Artificial light at night affects larval growth without altering survival or pupation in spongy moth (Lymantria dispar dispar)

Artificial light at night (ALAN) can disrupt circadian rhythms in nocturnal insects, but its effects on immature stages remain understudied. However, this knowledge is crucial, as a change in the development of insects can have ecological and economic consequences. For example, the spongy moth (Lymantria dispar dispar), native to Europe and Asia, has become an invasive species in North America, causing extensive defoliation of trees in forests and urban landscapes. We investigated how LED light color influences larval development and pupation in the spongy moth. Larvae were reared under three light conditions simulating outdoor lighting: (1) neutral white LED (3700 K), (2) amber LED (2200 K), and (3) a dark control (no light). Results showed no significant differences in larval survival, growth patterns, or pupal stage outcomes between treatments. However, larvae exposed to 3700 K light reached significantly higher body masses at the final instar than those under 2200 K and tended to be heavier than the control caterpillars. This is accompanied by a consistently higher weight gain at 3700 K in all larval stages. These results suggest that ALAN can influence larval growth—a crucial factor for fitness and population dynamics. Understanding the effects of artificial light at night (ALAN) on fitness related traits—especially in pest species like the spongy moth—is critical given the increasing prevalence of nighttime illumination. 1. Introduction Artificial light at night (ALAN) is a rapidly growing environmental pollutant and is increasingly recognized as a key driver of environmental change in the 21 st century (Hölker et al., 2021). Nocturnal insects are particularly susceptible to ALAN, as many species are highly adapted to natural, low-light night environment (Gaston et al., 2015; Owens et al., 2020; Owens & Lewis, 2018; Van Langevelde et al., 2018). ALAN has been shown to interfere with circadian rhythms and temporal synchronization, resulting in altered activity patterns and behavioral desynchronization (Jiang et al., 2023; Levy et al., 2024). Specifically, exposure to ALAN impairs mating behaviors (Botha et al., 2017), reduces foraging efficiency (Van Langevelde et al., 2017), alters species interactions (Ceppi et al., 2025; Cieraad et al., 2023) and composition (Bolliger et al., 2022; Van Koppenhagen et al., 2024), and decrease fecundity (Honnen et al., 2019). Despite these well-documented effects on adult insects, a critical knowledge gap remains regarding the impact of ALAN on earlier life stages, such as larvae and pupae, which may be equally or even more vulnerable to light pollution. Addressing this gap is essential for a comprehensive understanding of the ecological consequences of ALAN on insect populations. In holometabolous insects, development is tightly regulated by environmental factors, with photoperiod and temperature playing especially critical roles (Saunders, 2009). Natural day–night cycles are among the most reliable environmental cues available to organisms, and nearly all species rely on this diel pattern to regulate circadian systems and anticipate daily environmental changes (Gaston et al., 2017; Grubisic et al., 2019). By extending perceived daylength, ALAN can interfere with photoperiodic regulation and influence larval growth, development time, diapause induction, feeding activity and metamorphosis (Haynes et al., 2023; Merckx et al., 2023; Schroer et al., 2019; Van De Schoot et al., 2025; Van Geffen et al., 2014). One of the earliest studies to examine the effects of different light colors of ALAN on moth development found that exposure to green or white light reduced maximum larval and pupal mass and led to an earlier onset of pupation (Van Geffen et al., 2014). Recent research has highlighted diverse and occasionally contradictory effects of ALAN on the development and survival of moth larvae. For example, Grenis & Murphy (2019) reported that exposure to ALAN in the nocturnal moth Apamea sordens (Lepidoptera: Noctuidae) led to a slower growth rate, prolonged development and a lower body mass. Similarly, ALAN accelerated larval development and pupal duration, but resulted in lower pupal mass and increased larval mortality in the garden tiger moth ( Arctia caja, Lepidoptera: Erebidae) (Van De Schoot et al., 2025). However, not all studies have found significant effects of ALAN on developmental traits. Haynes et al. (2023) observed neither an influence of ALAN on the development time from larvae to pupae nor the pupal mass in the monarch butterfly ( Danaus Plexippus, Lepidoptera: Nymphalidae). Despite these few case studies, larval and pupal responses to ALAN remain underexplored, particularly under controlled conditions and with commercially relevant lighting technologies. Addressing these knowledge gaps is critical to fully understanding the consequences of ALAN, as developmental impacts can lead to broader ecological or ecosystem consequences, many of which remain poorly understood (Grubisic & Van Grunsven, 2021; Owens et al., 2020; Sanders & Gaston, 2018). In this study, we conducted a controlled laboratory experiment to test how different LED streetlight spectra affect the larval development and pupation of the spongy moth Lymantria dispar dispar (Lepidoptera: Erebidae), an economically important species that has caused severe damage to trees in North America and Europe. This species has nocturnally active larvae that undergo multiple molts before pupation. From the second instar to adult emergence, individuals were reared under three nighttime light conditions: (1) neutral white LED light (3700 K), (2) amber LED light (2200 K), and (3) a dark control with no nighttime lighting. The LED treatments represent commercially available streetlight spectra currently used in outdoor lighting. Based on a previous study (Van De Schoot et al., 2025), we hypothesized that caterpillars exposed to ALAN will experience higher mortality rates compared to those raised in darkness, potentially due to increased stress or disrupted circadian rhythms (H1). In line with previous studies performed on nocturnally active caterpillars (Grenis & Murphy, 2019; Van Geffen et al., 2014), we also expected that ALAN will reduce larval weight gain over time, reflecting slower growth rates, which in turn should result in lower maximum larval and pupal masses (H2). We further anticipate that overall development will be prolonged under ALAN, with caterpillars taking longer to reach pupation and experiencing extended pupal durations, as slower-growing individuals typically require more time to mature (H3). By quantifying different traits, our study investigates whether exposure to ALAN—and the spectral characteristics of that light—affects the growth, development, and survival of nocturnal moth larvae. Our findings provide new insights into the potential population-level consequences of different streetlight types on nocturnal insects. 2. Methods 2.1 Study species The spongy moth ( Lymantria dispar dispar, Lepidoptera: Erebidae) is an invasive, univoltine nocturnal moth native to much of the Palearctic region and introduced to eastern North America. It is a notorious forest pest with an exceptionally broad host range. In many populations, larvae exhibit a characteristic diel movement—ascending trees at night to feed and descending in the morning to seek shelter in bark crevices or under rocks. Under certain conditions, populations can grow explosively and increase by several orders of magnitude within a few years. During outbreak events, they can completely defoliate forests, which not only suppresses tree growth but may also disrupt water balance and lead to increased tree mortality (Rindos & Liebhold, 2023). The spongy moth has a well-documented life cycle (Boukouvala et al., 2022; Ponomarev et al., 2023), making it an ideal model species to rear and study the impacts of ALAN in a controlled laboratory setting. 2.2 Laboratory set up and light treatments The study was conducted in two climate chambers (3.8 m × 3.6 m × 2.15 m; L × W × H) automatically programmed to maintain a constant temperature of 26 °C and 70% relative humidity. To further monitor for any major deviations from the desired climate settings, three data loggers (EasyLog EL-USB-2, Lascar Electronics) were placed in each chamber and checked regularly. In each climate chamber, three tables (1.6 m × 0.6 m × 0.75 m; L × W × H) were arranged, one per treatment. Caterpillars were distributed on the tables and exposed to three nighttime light treatments, which consisted of two different color temperatures: neutral white (3700 K) and amber (2200 K) (400 ± 40 lux measured in the middle of the table; see spectral distribution in Supplementary Materials Fig. S1A), as well as a dark control without any light present at night. To prevent light interference between the treatments, light-impermeable tarpaulins (anthracite, 2 m x 3 m, 270 g/m²; kellerfahnen.ch) were installed between the tables. For the nighttime light treatments, we employed state-of-the-art LED streetlight luminaires (Izylum 1, Schréder Swiss AG). We therefore installed two streetlights on aluminum poles (1.7 m) in each climate chamber, which were secured into a wooden base for stability. The control treatment remained dark (to see the experimental set up, see Supplementary Materials, Fig. S2). Each streetlight setup was positioned at the center of the longer side of its corresponding table, placing the head directly above the midpoint. The luminous flux of all streetlight luminaires was standardized to 1500 lumens to ensure consistency across treatments, with calibration conducted by the Swiss Metrological Institute METAS in Bern (Supplementary Materials, Table S1). During the day all caterpillars received the same light treatment. To simulate daylight conditions during the day, we used linear LED lights (SANlight FLEX II-25; see spectral distribution in Supplementary Materials Fig. S1B). These linear LED lights were mounted on the ceiling to ensure a uniform height across all tables. The daylight LEDs were installed at the same height as the streetlights and centered above each table to ensure even illumination and to avoid any shadowing effects caused by the streetlight heads during the night (Supplementary Materials Fig. S2). All treatments received 15 hours of simulated daylight per 24-hour period, from 06:00 to 21:00. During the remaining 9-hour night period, the control treatment remained dark (15:9 h light:dark), whereas the two light treatments received continuous illumination from the streetlights throughout the entire night phase (15:9 h light:ALAN). The installation and maintenance of the LED streetlights were carried out by technical experts from the Electricity Supplies of the Canton of Zurich (EKZ), ensuring consistent and uninterrupted application of the light treatments throughout the study. 2.3 Caterpillar handling The spongy moth caterpillars used in this experiment originated from egg masses of a multi-generational population raised under a natural day-night cycle in a controlled laboratory setting in Munich, Germany (Bayerische Landesanstalt für Wald und Forstwirtschaft LWF). For our study, approximately 1,500 eggs were randomly selected and distributed into cylindrical containers (40 mm height × 86 mm diameter). The eggs were kept under control treatment conditions. Upon hatching, the caterpillars were reared in groups of ~50 individuals and fed an artificial diet (Frontier Agricultural Science, F9630B). Once caterpillars reached the second instar, 50 individuals were randomly assigned to each treatment. This procedure was carried out in both climate chambers, resulting in 100 caterpillars per treatment and a total of 600 caterpillars. Once caterpillars reached the second instar, they were transferred to individual containers (40 mm height × 86 mm diameter) closed with a white fine-mesh fiberglass fabric. All containers contained a cube of artificial diet (2 cm × 2 cm × 2 cm). The cube was replaced weekly to prevent it from drying out. Furthermore, the caterpillar containers were evenly distributed across four equally sized sectors marked on the tables (Supplementary Materials Fig. S2). Because light intensity decreases toward the edges of the table (to about a third of the measured intensity in the middle of the table) and microclimatic variation (e.g., humidity, temperature) might occur within the climate chambers, the containers were regularly rotated among these sectors within each treatment to ensure comparable conditions and minimize potential spatial effects. The rotation was performed weekly after the diet replacement. The individual caterpillar weighing started once caterpillars reached the second instar (upon transfer to individual containers) and then at each subsequent instar change. Weighing was conducted using a precision balance (ME403, Mettler Toledo), with caterpillars gently transferred onto a pre-weighed weighing boat (see Supplementary Materials Fig. S3). The experiment ran from 21 February to 12 June 2024, totaling 112 days. 2.4 Statistical analysis All statistical analyses were performed in R version 4.2.2 (R Core Team 2022). Linear mixed-effect models (LMM) were fitted using the lme4 package (Bates et al., 2015; version 1.1.30) to assess the effects of three ALAN treatments on the body mass of caterpillars. The model included the fixed effects of treatment (3700 K, 2200 K, dark control), instar stage (ordered factor), the treatment*instar interaction, and the climate chamber as a covariate, with individual caterpillars included as a random effect to account for repeated measures. Body mass was log-transformed to improve normality and homoscedasticity of residuals. To test for differences between treatments at each larval stage, we estimated marginal means of body mass for each treatment within each larval instar using the emmeans package (Lenth, 2025; version 1.8.0). We then performed a Post hoc analysis consisting of pairwise comparisons between treatments within each instar level using the contrast() function with a ”pairwise” method. To control for repeated measures, p-values were adjusted using the False Discovery Rate (FDR) method. Pupation measurements, including pupation mass, pupation duration, and total developmental time to pupation, were analyzed using linear models (LM). Each response variable (pupation mass, pupation duration, and total development time) was log-transformed to improve normality and homoscedasticity of residuals. The fixed effects included treatment, climate chamber, and the number of larval stages completed before pupation. The explanatory variables entering the models were checked for overall model performance using the DHARMa package (Hartig et al., 2024; version 0.4.6). Model performance was further assessed using R2 from the MuMIn package (Bartoń, 2024, version 1.47.5) and the Akaike information criterion (AIC) for the full model. To test treatment effects on mortality, we used a Cox proportional hazards model using coxph() from the survival package (Therneau, 2024; version 3.4.0) with time to death and death status (1 = died, 0 = survived until pupation) as the response, and treatment and climate chamber as predictors. A separate Cox model was fitted for pupation timing, using time to pupation and pupation status (1 = pupated, 0 = did not pupate) as the response. This model included treatment, climate chamber, and the number of larval stages completed before pupation as predictors. Kaplan–Meier survival curves were generated with the survfit() function from the survival package based on survival objects created using the surv() function, and plotted using ggsurvplot() from the survminer package (Kassambara et al., 2024; version 0.5.0). 3. Results 3.1 Larval mortality —————————————- Figure 1 —————————————- No significant differences in caterpillar mortality were observed between the light treatments over the entire experimental period of 112 days (χ² = 2.10, p = 0.35; Fig. 1). The proportion of caterpillars that survived to pupation was similar across treatments, with 75% mortality in the 3700 K group, 63% in the 2200 K group, and 72% in the control. For detailed regression results, see the Supplementary Materials (Tables S2) 3.2 Body mass —————————————- Figure 2 —————————————- Caterpillar body mass, measured across larval stages 2 to 8, increased consistently over time, with no statistically significant differences among treatments at most stages. However, caterpillars exposed to 3700 K light reached significantly higher mean body mass at larval stage 8 compared to those exposed to 2200 K (3700 K: 442 mg vs. 2200 K: 287 mg; t(479) = 2.54, p = 0.03), suggesting a potential influence of cooler color temperatures on late-stage growth, with a general trend of higher mass across all larval stages in the 3700 K group (Fig. 2). For detailed regression results, see the Supplementary Materials (Tables S3 & S4). 3.3 Pupation —————————————- Figure 3 —————————————- There were no significant effects of ALAN on any measured pupation parameters. The cumulative proportion of caterpillars that pupated over time was consistent across all treatments, with most individuals in each group completing pupation within a comparable time frame of 43 - 95 days (χ² = 1.45, p = 0.49). However, we observed a slight delay in the onset of pupation in the 3700 K group, and it also took them longer until all the caterpillars reached pupation compared to the other treatments (Fig. 3A). This is also reflected in a slightly later, albeit statistically insignificant, mean pupation age in the 3700 K group (Fig. 3B). Pupal mass did not differ significantly between treatments, although caterpillars exposed to ALAN tended to have slightly higher masses (3700K: 228 mg, +23% vs. control; 2200K: 198 mg, +7% vs. control; Fig. 3C). Similarly, the duration of the pupal stage did not show any significant difference between the treatments (Fig. 3D). For detailed regression results, see the Supplementary Materials (Tables S5-S11) 4. Discussion Our study examined the effects of ALAN—using two commercially available LED streetlight with two color temperatures (2200 K, 3700 K)—on the development of the economically important forest moth Lymantria dispar dispar . Caterpillars exposed to cooler 3700 K LEDs reached significantly higher body masses at the latest instar stage compared to those reared under warmer 2200 K lights. This result is accompanied by a broader trend of increased weight gain at 3700 K across all larval stages. However, we found no statistically significant effects of LED color on mortality, pupal mass, age at pupation, or pupal duration. Our findings suggest that while ALAN does not strongly affect overall life-history parameters, it can nonetheless influence key growth traits during the larval stage, potentially impacting population dynamics in this pest species. Survival rates were broadly similar across treatments, with no statistically significant differences in overall mortality. This is in contrast to our first hypothesis (H1), stating higher caterpillar mortality under ALAN compared to the dark control, based on recent findings by van de Schoot et al. (2025). The absence of mortality effects underlines that ALAN effects were subtle, which is consistent with a study of Grenis & Murphy (2019), where they also observed no impact of light treatment on larval survival. In contrast to our second hypothesis (H2), which predicted lower body masses in caterpillars exposed to ALAN due to potentially reduced growth rates, we observed a clear trend of higher body masses across all larval stages under 3700 K light. This contrasts with previous studies on other nocturnal moth species, such as Apamea sordens (Grenis & Murphy, 2019) and Mamestra brassicae (Van Geffen et al., 2014), which reported slower growth rates and reduced body mass under ALAN. Specifically, van Geffen et al. (2014) found that caterpillars exposed to white light reached lower body masses than those under red light, while our results indicate the opposite effect, with warmer light supporting higher growth rates than colder color temperatures. This unexpected positive effect of 3700 K ALAN on caterpillar growth is surprising, given that spongy moth caterpillars are predominantly nocturnal (the first three instar are thought to be diurnal). One would expect that certain light spectra closer to our daylight treatment—such as 3700 K—suppress their natural feeding activity (see spectral distribution of all the treatments in the Supplementary Materials Fig. S2 & S3). It is possible that differences in 3700 K and daylight regimes were subtle enough that caterpillars were not able to differentiate between day and night based on light cues. As a result, caterpillars may have adjusted their feeding patterns independent of light-cues which may have led to increased feeding activities. In contrast, the greater spectral difference to the daylight treatment under 2200 K light may have preserved day–night distinctions better than in 3700 K, resulting in growth patterns more similar to the dark control. Overall, our findings highlight the need for detailed behavioral observations under different light settings to better understand how ALAN influences feeding rhythms and activity patterns in nocturnal caterpillars. Our third hypothesis (H3), which states that ALAN causes the caterpillar to grow more slowly and take longer to reach pupation, was also not confirmed. We found no significant effect of ALAN on the timing or duration of pupation, in contrast to studies that found an earlier onset of pupation (Van Geffen et al., 2014) as well as a shorter pupation duration (Van De Schoot et al., 2025; Van Geffen et al., 2014). Despite a slight, though statistically insignificant, delay in the onset of pupation for caterpillars exposed to 3700 K light, the overall timing and duration of pupation were consistent across treatments, suggesting that the effects of ALAN on development duration in this experiment were minimal. Taken together, our findings suggest that although we did not find a significant change in developmental timing, we did detect some influence of ALAN on larval growth patterns. One possible reason for the limited overall effects and the differences to previous studies could be the use of a long-term laboratory population, which may differ in sensitivity to our treatments compared to wild populations. However, since these individuals were kept under a simulated natural day-night cycle and had never been exposed to artificial light at night before, strong adaptation to such conditions seems unlikely. Still, the observed increase in larval mass under 3700 K light is noteworthy given that insect body mass is widely accepted as a proxy for lifetime fitness, as it directly influences reproductive output and overall survival (Beukeboom, 2018). Heavier larvae are often expected to pupate earlier, as they reach the critical weight threshold for metamorphosis faster (Price et al., 2011; Schoonhoven et al., 2005). However, in our experiment, despite observing a trend towards higher larval mass under 3700 K ALAN, we found no corresponding acceleration in pupation timing. This disconnect suggests that factors beyond mass, such as hormonal regulation or energy allocation, might play a more significant role in determining pupation age in spongy moths and therefore needs more attention in future studies. Nevertheless, the larger body size observed at late larval stages under 3700 K could have critical implications for adult fitness. In capital breeders like spongy moths, mass gained as larvae is particularly important, as unlike income breeders they do not feed as adults. As a result, the energy acquired during larval development is the only reserve available for adult moths to find a mate and reproduce (Jervis et al., 2005). This could translate into increased fecundity, as larger females typically produce more eggs (Loewy et al., 2013), potentially amplifying population growth in potential pest species. Moreover, larger males might gain advantages in flight endurance, longevity, and sperm competition (Iyengar & Eisner, 1999). Though our observed mass differences compared to the control were not statistically significant, the trend suggests that even small shifts in larval growth under ALAN could contribute to accelerating mass outbreaks in spongy moth populations, with potentially far-reaching ecological and economic consequences. Furthermore, future studies should not only investigate laboratory-reared individuals but also include wild populations to determine whether these findings hold under natural field conditions. Overall, our results highlight the need for further research into the species-specific effects of ALAN on nocturnal Lepidoptera, as well as the broader ecological and economic risks associated with light pollution. Understanding the full range of potential ALAN impacts on the growth, fitness, and population dynamics of insect in general and specifically pest species like the spongy moth will be essential for developing effective conservation and pest management strategies in an increasingly illuminated world. Data availability statement The data supporting the findings of this study can be freely accessed at Figshare: https://dx.doi.org/10.6084/m9.figshare.29341487 Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Author contribution statement NVK: Conceptualization (lead); data curation (lead); formal analysis (lead); investigation (lead); methodology (lead); project administration (equal); visualization (lead); writing – original draft preparation (lead). MMG: Conceptualization (supporting); funding acquisition (supporting); formal analysis (supporting); methodology (equal); resources (equal); validation (equal); writing – review and editing (equal). ME: Formal analysis (equal); methodology (supporting), writing – review and editing (equal). DSM : Conceptualization (equal); investigation (equal); methodology (supporting); writing – review and editing (supporting). JH: Funding acquisition (supporting); resources (lead); validation (equal); writing – review and editing (supporting). JB: Conceptualization (equal); funding acquisition (lead); methodology (equal); project administration (lead); resources (equal); supervision (lead); validation (equal); writing – review and editing (equal) Acknowledgments We gratefully acknowledge Stephan Blum and Anita Rosenberger of the electricity company of Zurich EKZ and thank Fabio Costa and Jolanda Klaver for their support during the experiment. We thank Astrid Bächli for her support in the entomology laboratory at WSL. We also thank the Swiss Institute for Metrology METAS for providing their facilities to calibrate the luminaires used in this experiment. The project ALANeX was funded by the WSL project Extremes. not-yet-known not-yet-known not-yet-known unknown References Bartoń, K. 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Kaplan-Meier survival curves for spongy moth (Lymantria dispar dispar) under three ALAN treatments (3700 K, 2200 K and dark). The shaded band represents the 95% confidence interval. Figure 2. Estimated mean body mass (± 95 % confidence intervals) of spongy moth (Lymantria dispar dispar) larvae across different instars (stages 2 to 8) under three artificial light at night (ALAN) treatments: 3700 K, 2200 K, and a dark control. Asterisks indicate significant differences between treatments p < 0.05 within a larval stage. Small, more transparent dots show body mass of individual caterpillars. not-yet-known not-yet-known not-yet-known unknown Figure 3. Pupation outcomes of spongy moth (Lymantria dispar dispar) caterpillars across three artificial light at night (ALAN) treatments (3700 K, 2200 K, and dark control). (A) Proportion pupated over time, (B) age at pupation, (C) pupal mass, and (D) pupation duration. Large dots represent estimated mean values (± 95 % confidence intervals), small, more transparent dots represent individual caterpillar values. Information & Authors Information Version history Peer review timeline Published Ecology and Evolution Version of Record13 Oct 2025Published Copyright This work is licensed under a Non Exclusive No Reuse License. Collection

Authors Metrics & Citations Metrics Article Usage 444views 277downloads Citations Download citation Nicola van Koppenhagen, Martin M. Gossner, Michael Eisenring, et al. Artificial light at night affects larval growth without altering survival or pupation in spongy moth (Lymantria dispar dispar). Authorea. 18 June 2025. DOI: https://doi.org/10.22541/au.175025589.90986806/v1 DOI: https://doi.org/10.22541/au.175025589.90986806/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu.