In a container of water crowded with mosquito larvae, killing only some of them can sometimes result in more adult mosquitoes emerging than would have otherwise. New research on three container-breeding mosquito species details the complex dynamics between changes in larval density and mosquito survival and offers insight into the optimal timing for mosquito-control treatments. (Photo by Steven A. Juliano, Ph.D.)

By Kristina M. McIntire, Ph.D., and Steven A. Juliano, Ph.D.

Among mosquitoes in the genus Aedes, species whose larvae develop in small water-holding vessels (referred to as “container Aedes”) are some of the most important vectors of human viral diseases, including dengue, yellow fever, Zika, chikungunya, and viral encephalitis. Their preferred habitats for oviposition (i.e., laying eggs) and larval development range from natural rot holes in trees to human-made containers such as discarded tires or plastic containers, cemetery vases, and other containers that collect water. These sorts of containers are often common near houses and workplaces, resulting in exposure of people to biting Aedes populations.

Among mosquitoes in the genus Aedes,several species’ preferred habitats for oviposition (i.e., laying eggs) and larval development are small, water-filled containers, both natural and human-made such as the examples shown here: cemetery vase (top left), tires (top right), and a tree hole (bottom left). These sorts of containers are often common near houses and workplaces, resulting in exposure of people to biting Aedes populations. Detritus such as plant matter and dead insects contribute feeding material for the developing Aedes larvae, such as shown in the closeup of water in a tire (bottom right), where a drowned fly is visible on the surface and mosquito larvae are visible below. (Top left and bottom right photos by Steven A. Juliano, Ph.D.; top right and bottom left photos by Kristina M. McIntire, Ph.D.)

Because there are no widely available vaccines for most of these illnesses, preventing or limiting disease transmission depends primarily on mosquito control. The success of control efforts depends on understanding how populations of the target species will respond, especially when control efforts do not result in 100 percent elimination of the target mosquito larvae. Understanding these intricate and sometimes counterintuitive dynamics is crucial not only to the field of ecology but also to public health.

Three potential relationships exist between initial larval density and number of survivors and the effect of imposing extrinsic mortality on a high-density population, reducing its density to a new, lower population density. Additive (red): Imposing mortality would result in fewer survivors compared to not imposing mortality, as shown by point 1. Compensatory (blue): Imposing mortality would result in the same number of survivors compared to not imposing mortality, as shown by point 2. Overcompensatory (black): Imposing mortality would produce a greater number of survivors compared to not imposing mortality, as shown by point 3. (Figure originally published in Evans et al 2023, Insects)

Case in point: Larval density—i.e., how many mosquito larvae are present in a container and thus competing with each other—can affect their rates of survival and eventual emergence as adults. In fact, in some circumstances a decrease in larval density can result in an increase in the number of adults that emerge.

A recent study in which Steven participated, published in late 2022 in the journal Insects, reported field studies showing that Aedes species and populations vary in the sensitivity of larvae to crowding. In some cases, numbers of surviving adults peaked at intermediate larval density and then declined at greater larval densities. In other cases, numbers of surviving adults reached an upper limit that did not change with increasing larval density. In still other cases, numbers of surviving adults continued to increase with increasing density.

These differences in sensitivity suggest that the population responses produced by larval density reductions due to mosquito-control efforts are also likely to vary among species, populations, and locations. When control efforts reduce larval density of a crowded population, the different survival responses could result in counterproductive outcomes of either no change in adult production (referred to as compensation), or an actual increase in adult production (referred to as overcompensation) compared to no density reductions.

Studies in the laboratory have tested species responses to density reductions in separate experiments, but it has been rare to compare different species when subjected to the same conditions. In a study that we both conducted with undergraduate research students at Illinois State University, published in September in the Journal of Medical Entomology, we compared the population responses of Aedes aegypti, Aedes albopictus, and Aedes triseriatus to experimental removal of 48.8 percent of larvae at different times following hatching.

In this work, we standardized starting larval density of single-species cohorts at 250 newly hatched larvae in 400 milliliters of water. Detritus in the form of oak leaf litter and insect carcasses (typical of that found in containers in nature) was added as a substrate for microbial growth, which forms the food supply of developing Aedes larvae. Two, six, and eight days after hatching, we chose 122 larvae at random for removal from some containers (density-reduction replicates), while removing no larvae from others (control replicates). When adults emerged, we counted them from each container, determined sex, recorded time from hatch to adult, and measured wings of females as a quantification of adult size.

In a study comparing the population responses of Aedes mosquitoesto experimental removal of larvae at different times following hatching, starting larval density of single-species cohorts was set at 250 newly hatched larvae in 400 milliliters of water, as shown in the experimental containers here. Detritus in the form of oak leaf litter and insect carcasses (typical of that found in containers in nature) was added as a substrate for microbial growth, which forms the food supply of developing Aedes larvae. (Photo by Steven A. Juliano, Ph.D.)

In a study comparing the population responses of Aedes mosquitoes to experimental removal of larvae at different times following hatching, larval density reduction significantly increased adult production in test containers compared to controls, but the three species differed in the details of that response. Shown in this chart of results from the study are least squares means (±SE) for effects of treatments on the number of total adults, number of females, and number of males produced for Ae. aegypti (A), Ae. albopictus (B), and Ae. triseriatus (C). Means for all adults, females, and males within a species indicated with an * are significantly different from the corresponding control by Dunnett’s test, α = 0.05. Numbers at the bottom of the bars for total adults indicate the number of replicate containers contributing to that least squares mean, and also to least squares means for females and males. (Figure originally published in Juliano et al 2023, Journal of Medical Entomology)

Larval density reduction significantly increased adult production in test containers compared to controls, but the three species differed in the details of that response. Both Ae. albopictus and Ae. triseriatus produced more adults when larvae were removed early in their development (day 2); however, the increase for Ae. albopictus was primarily due to greater production of males, while the increase for Ae. triseriatus was due to greater production of females. Aedes aegypti adult production was not significantly affected by larval density reduction. Strikingly, none of these treatments significantly reduced adult production for any species. Additionally, development times for both sexes in Ae. aegypti were significantly shorter when larvae were removed on day 8, indicating potentially reduced generation time. There were no effects of larval density reduction on female wing length.

These results are important for two related reasons:

• First, we cannot assume that eliminating up to 50 percent of these Aedes larvae in a population will reduce the number of biting adults. Thus, it appears that successful control of these species requires killing a very high proportion of the larval population, at least for situations where containers are heavily colonized and larvae are crowded (as was the case in our experiment).
• Second, we cannot assume that these different Aedes species will respond uniformly to mosquito control efforts. However, the survival patterns among species suggest that control efforts would be most successful by targeting populations when larval densities are low, such as early in the active season, before the mosquito population builds up.

In a container of water crowded with mosquito larvae, killing only some of them can sometimes result in more adult mosquitoes emerging than would have otherwise. New research on three container-breeding mosquito species (two of which are shown here as adults) details the complex dynamics between changes in larval density and mosquito survival and offers insight into the optimal timing for mosquito-control treatments. (Photos by Banugopan Kesavaraju)

The counterproductive outcomes of compensation or overcompensation could result from the combination of negatively density-dependent survival (i.e., lower percent survival at high density) and density reductions that may arise via mosquito control. Thus, these results suggest that approaches to mosquito control that cause mortality late in larval life, after the action of density-dependent competition among larvae, seem desirable. Detailed preliminary investigations of local target populations and their responses to larval density may help to improve the effectiveness of mosquito control.

Kristina M. McIntire, Ph.D., is a postdoctoral fellow in the Department of Ecology and Evolutionary Biology at the University of Michigan and earned her B.S. and Ph.D. in biology at Illinois State University. Email: kris.m.mcintire@gmail.com. Steven A. Juliano, Ph.D., is a distinguished professor emeritus in the School of Biological Sciences at Illinois State University. Email: sajulian@ilstu.edu.