Moth Larvae Defy Expectations at Alpine Mega-Project Construction Sites

Deep beneath the Alps, tunnel boring machines grind through rock day and night, floodlit by banks of high-intensity lamps that turn midnight into a pale imitation of noon. The Turin-Lyon high-speed railway, one of Europe’s largest infrastructure undertakings, has transformed stretches of the Susa Valley in Italy and the Haute Maurienne-Vanoise Valley in France into zones of relentless construction. For ecologists, these sites represent a rare and troubling experiment: what happens when powerful artificial lighting is suddenly imposed on remote, biodiverse mountain landscapes that have known near-total darkness for millennia? A new field study, published in Ecology and Evolution, set out to answer that question for one of the most light-sensitive groups of organisms on Earth, the larvae of nocturnal moths, and the results challenge some of the field’s most widely held assumptions.

Artificial light at night, often abbreviated as ALAN, has emerged as one of the fastest-growing forms of environmental change on the planet. Global artificial illumination is expanding at roughly six percent per year, and its biological footprint extends across mammals, birds, reptiles, amphibians, fish, invertebrates and plants. For insects, the concern is acute. Around 64 percent of invertebrate species are nocturnal, and many are declining worldwide. Moths, in particular, have become a flagship group for light pollution research because they respond rapidly to environmental shifts in both community composition and phenology. Previous work has shown that ALAN can disrupt oviposition, slow larval growth and feeding, alter pupation, interfere with adult activity and reduce reproductive success. It can even impair dispersal, effectively fragmenting habitats for species that never touch a bulldozer.

Yet almost all of this evidence comes from urban or suburban settings, where light pollution is intense and biodiversity is already diminished. The research team, led by scientists at the University of Turin and co-funded through a dedicated PhD pathway established with the tunnel operator TELT, wanted to know whether the same patterns hold in high-value natural areas. They chose the larval stage of nocturnal Lepidoptera as their focal system for good reason. Larvae represent the longest phase of a moth’s life, depend entirely on vegetation for food, and move very little, making them exceptionally sensitive indicators of local conditions. Moreover, adult fitness depends heavily on larval performance and pupal weight, which ultimately determine survival and reproductive output. If construction lighting were harming moth populations, the signature should appear in the caterpillars.

The study design paired three illuminated construction sites along the railway corridor with nearby unlit control areas matched for exposure, altitude, land use, geomorphology and vegetation. The Puits d’Avrieux site in France, at 1110 to 1334 metres above sea level in a dry ecotone habitat, recorded the highest peak light intensity at 35.2 lux, followed by La Maddalena in Italy at 20.6 lux and Villarodin-Bourget-Modane at 13.0 lux. Because construction had begun years before the study, in 2007, 2013 and 2017 respectively, the team employed an After-Control/Impact design, a variant of the classic BACI approach that uses multiple control areas to account for spatial variability when no pre-impact baseline exists. Sampling covered 187 points, each corresponding to a single standardised plant, surveyed monthly from June to October 2022 using vegetation beating and net sweeping.

In total, the researchers collected 248 moth larvae and identified 98 species across 11 families and 42 genera, with Geometridae the most species-rich family. The silver birch, Betula pendula, hosted the largest number of larvae, followed by barberries, while the highest species richness appeared at a French control site. Larvae were weighed in the field with a precision balance and released, allowing the team to model three separate responses: presence or absence, species richness per plant, and total larval biomass per area. Generalised linear mixed models with zero-inflated distributions handled the sparse, heavily zero-dominated count data, and forward selection using Akaike’s Information Criterion identified the best predictor sets for each response variable.

The headline finding was striking by its absence. Artificial light at night, tested both as a univariate predictor and within the full modelling framework, showed a negative but non-significant effect on larval occurrence and species richness, and was not retained in any final model. Instead, the significant drivers were habitat and environmental factors. Larval presence was strongly shaped by sampling area, mowing regime, plant species and wind. Recently mown areas actually supported higher larval occurrence than unmown ones, a result the authors attribute to the vegetated margins adjacent to mown patches, which previous research has shown can buffer management impacts on moth diversity. Plant identity mattered enormously: compared with dog rose, larvae were significantly more frequent on barberries, birch and brambles, consistent with known host plant preferences.

Even more counterintuitive, the only statistically significant contrast between a construction site and its own control favoured the construction site. Larval occurrence and species richness were both higher at the illuminated Puits d’Avrieux site than at its control area Soleil, a comparison that held across both the presence-absence and richness models. No other construction site differed significantly from its controls. The authors suggest that the xerothermic, sun-exposed conditions of the Avrieux area, with its warm, dry climate, may favour adult moth activity, which correlates strongly with temperature and elevation, ultimately boosting local larval abundance regardless of the floodlights overhead.

Biomass told a murkier story. Forward selection identified the null model as the best fit for larval biomass, meaning none of the tested covariates, including light, explained the variation. The authors caution that this reflects limited statistical power rather than proof of no effect: most sampled larvae were tiny early instars, many could not be weighed reliably, and the dataset contained only 40 biomass observations with many zeros. Mean larval weights ranged from a mere 0.018 to 0.157 grams. The summer of 2022 compounded the challenge, as severe drought and heat caused premature leaf yellowing and defoliation in roses, brambles and oaks across several sites, forcing larvae to seek alternative food and adding a powerful confounding stressor to the experiment.

The team is careful not to overinterpret their results. Sparse count data likely limited the ability to detect a genuine light effect, and the characterisation of ALAN itself was incomplete: light was measured only as lux values, without capturing spectral composition, wavelength, colour temperature or photoperiod, all of which are known to influence insect physiology independently of intensity. Tree canopies partly obstructed the construction lighting, producing a patchwork of fully illuminated and shaded plants that may have diluted any detectable signal. Larval behaviour added further noise, as caterpillars grip their host plants more tightly after the first beat of a sampling net, particularly in wind, potentially leading to underestimates of their true abundance.

What the study ultimately delivers is a sobering lesson in ecological complexity. Moth larvae are herbivores, pollinators of nocturnal flowers and critical prey for birds and bats, so their fate ripples through entire food webs. Yet in this real-world mega-project setting, vegetation structure, microclimate and habitat management overwhelmed any measurable effect of construction lighting on the larval stage. The authors argue that this does not exonerate ALAN, whose documented harms to adult moths remain substantial, but it does highlight how habitat complexity may buffer or amplify light impacts in ways that urban studies cannot capture. Expanding research into semi-natural, high-naturalness areas, they conclude, is essential for understanding the full spectrum of light pollution effects, particularly for life stages with limited mobility, and for disentangling the synergistic stresses of noise, machinery and habitat loss that accompany humanity’s largest engineering endeavours.

Subject of Research: Effects of mega-project construction and artificial light at night on nocturnal moth larvae in the Alps

Article Title: Nocturnal Lepidoptera Larvae in Mega‐Project: Habitat and Environmental Factors Over Construction Effects

Article References: Trombin, D., Mazzei, P., Piccini, I., Dessì, L., Grieco, P., Matechou, E., & Bonelli, S. (2026). Nocturnal Lepidoptera Larvae in Mega‐Project: Habitat and Environmental Factors Over Construction Effects. Ecology and Evolution, 16(10), Article e74296. https://doi.org/10.1002/ece3.74296

Image Credits: AI Generated

DOI: 10.1002/ece3.74296

Keywords: artificial light at night, light pollution, Lepidoptera larvae, mega-projects, Turin-Lyon railway, Alpine ecology, moth declines, biodiversity, BACI design, habitat management, insect conservation, Ecology and Evolution