Like a Moth to a Flame: How Artificial Lights Affect Moths’ Behavior

Posted by Jacqueline Mosko on October 19, 2023 in Animals, Behavioral Neuroscience | Leave a comment

You have probably heard the expression, “drawn to it like a moth to a flame.” This expression calls upon the poorly understood attraction of moths to sources of artificial light, despite the fact that these lights often harm or even kill them. Society has deemed that moths are a useful simile for describing an irresistible attraction to something, even if it is wrong. This phrase has permeated pop culture – as Taylor Swift recently penned in an updated lyric to her song Better Than Revenge (Taylor’s Version), “…he was a moth to a flame, she was holding the matches” [1]. Prior to the rise of modern music, the people of ancient Greece saw a mention of “the fate of the moth in the flame” from the poet Aeschylus in 500 BC [2]. 

Such repeated exposures may begin to make you ponder the phenomenon at the root of this phrase – why are moths attracted to flames and light? In today’s urban environments, this question is particularly relevant when we witness moths flying into artificial light sources such as street lamps, house lights, and other man-made fixtures. Do lights resemble something they once benefitted from pursuing during flight? Are artificial lights simply distracting? Is a moth’s attraction to light a genetically encoded flaw, their very own self-destruct button a la the Death Star? Lucky for curious readers, entomologists (researchers specializing in the study of insects) have been working on determining the answers to this question. 

I Love Lamp

When moths fly towards artificial light sources, they are often injured, sometimes even dying upon contact with the hot surfaces. In fact, people have been using lights as a pest control method for thousands of years [2]. The act of moving towards light that moths demonstrate may also be referred to as phototaxis – photo for the involvement of light, taxis referring to the motor component of the process. When moths or other insects experience attraction and move towards light, it is known as positive phototaxis. Conversely, insects who are repelled by light exhibit negative phototaxis (for example, cockroaches, earthworms, and other creepy crawlers who prefer dark spaces!). Phototactic moths have been at the center of much of the research examining the relationship between lights and insects’ motor behaviors. 

In addition to phototaxis, researchers have worked to characterize kinematic features of moth flight. Generally, kinematics is the study of motion, with particular attention paid to the geometry of movement. In regard to moths’ flight activity, researchers are particularly interested in studying details of kinematics that help describe the way moths position their bodies during flight, as well as describing how they move through space in terms of speed and trajectory (the path followed by a flying object – in our case, the moth) [3]. One particularly important kinematic feature is called transverse orientation. Transverse orientation is accomplished by flying while maintaining a fixed angle between an insect’s body and a source of light [4]. The word ‘transverse’ refers to the plane the insects fly in, keeping a parallel relationship between their backs and the moon in the sky. This is incredibly important for insects’ ability to navigate. For generations, moths have used the light of the moon as a key tool for spatial navigation throughout their worlds [2]. Given that moths have navigated by using the moon, a reliable and very distant light source, it is unsurprising that the sudden emergence of artificial light – brighter, closer light sources – may disrupt this behavior. 

In recent work, a group of researchers utilized today’s advanced technology, including incredible high-speed cameras and 3D tracking software, to assess the flight paths of moths in a laboratory environment when exposed to various light sources. One of many variables examined in this study was the position of the light source. As aforementioned, the moon is a distant light source moths have evolved to search for above themselves. Prior to the invention of artificial lights, moths would scarcely encounter bright lights below their typical cruising altitude (except for, perhaps, the occasional ancient flame). In this study, it was found that encountering light sources from below caused moths to invert their body position, attempting to keep their backs to the light. This resulted in a sharp change in their trajectories that led to “erratic” flight before crashing (Fig. 1) [5]. Recalling the phenomenon of transverse orientation, the principle of maintaining a fixed angle between a moth’s body and sources of light such as the moon, this result calls into question the ability of moths to discriminate directionality or quality of light during flight. Unfortunately, it appears that this lack of discriminatory ability can lead moths into a tailspin – though, it is important to remember that they did not have access to natural moonlight during this test, only the reflected lights carefully positioned by experimenters. This finding replicated some of the disoriented crashing behavior colloquially observed in moths who flit around artificial light sources, seemingly losing their bearings before occasionally falling victim to hot lights. Simply put – manmade lights have the ability to cause moths to enter a tailspin while trying to maintain their ethological navigation behaviors.

Blinded by the (city) lights

Recently, researchers have begun to assess how moths are using lights to navigate in urban settings. It should come as no surprise that artificial lights have disrupted insects’ evolutionarily perfected navigation strategies. In the grand scheme of things, the invention of the light bulb is still quite recent. While moths have been around for approximately 300 million years, the incandescent light bulb (picture the antique bulb which emits light from its internal wires as a result of electric heat) was only patented in 1879 [6, 7]! Whereas the moon is moths’ intended source of light for nighttime navigation, light sources such as streetlamps have been observed to serve as beacons for moths who are unable to locate celestial light [8]. To test the likelihood of moths to exhibit phototactic behavior in a controlled environment, one study released groups of moths in the center of a ring of streetlamps (Fig. 2). In this environment, there were six streetlamps that were each 85 meters away from the release site. To determine the effect of the streetlamps on the flight behavior of the moths, they conducted this experiment in both “streetlamp on” and “streetlamp off” conditions. Researchers began by asking how many moths would fly directly to streetlamps and end their flight on the lamps, rather than flying past the lights. Surprisingly, by filming and tracking moths’ flight paths, they found that only 4% of the moths exhibited phototactic behavior when it was defined as terminating a flight path at a light source (Fig. 2, left) [8]. This may seem like it challenges the theory of lights interfering with moths’ flight, but the researchers took a closer look at what was occurring in the behavior of the moths that did not end their flights at the lamps. They quickly realized that even though only a small percentage of moths terminated their flight paths at the lamps, a larger portion of the moths incorporated the lamps into their flight paths (entering a 10-meter radius of a lamp) during the “light on” condition, before continuing their journey past the lamps (Fig. 2, right). To determine if distance from the lamps may be responsible for the low attraction rate, a follow up experiment was conducted by the same group where moths were released just 10 meters away from a streetlamp, and 100% of these moths exhibited the expected behaviors; they all flew towards the lamp, began to circle it, and eventually crashed, displaying the tailspin phenomenon described by the previous study (Fig. 1). Thus, even though not every moth was drawn to the streetlamps from a greater distance, it was seen that the lamplight significantly impacted moths’ flight trajectories and behavior when they were released within 10 meters of the lamps [8]. This study underscored the importance of moths’ proximity to artificial light sources for the occurrence of crashing behavior, but also demonstrated that smaller portions of moths tend to fly towards lights even from greater distances. 

Though this finding was interesting on its own, the same study delved further into flight behavior by looking at flight qualities aside from trajectory. As important as changes in the destination of a flight are, changes in the intention of a flight (what a moth intends to do while flying) can be equally telling. To investigate how artificial light affects flight quality in moths, researchers studied Hawk Moths collected from outside of the experimental field, as well as locally collected Lappet Moths. The expectation was that Hawk Moths would flee the experimental field, as it was unfamiliar terrain. Conversely, it was hypothesized that the Lappet Moths would more frequently display hunting and goal-oriented flight behavior to search for resources, since they were in a familiar area. The juxtaposition of these two species with different hypothesized behaviors allowed experimenters to examine the effect of light on many different types of flight: retreat, hunting, and mating-driven flights. In this experiment, researchers also took into account the role of the moon in altered flight behavior, due to its theorized role in phototaxis and transverse orientation. 

The experiment employs a scale of tortuosity to quantify moth’s flight behavior, which is a commonly used measure of deviation from a straight, expected path in insects and other flying objects [9]. In Hawk Moths, researchers observed that artificial lights significantly increased the tortuosity of flight when the moon was above the horizon (i.e., was visible). There was also a trending increase in tortuosity when the moon was not visible, and streetlamps were on. This essentially demonstrated that in moths carrying out escape-oriented flights in unfamiliar terrain, presence of artificial lamp light significantly impacted flight when the moon was above the horizon, as well as affecting moths’ flight even in the absence of the moon. In Lappet Moths, no change in behavior was exhibited when the moon was visible, but tortuosity was significantly increased when the lights were on in the absence of the moon (Fig. 3) [8]. Altogether, this showed that moths were most likely to be affected by artificial light when engaging in behaviors which typically show a steady and fixed flight path, such as retreating from a foreign area. The Hawk Moths’ deviation from their expected path in presence of streetlamps is a poignant example of this phenomenon. In Lappet Moths, although the difference in measures of tortuosity when lamps are on versus off appear smaller, this is accounted for by the differences in flight intention. These moths were flying in less straight paths, with the intention of finding food, mating, and performing other goal-oriented behaviors, but even so, tortuosity was increased when lamps were on in the absence of the moon – indicating that artificial light can impact these behaviors as well [8]. The differences in behavior between species unmistakably underscore the varied importance of the moon’s position in relation to artificial light and is ripe for future study in this field. 

I Can See the Light

These species-dependent ethological differences in moths’ behavior, in response to light in the presence or absence of the moon, shed light on the theories of transverse orientation’s role in the classic moth-flame paradigm. Whereas people believed for centuries that moths were flocking to flames and lights due to an innate attraction, it seems that the more realistic and scientifically backed theory is that moths are more confused by lights than they are attracted to them. The key tenant of current research is that transverse orientation is the driving force behind moths’ observed behaviors. Considering the high likelihood of moths to enter a tailspin in attempt to maintain their body position in relation to artificial lights in both a lab and ethologically relevant setting, it is not difficult to accept the explanation that moths have trouble navigating urban settings where any light could be mistaken for the celestial body they’ve navigated by for eons.

Citations

[1] Taylor Swift. (2023). Better Than Revenge (Taylor’s Version). On Speak Now (Taylor’s Version). 

[2] Macnaughton, A. (2011). Moths and Artificial Night Lighting. 2011 SOURCES OF KNOWLEDGE FORUM. 

[3] Encyclopædia Britannica, inc. (2023). Kinematics. Encyclopædia Britannica. https://www.britannica.com/science/kinematics 

[4] NPR. (2007). Why are Moths Attracted to Flame? All Things Considered. Retrieved October 19, 2023, from https://www.npr.org/templates/story/story.php?storyId=12903572. 

 [5] Fabian, S. T., Sondhi, Y., Allen, P., Theobald, J., & Lin, H.-T. (2023). Why flying insects gather at artificial light. bioRxiv. https://doi.org/10.1101/2023.04.11.536486 

[6] Kawahara, A. Y., Plotkin, D., Espeland, M., Meusemann, K., Toussaint, E. F., Donath, A., Gimnich, F., Frandsen, P. B., Zwick, A., dos Reis, M., Barber, J. R., Peters, R. S., Liu, S., Zhou, X., Mayer, C., Podsiadlowski, L., Storer, C., Yack, J. E., Misof, B., & Breinholt, J. W. (2019). Phylogenomics reveals the evolutionary timing and pattern of butterflies and Moths. Proceedings of the National Academy of Sciences, 116(45), 22657–22663. https://doi.org/10.1073/pnas.1907847116 

[7] MATULKA, R., & WOOD, D. (2013, November 22). The history of the light bulb. Energy.gov. https://www.energy.gov/articles/history-light-bulb 

[8] Degen, J., Storms, M., Lee, C. B., Jechow, A., Stöckl, A. L., Hölker, F., Jakhar, A., Walter, T., Walter, S., Mitesser, O., Hovestadt, T., & Degen, T. (2022). Streetlights affect moth orientation beyond flight-to-light behaviour. bioRxiv. https://doi.org/10.1101/2022.10.06.511092 

[9] Bustamante, J., Jr, Ahmed, M., Deora, T., Fabien, B., & Daniel, T. L. (2022). Abdominal Movements in Insect Flight Reshape the Role of Non-Aerodynamic Structures for Flight Maneuverability I: Model Predictive Control for Flower Tracking. Integrative organismal biology (Oxford, England), 4(1), obac039. https://doi.org/10.1093/iob/obac039