount and quality of juvenile habitat. In 2018, Danielle Bodner and colleagues found that there was a greater density of adult mosquitoes in blocks with more discarded containers, which could hold water and thus provide habitat for mosquito larvae. In 2017, Eliza Little, LaDeau, and colleagues discovered that when the percentage of abandoned buildings was higher, that correlated with both a larger amount of juvenile habitat in the form of discarded containers and increased densities of both adults and juveniles. (The 2017 study is highlighted in a previous Entomology Today article by Meredith Walker.) In light of these findings, LaDeau and her colleagues hypothesized that there might be variation in mosquito body size related to variation in the amount of juvenile habitat, with the assumption that the amount of juvenile habitat correlated with the proportion of buildings abandoned in a block.

To test this hypothesis, LaDeau and colleagues trapped female Asian tiger mosquitoes from 13 blocks of mostly residential neighborhoods in Baltimore, Maryland. The blocks were the same blocks used in the Little et al. 2017 study. Because wing length is a convenient measure that correlates closely with body size, they measured the wing length of female mosquitoes. They collected a total sample size of 1,097 females.

As an indirect measure of juvenile habitat availability, they categorized blocks according to the proportion of buildings that were abandoned. “Abandonment levels clustered into three groups,” LaDeau explained. “Low abandonment (0–5.5%), intermediate abandonment (18–30%), and high abandonment (42–48%). However, not all blocks in a neighborhood had the same abandonment level: the highest proportion of abandoned blocks came from low socioeconomic status (SES) neighborhoods, and all blocks from high SES neighborhoods had low abandonment. But median SES neighborhoods had blocks split across all three abandonment categories.”

LaDeau and her colleagues discovered that the wing lengths of sampled female mosquitoes were significantly larger in high abandonment blocks than in low abandonment blocks. The significance of this was twofold. First, this was consistent with the conclusion that female body size was higher with more abundant juvenile habitat in the high abandonment blocks. Second, with increased body size, there would be higher survival rates and greater population growth, which would lead to more females that could take bloodmeals from humans. That would potentially create an increased the risk of disease transmission in higher abandonment blocks, creating a health disparity resulting from infrastructure abandonment in low SES blocks. This finding paralleled research by LaDeau, Bodner, and colleagues in 2013 looking at the abundance of seven mosquito species in Baltimore and Washington, D.C. That study found that Asian tiger mosquitoes and Culex pipiens mosquitoes (a primary vector of West Nile virus) were more abundant in low SES neighborhoods in those cities.

Potential Solutions

What are some potential solutions to address this problem of larger female mosquitoes in blocks with more abandoned buildings? One obvious measure would be to hold community clean-ups, where citizens could get together and pick up trash. Another potential solution would be education: the more residents know about the risks of mosquitoes and the easy steps that can be taken to reduce juvenile mosquito habitats in discarded containers, the less habitat there would be for mosquitoes. However, in a 2016 study of the effect of printed educational material about mosquito habitats and mosquito-borne disease risks, Danielle Bodner, Shannon LaDeau, and colleagues found that households that received educational material were less concerned about mosquito-borne diseases than households that did not receive educational materials. This unintended consequence highlights the complexity of public-education efforts.

In regard to which specific facets of Aedes albopictus ecology are most important to study to better understand the influence of juvenile habitat on population dynamics, LaDeau said, “We have little quantitative understanding of how long adult mosquitoes survive and how far they generally disperse from juvenile habitats. Both of these traits have important influence on how likely a mosquito is to be able to transmit a pathogen.”

When asked about important next steps in this research, LaDeau said, “Work is needed to identify the mechanisms that cause the differences in size, and the consequences of being different sizes. The literature is mixed regarding how, why, and when bigger means higher fitness or greater vector competence. The relationship can be context dependent, with bigger size having different consequences at different temperatures, for instance. A majority of work has been done on two to three mosquito species only and they all behave a little differently.”

These are intriguing findings that show the interplay between the amount of juvenile habitat in neighborhoods and an important life history characteristic—female body size. Going forward, it will be valuable to learn more about how other life history characteristics vary with abundance of juvenile habitat, and how those variables in turn affect vector competence. It will also be vital to learn more about which practical solutions might be most effective to try to reduce the availability of juvenile habitat and exposure to biting mosquitoes. As was shown by the unexpected discovery on the unintended consequences of education efforts by Bodner and colleagues, investigating solutions is particularly complex and challenging—but it’s nonetheless important.

John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. Director of Science View Productions and Adjunct Professor at the College of the Holy Cross, Dr. Roche has published over 200 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com.