Light-Polluted Nights and Heat Push Day-Active Tiger Mosquitoes Into the Dark

The Asian tiger mosquito, Aedes albopictus, is a creature of daylight. Across most of its vast invasive range, females seek blood in the early morning and late afternoon, retreating to sheltered resting spots once the sun goes down. But a new field study from the greater St. Louis area suggests that two hallmarks of the human-altered environment—artificial light at night and increasingly extreme daytime heat—are quietly rewriting that schedule. On hot days, at sites bathed in urban glow, wild tiger mosquitoes were caught actively hunting for hosts in the middle of the night, a behavior that could reshape when and how people are exposed to the viruses this species carries.

The research, published in BMC Environmental Science, was led by Lauren E. Johnson of Washington University in St. Louis together with colleagues at Tyson Research Center. During the summer of 2023, the team enrolled 58 households across the St. Louis metropolitan region, deliberately selecting sites along a wide gradient of artificial sky brightness ranging from roughly 1,020 to 10,500 microcandelas per square meter. At each site, they deployed BG-Sentinel traps programmed to run from 10 p.m. to 4 a.m., capturing host-seeking females and mate-seeking males. The traps mimic a human host by releasing carbon dioxide from dry ice and artificial human skin odors, while their high-contrast black and white design appeals to the visually oriented tiger mosquito. Each morning, the researchers collected, sexed, and identified the catch, and they also sampled mosquitoes with aspirators in the early evening to estimate baseline abundance at every location.

Temperature proved to be a critical piece of the puzzle. Field loggers and National Weather Service records showed that daytime maximum temperatures during the study swung by as much as 13 degrees Celsius, from 25.5 to 38.3 degrees. That upper range matters because laboratory work on St. Louis tiger mosquitoes has pegged their critical thermal maximum—the temperature beyond which survival is threatened—at around 36.7 degrees. In other words, the mosquitoes in this study were frequently confronting conditions hot enough to make daytime activity genuinely dangerous, since a blood meal itself can raise a female’s body temperature to lethal levels.

To untangle the effects of light pollution and heat, the team turned to generalized additive models fitted with negative binomial distributions, an approach well suited to non-linear relationships and overdispersed count data. They also included percent impervious surface cover, a standard proxy for urbanization drawn from the national land cover database, to ensure that any apparent effect of artificial light was not simply a stand-in for dense development. A spatial autocorrelation test using Moran’s I confirmed that neither daytime nor nighttime abundance was clustered geographically, allowing the analysis to proceed without spatial corrections.

The results were striking. The best-fitting model explained 61.9 percent of the variance in nighttime mosquito abundance, and the two dominant predictors were nearly tied: daytime abundance accounted for 28.22 percent of explained variance, while the interaction between artificial light at night and temperature accounted for 28.17 percent. Impervious surface, by contrast, contributed a mere 2.86 percent, suggesting that urbanization per se is a far weaker driver of nocturnal activity than the light and heat conditions themselves. Every term in the model was statistically significant.

The shape of the light response was decidedly non-linear. Nighttime captures were essentially zero at the darkest sites, but abundance climbed sharply once artificial sky brightness crossed roughly 3,500 microcandelas per square meter, remaining elevated all the way to the brightest locations. This threshold pattern mirrors laboratory experiments showing that below a biologically meaningful intensity, artificial light has little effect on tiger mosquito behavior, but once that intensity is reached, nighttime activity switches on dramatically. The authors note that this kind of threshold effect would likely be missed by studies that treat light pollution as a simple presence-or-absence variable, underscoring the value of sampling along a continuous gradient.

Temperature determined whether that threshold mattered. When the researchers colored their data points by whether the preceding day’s maximum temperature had reached or exceeded the females’ critical thermal maximum, a clear pattern emerged: nearly every sampling event following such a scorching day showed some nighttime activity, and the few exceptions occurred almost exclusively at sites below the 3,500-microcandela threshold. Model predictions told the same story. At 40 degrees Celsius, predicted nighttime abundance rose steeply with light pollution; at 35 degrees the increase was gentler; and at or below 30 degrees, the model predicted essentially no night activity regardless of brightness. Dark sites showed little to no nocturnal capture even on the hottest days, indicating that heat alone was not enough—the mosquitoes needed light to exploit the night.

The most likely interpretation is that artificial light unlocks a temporal refuge from thermal stress. Diurnal species that use light as an exogenous cue for navigation and timing may, under light-polluted skies, gain access to a nighttime niche that would otherwise be inaccessible. By shifting host-seeking into cooler, illuminated hours, female tiger mosquitoes can avoid the lethal costs of blood-feeding during heat extremes while still securing the nutrition needed for egg production. Similar heat-driven nocturnality has been documented in other day-active animals, including the white-lipped peccary, but the mosquito study is among the first to demonstrate the phenomenon in a wild disease-vector population and to identify light pollution as the enabling condition.

The findings carry real public health weight. Aedes albopictus is a competent vector of dengue, chikungunya, and Zika viruses and one of the most abundant urban mosquitoes in the temperate United States. If hot weather and bright nights routinely extend its biting window into the hours when people sit on porches, sleep with open windows, or walk lit streets, the geometry of vector-host contact changes. Notably, the effect runs in the opposite direction for nocturnal, light-avoiding Anopheles mosquitoes, in which even brief light exposure suppresses biting for hours. Species-specific responses like these could reshape disease transmission dynamics in densely populated urban areas where both heat waves and light pollution are intensifying, and they argue for control strategies timed to these expanded activity periods rather than the traditional dawn-and-dusk schedule.

The study also bridges a persistent gap between laboratory and field. Controlled experiments had long shown that artificial light at night activates nighttime biting in Aedes aegypti and Aedes albopictus and disrupts clock gene expression, but evidence from wild populations remained thin; the closest precedent was a modeling study in a large Chinese city that reported high nocturnal activity without pinpointing a cause. By sampling 58 sites across a broad brightness gradient and pairing the data with precise temperature records, the St. Louis team confirmed that laboratory observations translate to real urban landscapes. Open questions remain, including whether the nocturnal shift represents a permanent expansion of the species’ activity period or a temporary behavioral adjustment, and how chronic light exposure might interact with seasonal processes such as diapause. What is already clear, however, is that as cities grow hotter and brighter, the tiger mosquito’s day is stretching into the night.

Subject of Research: Effects of artificial light at night and heat on the nocturnal activity of the tiger mosquito Aedes albopictus

Article Title: Hot days and light-polluted nights increase nighttime activity of the diurnal tiger mosquito (Aedes albopictus)

Article References: Johnson, L. E., Tayon, L. L., Uder, E. R., Dobbs, K. G., Radomski, T., Medley, K. A., & Westby, K. M. (2025). Hot days and light-polluted nights increase nighttime activity of the diurnal tiger mosquito (Aedes albopictus). BMC Environmental Science, 2(1), Article 15. https://doi.org/10.1186/s44329-025-00029-3

Image Credits: AI Generated

DOI: 10.1186/s44329-025-00029-3

Keywords: Aedes albopictus, tiger mosquito, light pollution, artificial light at night, heat waves, vector-borne disease, urban ecology, circadian rhythms, host-seeking, climate change, public health, mosquito control