The experiment was designed around the biology of the fall webworm, a North American moth whose gregarious caterpillars spin the familiar silken tents seen on hundreds of tree and shrub species. The species is a true edge specialist: its webs appear along streams, forest margins, roadways, and sidewalks, precisely the places where humans install streetlights. Adults are short-lived, surviving roughly four days after emerging from their cocoons, and they do not feed. They are considered nocturnal and are thought to mate during twilight, often remaining in copulation past sunrise into the afternoon, which makes them an ideal subject for testing whether artificial illumination disrupts the narrow window in which reproduction must occur.

To run the test, the team, led by Mykaela M. Tanino-Springsteen and Shannon M. Murphy, used moths from a laboratory colony established in 2019 with wild larvae collected in Colorado and regularly supplemented with wild-caught individuals to maintain genetic diversity. Virgin, unrelated males and females were paired within twenty-four hours of eclosion and placed in clear plastic mating chambers. Each evening the chambers were moved to a partitioned greenhouse on the University of Denver campus, with half assigned to a room illuminated by ceiling-mounted high-pressure sodium luminaires, the same technology widely used in street lighting, and half to a room shielded from the lights and most surrounding urban skyglow. The lamps were programmed to switch on at 8:30 in the evening and off at 6:00 in the morning, mirroring the schedule of real streetlights in the surrounding Denver neighborhood.

The researchers deliberately avoided calling the darkened room a control, because even that side of the greenhouse received some ambient light pollution through its single wall of south-facing glass windows. The two rooms shared the same temperature and humidity, and the mating chambers were arranged in a single layer on benches about five feet below the light source, all equidistant from the lamps. The team also confirmed that the chambers retained heat equally in both rooms, ruling out thermal differences as an explanation for any behavioral divergence. Over four consecutive nights in late June 2023, the researchers checked each morning for mating pairs and recorded any deaths.

The mating result was dramatic. Of the seventy pairs in which both individuals survived the night, eighteen percent of those in ambient light mated, while zero percent of those under the streetlights did, a statistically significant difference detected with Pearson chi-square tests. Because fall webworms typically mate within a day when they are going to mate at all, and because they remain coupled after sunrise, the researchers were confident they were observing essentially all mating events. The complete shutdown of reproduction under artificial light suggests that these moths require low-light conditions to initiate copulation, and that the spectral signature and intensity of sodium streetlights are enough to suppress that behavior entirely.

Survival told a subtler story. Across all 186 moths analyzed, survival was 92.4 percent in the ambient treatment compared with 83 percent under ALAN, a trend that fell just short of conventional statistical significance with a p-value of 0.0512. Males and females did not differ in their survival responses. Recognizing that the near-significant result might reflect an underpowered sample, the team conducted a post-hoc power analysis, which showed that detecting a twenty percent difference would require about 148 moths per treatment, only modestly more than the 92 to 94 moths they actually tested. The authors caution that the probability of a type-II error is high and that ALAN likely does reduce survival, even though the effect size, a Cohen’s d of 0.29, is small.

Why would light at night block mating so completely? The study outlines two leading mechanistic hypotheses, neither yet tested directly in fall webworms. The first involves pheromones, the chemical signals female moths emit to attract males. In related work on the cabbage moth (Mamestra brassicae), researchers found that ALAN altered the chemical composition of female sex pheromones and reduced overall pheromone production, which could cripple mate attraction. The second involves circadian rhythms: for many moth species, the timing of female pheromone release, male responsiveness, and copulation is governed by a twenty-four-hour clock calibrated to natural light cues. If streetlights erase those cues, females may never call and males may never respond, leaving both sexes reproductively idle despite being physically capable of mating.

The survival effect, if confirmed, could arise through several pathways documented in other insects. Flight-to-light behavior, the fatal attraction that draws moths toward lamps, can exhaust individuals or expose them to predation, and previous work has shown it can be sex-biased in Lepidoptera. Short-wavelength blue light has proven lethal to fruit flies, flour beetles, and mosquitoes in laboratory studies, though the effects of different light spectra on fall webworms have never been examined. ALAN has also been shown to suppress melatonin production and impair immune function in crickets, and earlier research from the same Denver laboratory found that light pollution reduced the performance of a grass-feeding caterpillar both directly and indirectly, by altering host plant traits such as toughness and carbon-to-nitrogen ratios.

The population-level implications are significant. Because fall webworms concentrate along habitat edges, and because urban and rural edges are exactly where streetlights cluster, the species may face chronic exposure during its brief reproductive window. If moths cannot escape illuminated zones, the complete mating inhibition observed in the greenhouse could translate into local population declines in lit areas, with cascading effects on the predators and parasitoids that depend on fall webworm larvae for food. There is also a hint of evolutionary hope in the literature: urban ermine moth populations show reduced flight-to-light behavior compared with dark-sky populations, and corn earworm moths have become less likely to be caught in light traps over time, possibly an adaptive response to decades of light pollution. The Denver team’s colony, however, was sourced largely from rural, dark environments, and they plan to test whether urban fall webworms mate more readily under ALAN than their rural relatives.

The study also carries a notable human dimension: the experiment was conducted in partnership with middle school campers from the University of Denver’s DU SciTech summer camp, part of an ongoing series of authentic research experiences in which students help investigate how ALAN reshapes natural environments. As for the science itself, the authors emphasize that ALAN likely touches every life stage of the fall webworm. Larvae require photoperiod cues to enter diapause, and laboratory-reared larvae exposed to constant light fail to enter that dormant state, a potentially lethal mistake for insects facing a Colorado winter. But it is the adult finding that stands out most starkly. Reproduction is the bottleneck through which every population must pass, and this experiment shows that for one common, widespread moth, the simple presence of a streetlight is enough to bring it to a complete halt. As cities continue to brighten the night, the dark refuges that nocturnal insects depend on are shrinking, and studies like this one make clear that the consequences may be measured in generations that are never born.

Subject of Research: Effects of artificial light at night on mating and survival of the fall webworm moth

Article Title: Artificial light at night inhibits mating and may reduce survival in a nocturnal moth

Article References: Artificial light at night inhibits mating and may reduce survival in a nocturnal moth. (n.d.). https://doi.org/10.1186/s44329-025-00030-w

Image Credits: AI Generated

DOI: 10.1186/s44329-025-00030-w

Keywords: artificial light at night, light pollution, fall webworm, Hyphantria cunea, moths, insect decline, mating behavior, pheromones, circadian rhythm, urban ecology, streetlights, Lepidoptera