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Lehtonen & Ulrika Candolin Title: The color of artificial light affects mate attraction in the common glow-worm Year: 2023 Version: Published version Copyright: The Author(s) 2023 Rights: CC BY 4.0 Rights url: http://creativecommons.org/licenses/by/4.0/ Please cite the original version: Kivelä, L., Elgert, C., Lehtonen, T. K., & Candolin, U. (2023). The color of artificial light affects mate attraction in the common glow-worm. Science of The Total Environment, 857, 159451. https://doi.org/10.1016/j.scitotenv.2022.159451 Science of the Total Environment 857 (2023) 159451 Contents lists available at ScienceDirect Science of the Total Environment j ourna l homepage: www.e lsev ie r .com/ locate /sc i totenv The color of artificial light affects mate attraction in the common glow-worm Linnea Kivelä a,b,⁎, Christina Elgert a,b, Topi K. Lehtonen a,b,c, Ulrika Candolin a,b a Organismal and Evolutionary Biology, University of Helsinki, PO Box 65, 00014 Helsinki, Finland b Tvärminne Zoological Station, University of Helsinki, J.A. Palménin tie 260, 10900 Hanko, Finland c Natural Resources Institute, Paavo Havaksen tie 3, 90570 Oulu, Finland H I G H L I G H T S G R A P H I C A L A B S T R A C T ⁎ Corresponding author at: Organismal and Evolutionary PO Box 65, 00014 Helsinki, Finland. E-mail address: linnea.kivela@helsinki.fi (L. Kivelä). http://dx.doi.org/10.1016/j.scitotenv.2022.159451 Received 7 June 2022; Received in revised form 22 S Available online 14 October 2022 0048-9697/© 2022 The Authors. Published by Elsevi • Light pollution is an emerging environ- mental threat to nocturnal organisms. • Glow-worms are dependent on darkness for mate finding. • Dummy female glow-worms were ex- posed to four colors of light in the field. • Long wavelength artificial light was less detrimental to mate attraction success. • Spectral tuning of outdoor lighting is a po- tential mitigation measure. A B S T R A C T A R T I C L E I N F O Editor: Daniel Wunderlin Keywords: Artificial light at night Environmental change Lampyridae Light pollution Mate choice Spectral tuning Artificial light at night, often referred to as ‘light pollution’, is a global environmental problem that threatens many nocturnal organisms. One such species is the European common glow-worm (Lampyris noctiluca), in which reproduc- tion relies on the ability of sedentary bioluminescent females to attract flying males to mate. Previous studies show that broad-spectrum white artificial light interferes with mate attraction in this beetle. However, much less is known about wavelength-specific effects. In this study, we experimentally investigate how the peak wavelength (color) of artificial light affects glow-worm mate attraction success in the field by using dummy females that trap males landing to mate. Each dummy was illuminated from above by either a blue (peak wavelength: 452 nm), white (449 nm), yellow (575 nm), or red (625 nm) LED lighting, or light switched off in the control. We estimated mate attraction success as both the probability of attracting at least one male and the number of males attracted. In both cases, mate attraction success depended on the peak wavelength of the artificial light, with short wavelengths (blue and white) decreasing it more than long wavelengths (yellow and red). Hence, adjusting the spectrum of artifi- cial light can be an effective measure for mitigating the negative effects of light pollution on glow-worm reproduction. 1. Introduction Light pollution, or artificial light at night (ALAN) (Davies andSmyth, 2017), is a potentially severe anthropogenic environmental challenge. Since electric lighting became common in the early 20th century, nighttime environments Biology, University of Helsinki, eptember 2022; Accepted 11 Octo er B.V. This is an open access artic have become increasingly more illuminated by artificial light, with light pollu- tion continuing to be a rapidly growing problem (estimated global increase of 6 % per year (Hölker et al., 2010b)). Currently, light pollution is estimated to affect 49.5 % of the land surface area between 59°N and 55°S (Gaston et al., 2021). Natural dark-light cycles have, in the time scale of evolution, remained consistent, and therefore their disturbances can have various ecological conse- quences (Longcore and Rich, 2004; Gaston et al., 2015a; Davies and Smyth, 2017). Awidediversity of organisms, fromplants to both terrestrial and aquatic vertebrates and invertebrates, have been impacted, with the effects ranging ber 2022 le under the CC BY license (http://creativecommons.org/licenses/by/4.0/). http://crossmark.crossref.org/dialog/?doi=10.1016/j.scitotenv.2022.159451&domain=pdf http://dx.doi.org/10.1016/j.scitotenv.2022.159451 mailto:linnea.kivela@helsinki.fi http://dx.doi.org/10.1016/j.scitotenv.2022.159451 http://creativecommons.org/licenses/by/4.0/ http://www.sciencedirect.com/science/journal/ www.elsevier.com/locate/scitotenv Fig. 1. The experimental setup for investigating the impact of color on mate attraction. A dummy female was placed under either a red, white, yellow, or blue artificial light source. In the control the light was turned off. The height at which the light was attached was customized for each treatment to even out differences in intensity caused by the color-altering foils. L. Kivelä et al. Science of the Total Environment 857 (2023) 159451 fromchanges in individual physiology andbehavior to those inpopulation ecol- ogy and community structure (Gaston and Bennie, 2014; Davies and Smyth, 2017). Nocturnal species, which constitute an estimated 30%of all vertebrates and 60 % of all invertebrates, are particularly vulnerable to artificial light (Hölker et al., 2010a). Indeed, light pollution has been proposed as a major driver of global insect declines (Owens et al., 2020). Light pollution may affect behavior by interfering with, for instance, foraging, reproduction, migration, or communication (Longcore and Rich, 2004). For example, positively phototactic insects, such as many moths, are attracted to artificial light sources. This response disrupts their normal behavioral patterns, sometimes with fatal consequences (Boyes et al., 2021). Artificial light at night may also obscure the light from natural light sources, disorienting species that navigate by themoon and stars. Sim- ilarly, it can disturb communication in bioluminescent species, such as glow-worms and other fireflies (Owens and Lewis, 2018). Due to the negative effects of light pollution highlighted by recent re- search, there is an urgent need for effective mitigation measures. Here, new technologies offer promising opportunities, for example by controlling the artificial light spectrum and intensity with Light Emitting Diode (LED) based lighting infrastructure (Gaston et al., 2015a; Davies and Smyth, 2017). However, increased efficiency and lowered costs of light sources can also result in increased light use (Hölker et al., 2010b). Furthermore, the ecological consequences of the changing spectral landscape of artificial light resulting from the application of these new technologies (Gaston et al., 2015a; Pagden et al., 2020) are poorly known. Given the global increase in light pollution, a better understanding of the ecological impacts of different types (e.g. spectral composition) of artificial light is required, especially for the needs of policy development and strategic planning of lighting systems (Hölker et al., 2010a; Gaston et al., 2015b). Glow-worms and other fireflies (Lampyridae), which use biolumines- cent signals to attract mates, are especially vulnerable to light pollution. Broad spectrumwhite artificial light has been shown to affect both glow be- havior and mate attraction success of female common glow-worms (Lampyris noctiluca). For example, females exposed to artificial light glow less and attract fewer males (Bird and Parker, 2014; Elgert et al., 2020a; Stewart et al., 2020; Van den Broeck et al., 2021a). The impact of artificial light on mate attraction increases with light quantity (i.e. intensity) (Elgert et al., 2020b; Van den Broeck et al., 2021a), but we still know little about the influence of light quality (e.g. spectrum). Such knowledge is essential, because the effects of artificial light on glow-worm reproduction and, in the long term, even population viability, may vary depending on the type of light, especially its spectral composition. The use of new technologies should allow adjustment of e.g., public artificial lights accordingly. In the current study, we investigated the effects of artificial light wave- length spectrum (i.e. color) on mate attraction success in the common glow-worm. In particular, we assessed mate attraction success both in terms of the proportion of successful female dummies and the number of mates they attracted and, based on previous findings, expected that the suc- cess is lower under exposure to artificial light than in natural darkness (Ineichen and Rüttimann, 2012; Bird and Parker, 2014; Elgert et al., 2020a; Van den Broeck et al., 2021b). Furthermore, we predicted that mate attraction success is related to the spectrum of artificial light, with light characterized by shortwavelengths (white and blue) having a larger im- pact than long wavelengths (yellow and red). This prediction was based on the previous observations that a light signal is less attractive to males when a blue component is added to it, whereas the addition of a red light compo- nent does not have an adverse effect (Booth et al., 2004). In addition, white artificial light has generally been considered more adverse to a range of animals than yellow light (Gaston et al., 2012; Longcore et al., 2018). 2. Materials and methods 2.1. Study species and area In the common glow-worm, sedentary and larviform females attract fly- ing males by emitting a constant greenish glow at night (Lewis, 2016). The 2 brightness of the glow correlates with female body size, which, in turn, cor- relates with fecundity (i.e. the number of eggs the female produces) (Hopkins et al., 2015, 2021). Furthermore, brighter females tend to be more successful in attracting males (Hopkins et al., 2015; Elgert et al., 2020b; Lehtonen and Kaitala, 2020). The glow-worm is a capital breeder, with a typical adult lifespan of less than two weeks and females dying after havingmated and laid their eggs (Lewis, 2016). Population surveys in- dicate that glow-worm populations have declined at least locally in the UK and probably also elsewhere (Gardiner, 2009; Gardiner and Didham, 2020). The reasons for the declines are likely to include climate change, habitat fragmentation, and urbanization, including light pollution (Gardiner and Didham, 2020, 2021; Lewis et al., 2020; Lehtonen et al., 2021). All work was conducted in Southern Finland in the proximity of Tvärminne Zoological Station (N 59°51′, E 23°14′), during the glow-worm breeding season, between 12th June and 4th July in 2020. This period was chosen based on earlier observations of glow-worm reproduction in the same area (e.g. Elgert et al., 2020a; Lehtonen and Kaitala, 2020). The experiment was run only on nights of good weather (N = 20 nights), be- cause poor weather conditions (low temperatures, wind and rain) restrict male activity (Dreisig, 1971). 2.2. Experimental setup We used dummy females (LED lures) that trapped males landing to mate. These were identical to those used in multiple recent studies (Elgert et al., 2020a, 2020b; Lehtonen and Kaitala, 2020), except for the brightness of the LED. In particular, the dummies were equipped with a green 5 mm LED that mimicked the glow of a female. The LED was attached to a halved 1.5 L bottle of transparent, non-glossy plastic, with the top half inserted up- side down to form a funnel trap (Fig. 1). The green LED was powered by two AA batteries and wired with resistors (2000 Ω) to adjust the intensity to 0.065–0.075 μWnm−1 (microwatts per nanometer), as measured with a spectrophotometer and integrating sphere (Borshagovski et al., 2020). Its peak wavelength was 562 nm, which matches the glow of female glow-worms (550–570 nm) (Schwalb, 1961; De Cock, 2004). Each dummy female was lit from above by an artificial light that simu- lated a light source, such as a streetlamp or yard light. The artificial light sources were composed of white LEDs covered with EUROLITE color-foil to alter their light spectra (Table 1). We had five treatments: blue, white, yellow, and red artificial light, and a control (the LED switched off). The Table 1 Light treatment details. Light intensities were measured at the vertical level of the dummy female. Intensity Color Filter Peak wavelength (nm) Photons/cm2/s Lux Blue Color-foil 165 “Daylight blue” 452 6.45 ∗ 1012 5.0 White Diffusion filter 129 “Heavy frost” 449 5.27 ∗ 1012 5.6 Yellow Color-foil 104 “Deep amber” 575 4.21 ∗ 1012 6.0 Red Color-foil 164 “Flame red” 625 1.27 ∗ 1013 8.0 Control None None 0 0 Fig. 2. Number of males attracted by imitation females in the unlit control, and in the blue, white, yellow, and red, artificial light treatments. The box plots show median values (horizontal black lines), interquartile ranges (colored boxes), upper quartiles (whiskers), and extreme values (black dots). Whiskers representing the lower quartiles are not visible, because in all treatments at least 25 % of the imitation females attracted zero males. Treatments with a different letter A-B were significantly different from each other (negative binomial GLMM). N = 60 in all treatments except N = 59 in the yellow light treatment. L. Kivelä et al. Science of the Total Environment 857 (2023) 159451 light source in the blue, white, and yellow light treatments consisted of a Fenix UC01 mini flashlight at medium output. Due to the stronger absorp- tion (i.e. a dimming effect) of the red color foil, the red light source needed to be more powerful to reach a similar intensity of light as that of the other treatments. Therefore, we used a VARTA Indestructible LED x5 headlamp at half output. We used opaque lampshades (made of plastic cups) to prevent the source of artificial light from being directly visible from the outside, and to direct its cone downwards. Each light source was attached to a pole at 100–120 cm above the ground level, i.e. 80–100 cm above the LED lure. To calibrate the light intensities and counter variation in the color foils' ab- sorption, the light sources were attached at the following heights above the ground: 100 cm (blue), 110 cm (yellow), 120 cm (white), and 120 cm (red) (Fig. 1).Wemeasured the resulting intensity and peakwavelength of the ar- tificial light in each treatment with a spectrophotometer and cosine correc- tor (Borshagovski et al., 2020) in an otherwise dark room to acquire precise values in the absence of external disturbances (Table 1). These intensities were comparable to the lower range of intensities of artificial lights we measured in the area (but not closer than 50 m from any of the replicates of this study) (Table S1). The spectrophotometer measurements were taken at 20 cm above the ground level, corresponding to the position of the dummy females (in relation to the ground) in the experiment. Such a position is ecologically relevant, with females often perching on vegetation above the ground level (Tyler, 2002). The dummy females and artificial lights were placed along a 1 km stretch of an unlit, forested road to Tvärminne Zoological Station, at 15 sep- arate sites where glow-worms had been observed in previous years, with the distance between adjacent sites at least 40 m. We randomized which treatment was conducted at a specific site each night. We ran 3 repli- cates of each of the 5 treatments per night. Over the course of the exper- iment, we completed 4 replicates of each treatment at each of the 15 sites. This resulted in 60 replicates per treatment (over 20 nights), except for the yellow treatment, in which one replicate was discarded due to battery failure. We activated the artificial lights and dummy females each night be- tween 23:45 and 00:15 and turned them off between 01:45 and 02:15. We turned off the lights in the same order as we had turned them on, with each light and dummy female being active for two hours. We then inspected the dummy females for any male glow-worms that they had attracted and trapped, which were counted and placed into plastic vials. We kept the males indoors until the following morning. Males were then markedwith a small dot of acrylic paint on their pronotum (first exoskeletal shield) and released. Themarkings allowed us to identify recapturedmales. To ensure the independence of data points, recaptured males were not in- cluded in the data analyses (detailed below). 2.3. Statistical analysis All statistical analyses were performed using R v. 4.2.0. (R Core Team, 2022) and RStudio v. 2022.02.3. (RStudio Team, 2022) for 3 macOS. We investigated the effect of the artificial light treatments (blue, white, yellow, red, dark control) on mate attraction success (see below) by using a generalized linear mixed model (GLMM) with a neg- ative binomial distribution (R package lme4 v. 1.1.29.; Bates et al., 2015), as appropriate for overdispersed count data (Zuur et al., 2013). The number of males attracted into each trap was denoted as a count re- sponse variable and the treatment (color of artificial light) as a categor- ical explanatory variable. To account for the effects of the night and site of each replicate, they were included as random effects. The model was selected based on Akaike information criteria (AIC, Akaike, 1973). The goodness of fit of the model was checked using the R package DHARMa v. 0.4.5. (Hartig, 2022). Aswe cannot rule out the possibility that the presence of amale in a trap gives odor cues that might attract other males, or that males move in swarms, we also analyzed the data using male presence/absence data. Here, we used a binomial GLMM (suitable for binary data), with whether the dummy female attracted at least one male as a binary response variable (0/1), the artificial light treatment as an explanatory variable, and each rep- licate's night and site as random effects. 3. Results The dummy females attracted a total of 624 male glow-worms during the experiment. Dummy females in the blue and white light treatments attracted significantly fewer males than dummy females in the yellow, red and control treatments (Fig. 2, Table 2). The yellow and red light treat- ments did not significantly differ from the control treatment and the white and blue treatments did not significantly differ from each other (Fig. 2, Table 2). Themedian number of males attractedwas two in the control, yel- low and red light treatments, and zero in the blue and white light treat- ments (Fig. 2). Over the whole experiment, 56.9 % (170/299) of the dummy females attracted at least one male. A significantly lower portion of dummies attracted at least one male in the blue and white light than in the yellow and red light treatments, and in the control (Tables 3 and 4). The proportion of attracted males did not significantly differ between the yellow, red, and control light treatments (Tables 3 and 4) or between the blue and white light treatments (Tables 3 and 4). Thus, results from the two analyses were qualitatively the same. Table 2 Estimates, z-values and p-values of pairwise treatment comparisons of numbers of attracted males using a negative binomial GLMM, with replicate night and site in- cluded as random factors. Pairwise comparisons of numbers of attracted males Treatment Estimate Z P Blue vs control −1.372 −6.601 <0.001 White vs control −1.891 −7.735 <0.001 Yellow vs control −0.203 −1.157 0.247 Red vs control −0.049 −0.287 0.774 White vs blue −0.519 −1.905 0.057 Yellow vs blue 1.168 5.461 <0.001 Red vs blue 1.3225 6.231 <0.001 Yellow vs white 1.687 6.768 <0.001 Red vs white 1.842 7.435 <0.001 Red vs yellow 0.1542 0.871 0.384 Table 4 Estimates, z-values and p-values of a binomial GLMMassessingwhether the dummy female attracted at least one male, with replicate night and site included as random factors. Pairwise comparisons between the different light treatments are shown. Pairwise comparisons of numbers of attracted males Treatment Estimate Z P Blue vs control −1.573 −3.769 <0.001 White vs control −1.491 −3.594 <0.001 Yellow vs control 0.1627 0.386 0.699 Red vs control 0.270 0.636 0.525 White vs blue 0.082 0.202 0.840 Yellow vs blue 1.735 4.075 <0.001 Red vs blue 1.843 4.294 <0.001 Yellow vs white 1.653 3.906 <0.001 Red vs white 1.761 4.128 <0.001 Red vs yellow 0.107 0.249 0.803 L. Kivelä et al. Science of the Total Environment 857 (2023) 159451 4. Discussion The results show that female glow-worm mate attraction success under artificial light depends on the peak wavelength of the light, independent of whether the success is measured as the number of attracted males or as mate attraction probability. In the field, dummy females exposed to rela- tively short wavelengths of artificial light (blue and white) had a signifi- cantly lower mate attraction success than those exposed to longer wavelengths of artificial light (yellow and red). Moreover, the mate attrac- tion success of dummy females under yellow and red artificial light did not differ significantly from that of dummy females in the control. Our result that white artificial light hinders the ability of female glow- worms to attract males is in line with previous findings (e.g. Bird and Parker, 2014; Elgert et al., 2020a; Stewart et al., 2020). We also found blue light to have a very similar effect (with its spectrum also being very similar, only narrower, Fig. S1). In contrast to our initial hypothesis, we also found that the mate attraction success of dummy females under yellow and red artificial light was similar to that observed in the control treatment. In particular, due to previous findings of a lower mate attraction success even under sodium lamps (Ineichen and Rüttimann, 2012; Van den Broeck et al., 2021b), we hypothesized that mate attraction success would be lower in all artificial light treatments compared to the control. In particular, Ineichen and Rüttimann (2012) found yellow‑tinted artificial light from high-pressure sodium streetlights to prevent mate attraction in the common glow-worm, albeit under streetlights with considerably higher intensity of artificial light than what was used in this experiment (46–64 lx vs 5–8 lx; for comparison, moonlight is typically only 0.01–0.6 lx (Kyba et al., 2017)). Moreover, Van den Broeck et al. (2021b) recently found that female glow-worms took longer to mate when exposed to light from low-pressure sodium streetlights, even when light intensity was relatively low. Why, then, did we not find a difference between our long wavelength treatments, red and yellow, or between them and the control? First, wave- lengthsmay differ in the extent they interactwith the greenish female glow. For example, adding a blue component to a femalemimicking stimulus was Table 3 Male attraction success of dummy females in the different light treatments. Treat- ments with a different letter (A or B) were significantly different from each other (logistic regression). N = 60 in all treatments except in the yellow-light treatment N = 59. Number of males attracted (frequency) Treatment 0 ≥1 Success % Significance Control 19 41 68.3 % A Blue 39 21 35.0 % B White 38 22 36.7 % B Yellow 17 42 71.2 % A Red 16 44 73.3 % A 4 found to reduce the attractiveness of the stimulus to male glow-worms, whereas adding a red component did not have an effect (Booth et al., 2004). Second, male glow-worms may not be able to perceive longer wave- length artificial light; thus such light might not markedly reduce the visibil- ity of female signals to them. In another lampyrid species, the firefly Aquatica ficta, male signaling behavior was altered as a result of exposure to artificial light of short andmidwavelengths, but not too longwavelength (≥597 nm), implying a low visual sensitivity to yellow and red light (Owens et al., 2018). Third, it is possible that yellow, and even red, artificial light did suppress female signal visibility, but to such a low degree that it did not significantly affect the number of males attracted (given the current sample sizes and study design). As in many recent studies (Hopkins et al., 2015; Elgert et al., 2020a; Lehtonen and Kaitala, 2020), the dummy female LEDs in the current study corresponded to the glow of a particularly bright glow-worm female, i.e. dummy females were notably brighter than the glow of an average female (A.-M. Borshagovski 2017–2018, unpublished data). Therefore, under long wavelength artificial light, the apparent brightness of dummy females to males, or other aspects of their detectabil- ity or attractiveness,may not have decreasedmarkedly (e.g. under a critical threshold level). Thefinal potential explanationwe provide for the high number of males attracted by dummy females in the long wavelength treatments is that males may be attracted by long wavelength (yellow to red) artificial light. Anecdotal observations suggest that glow-wormmales domake approaches towards red light, although to a lesser extent than towards green light (Schwalb, 1961). More generally, positive phototaxis (attraction) in re- sponse to artificial light is common in insects, with many moths, beetles, flies, and aquatic insects showing attraction especially to short wavelength (blue and UV) light (Park and Lee, 2017; Owens and Lewis, 2018). How- ever, observations of attraction to long wavelengths of light are rarer. Ex- amples include certain pest insects being attracted to red light, and nymphs of certain aquatic insects showing attraction to mid wavelength (green and yellow) light (Park and Lee, 2017; Kühne et al., 2021). Further- more,fireflies of the genusDiaphanes have been found to be attracted to red LEDs (Pacheco et al., 2016). Hence, if long wavelength light attracts male glow-worms, the highmale attraction success of dummy females in the yel- low and red light treatments may have been due to males being lured to their vicinity by the light. Being in the proximity may then have increased the probability of males noticing the dummy female within the cone of ar- tificial light. Such a scenario could compensate for any reduction in the vis- ibility (or apparent brightness) of the dummy female under the artificial light. Previous studies have shown that the intensity of artificial light can also affect glow-worm mate attraction success (Elgert et al., 2020a, 2020b; Van den Broeck et al., 2021a). Here, our treatments had minor differences in light intensity, when measured both as photons/cm2/s and lux (Table 1, Fig. S2). However, these do not explain the differences in mate attraction, because the treatments with both the lowest (yellow) and highest (red) light intensity value attracted the greatest numbers of males (Fig. S2), L. Kivelä et al. Science of the Total Environment 857 (2023) 159451 showing the minor light intensity differences did not explain treatment dif- ferences in mate attraction. Spectral tuning of artificial lights has been suggested as a mitigation measure for reducing the harmful impacts of nighttime illumination, with yellow light often presented as a less detrimental alternative to white light (Gaston et al., 2012; Longcore and Rich, 2016). The suggestion is sup- ported by our result that the mate attraction success of dummy females under red and yellow artificial light was not significantly reduced, unlike that of dummy females under blue and white light, indicating a lesser im- pact of the former on glow-worm reproduction. Nevertheless, we may need to be cautious about recommending yellow artificial lighting instead of blue-white light in areas inhabited by glow-worms (or other firefly spe- cies). For example, in the firefly Photinus obscurellus, all tested colors of ar- tificial light (cool white, warmwhite, blue, amber, and red) suppressed the courting activity of both sexes, with bright amber light having the greatest negative impact (Owens and Lewis, 2021). Furthermore, we cannot yet judge with confidence whether long wavelength artificial light is truly in- nocuous to glow-worm mate finding and attraction. For instance, we do not know how long-wavelength artificial light may affect female signaling behavior or the extent it may attract male glow-worms away from females. Moreover, the impact of artificial light is likely to be more severe when a wide area is illuminated, while our setup was akin to the use of spotlights (e.g. in yards and gardens) that only illuminate a small area. Thus, we need more research into the effectiveness of spectral tuning as a mitigation measure for this species. In addition, other species may differ in their light sensitivities, which poses a challenge to finding a universal solution. Addi- tional measures to consider include reducing light intensity, limiting direct glare through shielding, and decreasing lighting duration, e.g., by using motion detection or timers (Longcore and Rich, 2016). 5. Conclusions To conclude, we found that long wavelength (yellow and red) artificial light did not significantly reduce mate attraction success in the common glow-worm, whereas short wavelength (blue and white) artificial light did. Hence, artificial light wavelength appears to be an important factor in determining the effects of light pollution on glow-worms, with potential implications for glow-worm conservation. Our findings indicate that favor- ing yellow over bluish and white lighting in the proximity of glow-worm habitats may be a useful method for mitigating the negative effects of light pollution on glow-worm reproduction. However, further investigation of how glow-worms respond to long wavelength light is still required. By showing that different artificial light spectra can have different effects, this study strengthens our knowledge regarding the ecological effects of light pollution. Funding This workwas supported by Societas pro Fauna et Flora Fennica (to LK), the Swedish Cultural Foundation in Finland (grant number 160603 to UC), the Maj and Tor Nessling Foundation (grant number 202000239 to CE). CRediT authorship contribution statement Linnea Kivelä: Conceptualization, Methodology, Formal analysis, In- vestigation, Writing – original draft, Writing – review& editing, Visualiza- tion. Christina Elgert: Conceptualization, Methodology, Investigation, Writing – original draft, Writing – review & editing. Topi K. Lehtonen: Conceptualization, Methodology, Formal analysis, Writing – review & editing. Ulrika Candolin: Conceptualization, Methodology, Writing – review & editing. Data availability The data are available at doi:https://doi.org/10.7910/DVN/QWQPLJ. 5 Declaration of competing interest The authors declare that they have no known competing financial inter- ests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We would like to thank Tvärminne Zoological Station for providing fa- cilities and accommodation. 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Introduction 2. Materials and methods 2.1. Study species and area 2.2. Experimental setup 2.3. Statistical analysis 3. Results 4. Discussion 5. Conclusions Funding CRediT authorship contribution statement Declaration of competing interest Acknowledgements Appendix A. Supplementary data References