A deadly passion: moths and their attraction to artificial light

Bee to the blossom, moth to the flame;
Each to his passion ….

~Helen Hunt Jackson

It’s a common saying – “like a moth to a flame” – which implies a strong, perhaps even dangerous attraction to something.  And it is certainly true that moths seem blindly attracted to man-made light, including flames, often with disastrous results.  But why?  The behavior is a mystery in science.

Moths are members of the Order Lepidoptera (Greek for “scale” and “wing” – perhaps to distinguish them from feather-winged birds) with 130,000 species across the globe.  They are an important organism in ecosystems, acting as pollinators, food sources, and sometimes pests.  Most moths are nocturnal, or active during evening hours, and exhibit “positive phototaxis,” meaning they move toward a light source.  (“Negative phototaxis,” on the other hand, means an organism moves away from the light; think cockroaches scurrying when the lights come on.)  This can be seen during the evening hours if you put up a white sheet with a strong light source behind it – moths, and other insects, will swarm on the fabric. Scientists have a few theories (see our module Theories, Hypothesis, and Laws for more information on how scientists use theories) as to why moths behave in this manner, but there is no consensus on the matter.

The most widely held theory is that moths use an internal navigation system, oriented to the moon, and a man-made light source disorients the creature.  The theory postulates that moths evolved with the moon as their chief point of reference for flying, using its light to maintain a steady angle of flight.  A light bulb or candle flame disorients the moth, confusing it and causing it to reorient itself to the artificial light.  But some scientists question if moths actually use the moon to navigate and there is no clear explanation for why the moth often crashes into the light source, with disastrous results for the creature.

Another theory is that artificial light produces some ultraviolet (UV) light that may mimic the same wavelengths of the moth’s preferred flowers.  (For more on light and wavelengths, see Light I).  Many moths are pollinators, attracted to a flower’s nectar and, like bees, transport some pollen in the process of feeding.  (See the module Adaptation for a discussion of Darwin’s theories on flower evolution and pollinating moths.)  Some flowers look different in UV light, a wavelength visible to many insects, and use patterns or markings to help attract and guide their pollinators.  This theory explains why the creatures are so attracted to the light source, but it has some problems because many moths use scent and CO₂ as a guide to flowers and artificial light does not mimic those elements.

Other theories for a moth’s attraction to artificial light are that the light mimics the infrared frequencies of a female moth’s pheromones, or that it is part of an escape route mechanism where moths fly toward light to escape danger, or that it triggers a response in these night-flying insects to hide in morning sunlight.  A final explanation, which is less of an explanation than a fact, is that there are many species of moths that are not all reacting to the light for the same reasons.

The beauty of diversity is that there can be many reasons behind a creature’s instinctual habit.  Since science is a process that is ongoing and evaluating theories is an important part of that process, scientists continue to develop and test ideas for why moths are attracted to artificial light. With new advances and creative experiments, we learn more about the possible reasons for a moth’s actions.  And, though we may never have a single reason for the cause, this process allows scientists to better understand the insect’s “passion” for the flame.

For more information:

– Like moths to a flame – BBC – discusses the impact artificial light is having on insect populations. http://www.bbc.co.uk/blogs/wondermonkey/2011/05/like-moths-to-a-flame.shtml
– Why are Moths Attracted to Flame? – NPR – interview with two entomologists on possible reasons for a moth’s attraction to artificial light. http://www.npr.org/templates/story/story.php?storyId=12903572
– A robot flower experiment reveals how hawkmoths see at dusk – The Verge – article discussing a scientific experiment on how moths see flowers at night. http://www.theverge.com/2015/6/11/8767059/moth-vision-night-robot-flower

Other resources:

– The Lepidopterist Society – http://www.lepsoc.org/index.php
– National Moth Week – http://nationalmothweek.org/
– Moth Pollination, USDA Forest Service – http://www.fs.fed.us/wildflowers/pollinators/animals/moths.shtml

Written by Eric Dillalogue

Eric Dillalogue holds a MS in Library and Information Science and a BA in English. He has worked in a variety of roles from service industry management, academic libraries, and grant administration. He has taught courses on information literacy, web research, and developmental reading. Eric joined the Visionlearning team as a project manager in 2014.