Why Are Moths Attracted to Light? Insect Phototaxis


Why Are Moths Attracted to Light
While moths are attracted to light and heat, they likely circle lamps because their navigation gets confused.

The phrase “like a moth to a flame” refers to a potential fatal attraction, since a moth doesn’t always escape the heat of a fire or the zap of a bug light. But, why are moths and other insects attracted to light? There are several theories that explain the behavior. Which ones are correct?

  • Moths and many other flying insects are attracted to light. This is called positive phototaxis. In contrast, some insects such as cockroaches avoid light. This is negative phototaxis.
  • There are many theories about why moths are attracted to light. The transverse orientation theory has wide support from scientific studies. This theory proposes that light confuses insect navigation, causing them to circle the light as they maintain the same angle with respect to the light source.
  • Regardless of the reason for the behavior, many flying insects feel attraction to light, heat, and electromagnetic fields.

Theories About Why Moths Are Attracted to Light

Phototactic Behavior Theory

This theory proposes that moths are attracted to light or positively phototactic. The theory describes the behavior, but does not explain its reasons. It may be for navigation or predator evasion.

Laboratory experiment exposing moths to varying light intensities and wavelengths measure whether or not there are changes in their attraction levels. Moths and many flying insects, such as certain species of flies, move toward light sources or display positive phototaxis. Meanwhile, cockroach and some species of beetles display negative phototaxis and avoid light.

Transverse Orientation Theory

This theory proposes that rather than being attracted to light, moth navigation gets confused by it. Moths and many other flying insects use light for navigation. Light rays from a distant source (Sun, Moon, stars) are parallel to each other, so these insects have evolved to receive and process light in a transverse orientation. When a moth flies in a straight line, the visual pattern of a distant light remains constant. But, the angle of light rapidly changes for a small artificial light. So, keeping the angle constant results in the insect spiraling around the light.

Experiments testing this theory measure the angle of insect flight. Moths fly at a consistent angle around an artificial light, supporting the theory. In the case of moths, specifically, the insects keep their dorsal surface (back) toward the light. Another experiment tested whether moths navigate by maintaining a fixed position relative to celestial light. In this experiment, moth behavior changed when constellations in a planetarium were altered.

Fatal Attraction Theory

This theory suggested that artificial lights are so attractive to moths that they cannot resist them, even to the point of exhaustion or death.

Moths and certain other insects feel attraction to light, heat, and electromagnetic fields. While wind blows some insects out of their death spiral, the theory suggests flames and lights are irresistible. However, insects don’t land on hot light bulbs. One study showed that younger moths were attracted to lights, while older moths continued pollinating night-blooming plants despite the lure of artificial lighting. So, the fatal attraction theory is overly simplistic.

Misdirected Sexual Attraction Theory

Another theory proposes that insects confuse artificial lights with the infrared light frequencies of pheromones of potential mates. Two pieces of evidence largely disprove this theory. First, males feel attraction to female pheromones, yet both male and female moths are attracted to lights. Second, male moth sensory organs detect pheromones directly, without needing an infrared cue.

Ultraviolet Light Attraction

Another theory proposes that moths seek ultraviolet light as a pollination guide. Insects perceive ultraviolet light and some flowers have ultraviolet patches that guide pollinators. So, perhaps a light looks like a big field of blooms.

Research indicates ultraviolet, blue, and green lights do attract bugs more than yellow, amber, or red lights. This is true even for LED lights, which emit very little heat. But, whether this relates to insect pollination or simply reflects the way insect vision works is unclear.

Night Blindness

This theory proposes that insects remain circling lights for protection.

While humans recover from temporary blindness from bright outdoor lights pretty quickly, it takes insects around half an hour to recover their sight. So, they may stay within range of the light to avoid the dangers of flying blind, such as collisions and predators.

Pros and Cons of Moths Attracted to Light

Pest Control

Insect attraction to light has implications for pest control:

  • Use of Light Traps: Light traps are common for controlling insects like moths. These traps efficiently capture large numbers of pests, reducing their impact on agriculture and horticulture.
  • Targeted Pest Management: Understanding the specific wavelengths of light that attract certain pests leads to the development of more targeted and effective pest control methods, minimizing the use of chemical pesticides.
  • Monitoring Pest Populations: Light traps are also useful for monitoring purposes, providing data on the types and numbers of species in a particular area. This information helps in implementing timely and appropriate pest control strategies.

Risks to Ecosystems from Light Pollution

However, there is a darker side to the insect attraction to light:

  • Disruption of Natural Behaviors: Artificial light disrupt the natural behaviors of nocturnal insects, affecting their feeding, mating, and migration patterns. This disruption leads to declines in insect populations. This, in turn, affects species that rely on them for food, such as birds and bats.
  • Altered Predator-Prey Dynamics: The unnatural concentration of insects around light sources alters predator-prey dynamics. Predators might exploit these gatherings for easy feeding, which further unbalancing ecological relationships.
  • Impact on Plant-Pollinator Interactions: Many insects, including some moths, play a crucial role in pollinating plants. The disruption of their natural activity patterns due to artificial light impacts pollination of both wild and cultivated plants.
  • Contribution to Insect Decline: Light pollution is one of the factors contributing to the global decline of insect populations. This decline has broader implications for biodiversity and the functioning of ecosystems.
  • Behavioral Changes and Energy Expenditure: Exposure to artificial light leads to increased energy expenditure in insects as they navigate the illuminated environment, potentially reducing their overall fitness and survival rates.

References

  • Boyes, D.H.; Evans, D. M. et al. (2020). “Is light pollution driving moth population declines? A review of causal mechanisms across the life cycle.” Insect Conservation and Diversity. 14(2): 167-187. doi:10.1111/icad.12447
  • Gorostiza, E.A.; Colomb, J.; Brembs, B. (2016). “A decision underlies phototaxis in an insect.” Open Biol. 6(12): 160229. doi:10.1098/rsob.160229
  • Jägerbrand, A.; Andersson, P.; et al. (2023). “Dose-effects in behavioural responses of moths to light in a controlled lab experiment.” Sci Rep. 13(1): 10339. doi: 10.1038/s41598-023-37256-0
  • Jonason, D.; Franzén, M.; Ranius, T. (2014). “Surveying Moths Using Light Traps: Effects of Weather and Time of Year.” PLoS ONE. 9(3): e92453. doi:doi.org/10.1371/journal.pone.0092453
  • Suver, M.P.; Mamiya, A.; Dickinson, M.H. (2012). “Octopamine neurons mediate flight-induced modulation of visual processing in Drosophila.” Curr. Biol. 22: 2294–2302. doi:10.1016/j.cub.2012.10.034