In this post I reflect back on what my hubs and I have learned about mothing over the past year or so, and a look into the science of why moths are attracted to light. We first started mothing back in October of 2019 and our first attempts were pretty pitiful, and we’ve come a long way. Thank goodness!

When we lived in San Jose, we’d drive 45 minutes up to Montebello Open Space Preserve, set up our crotchety system on the side of the road and wait. October isn’t that cold in central California, but at night it’s chilly for sure. All bundled up we would stand, and wait thinking moths would flock to our sheet within minutes. WRONG! Not only were we impatient, it was sort of the end of the prime mothing months. We saw more cars driving by with drivers that I guarantee were highly confused to what two ding dongs were doing on the side of the road than interesting moths. One ranger paused in her truck, confused as heck and curious why we had extremely bright headlamps intensely focused at our white bed sheet draped over the car. I don’t blame her. 

So flash forward to nowadays. Even though we are nomads in California, moving locations monthly we have a more permanent mothing set up. One day when we live in our own space we’ll set up a truly permanent, water-proof, stable contraption to maximize night-flying insects. But until then this is what our set up looks like and what we’ve learned. 

We either use existing structures at our host’s property, like door frames, or sliding doors. But when physical structures are absent we use our Subaru Forester. The sheet can be closed shut on by door frames, making for a secure hold. We’ve also used the roof rack with gear ties and rubber bands.

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Mothing Newbs

It was a humble beginning, but everyone starts somewhere! Here’s a few moths we found when we first began. We’ve seen a lot more diversity since, which you’ll see below.

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Moth Set Up Components:

We are truly still amateurs, but we have steadily mothed over the last year, and take our gear wherever we go. However I’m sure in a year we’ll have a better set up, but I assume the core type of gear won’t change. We have tried to use a giant portable battery the kind you could jump your car with but that thing was so heavy and obnoxious it wasn’t worth it. For now here’s what we use:

• Old or Thrifted Bed Sheets, we’ve tried fitted and non, makes no difference, both get nasty eventually
• UV Light Options:
  • Small tube capable of being powered by a phone battery pack via usb connection, good for backpacking,
  • Large “Cannon” 100W Light
• Any trinkets to get the sheet to stay up: rubber bands, thumb tacks, gear ties
• Extension Cords
• Portable Battery Pack (for mothing while backpacking)

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Ecuador Dec 2019-Jan 2020

For our second trip to Ecuador, while still very much bird focused, included documenting plant and insect diversity. Here are some photos of moths attracted to our small UV light. The colors, shapes, and, patterns were INCREDIBLE! Just fantastic! The last two photos are of giant silk moths, that were about the size of an adult hand! Huge! MUY BUENO!

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Why are moths attracted to the light?

Anyone with a porch light has probably observed a fluttering flock of moths having a party around the light at darkness, with a few compromised comrades on the floor beneath. So, yes indeed they are attracted to the light. But they obviously don’t gain anything from the being zapped. What used to be the commonly accepted theory that they are attracted to the moon – or a fake moon aka porchlight – is not actually the full picture. Actually, a fully fleshed out theory is still unknown! 

Before explaining the moon theory, moths detect light through receptors in their eyes. Most insects, including Lepidoptera (the Family that moths belong to) have photoreceptors that detect UV, blue and green light. But some moths can detect red, and others only detect UV and green wavelengths. So moths are a total mixed bag and more research is fully needed to have a complete understanding.

Back to the moon theory. What researchers think is that the moon and stars is actually a tool, which moths use to orient themselves. Since the moon is pretty stationary, always “above” the moth, the moth can fly straight using the parallel light rays as guidance. But us dang humans throw in millions of lamps which spread light rays in a circular fashion, and the moths get confused! Moths flying near the light are flying at all different angles to the source which causes them to turn towards the light. This can cause the spiraling flight patterns you may have observed. But again, it’s not fully understood.

Moths aren’t the only creatures attracted to light, a type of movement called positive phototaxis. This can be observed in bacteria under a microscope, jellyfish to avoid predation, and in many other insects, such as our lovely moths. 

Moths From 2020 Summer in BC

While living in British Columbia in the summer months of 2020 we draped the sheet across a large piece of plywood which worked great as a semi-permanent set-up. It was heavy to move but worked quite well, as the plywood provided a great solid and flat surface to observe and photograph the moths on.

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Why UV Light?

Exactly why moths are attracted to UV wavelengths isn’t totally understood. We just know it works. Using light to attract, and trap nocturnal Lepidoptera is a well known and studied technique. What we do know is that the smaller wavelengths of light emitted by UV lamps/bulbs attract a wide variety of species, and quantity of those species compared to other wavelengths.

Using this light trapping technique allows many different questions to be asked. Such as how biodiversity changes in urban vs wild landscapes, differences in biodiversity in different landscapes with differing artificial light influences and how climate change will affect moth communities. But what we do know is that there is a size bias. A 2011 Study by Langevelde et al. found that smaller wavelengths attract generally larger moths on average. So moths with “larger body mass, larger wing dimensions and larger eyes” will come to the light at a higher proportion. While we’d love to see all the moths in the vicinity, we aren’t conducting a rigorous study. . . yet! But it would be interesting to see who shows up to a light with longer wavelengths.

In a nutshell, UV LED lights are easily accessible, portable, and according to a 2017 article by Infusino et al. they are “resistant to mechanical damage, easily protected from heavy rain and energy efficient.” When compared to other traditionally used light trapping sources/lamps UV LEDs are just as effective in attracting diversity and quantity of moths. Any differences in diversity and abundance is mostly a reflection of the habitat. So for any type of field work, this set up makes sense. Since we moth while backpacking in different habitats, I’m glad to know this set up is backed by science.

For a great introduction article about how to moth, this Science Friday article has some great animations.

Winter in California, 2020/2021

Perhaps I have a slight bias for the red and pink colored moths, but Ooo La La, these winter time moths are stunning! On the California Coast winter doesn’t mean no bugs! There are oodles!

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Impact of Artificial Lights

So the obvious thought would be, duh all these artificial lights we as humans turn on at night must affect moths, but what does the science say? Well there is even an acronym for this historically recent lighting up of the night. ALAN! ALAN stands for artificial light at night. Dang it ALAN! And the simple answer is yes, but by how much and in what ways is still unknown. A 2020 study by Boyes et al. found moths are affected by ALAN at the individual behavior and physiological level. Whether ALAN affects moths at the population level is still being determined. But what the researchers did find was “that ALAN can disrupt reproduction, larval development, pupal diapause, with likely negative impacts on individual fitness, and that moths can be indirectly affected via host plants and predators.” What a list! Dang it ALAN!

“When we try to pick out anything by itself, we find it hitched to everything else in the Universe.”

John Muir

Since widespread insect declines have been documented, and light pollution only increasing globally, this added effect could be combining with factors such as habitat decline, and climate change to negatively affect moths populations overall. It may only be the beginning of our understanding of all the different ways humans are causing insect decline. Depressing. So any precautionary strategies or techniques we can do should be done to curb an insect apocalypse.

So are we to blame? Are we inadvertently killing moths? Disrupting their precious innocent lives? Or is documenting their presence helping the broader scientific community? I’d strongly argue that it’s the latter. Only when we mothed on the Sunshine Coast in September of 2020 during an irruption of Hemlock Looper Moths did I feel guilty, as there were quite a few casualties. Attracting thousands of them to an Airbnb porch was not something we’d expect to happen. But by braving the cold of the early morning and late night (thank you Trevor), and by documenting and uploading our observations to places like iNaturalists we have helped researchers recognize range expansions, and contributed some iNaturalists firsts. This last batch of photos below showcase them.

Moths that are the first for iNaturalist

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If you find a moth, and want to know what it is, try posting to either iNaturalist or Bug Guide. The App Leps by Fieldguide is a also good starting point.

Well I hope this gives an small insight into our mothing adventures. Thank you to our families that have experienced the moth madness, as well as to all our hosts as we travel throughout California allowing us to set up a weird blue light to attract the little mostly grey moths. We are now off to the Southern California desert region, and we’re excited to see who will visit our lights there! Stay tuned!

Happy Mothing!

• Resources:
  • Phototaxis Wiki Page
  • Science Friday ,Go Mothing! Article
  • Science Friday, Why Are Moths Attracted To Light? Article
  • Infusino, Marco, et al. “Assessing the efficiency of UV LEDs as light sources for sampling the diversity of macro-moths (Lepidoptera).” European Journal of Entomology 114 (2017).
  • Altermatt, Florian, Adrian Baumeyer, and Dieter Ebert. “Experimental evidence for male biased flight‐to‐light behavior in two moth species.” Entomologia experimentalis et applicata 130.3 (2009): 259-265.
  • van Langevelde, Frank, et al. “Effect of spectral composition of artificial light on the attraction of moths.” Biological conservation 144.9 (2011): 2274-2281.
  • Brehm G (2017) A new LED lamp for the collection of nocturnal Lepidoptera and a spectral comparison of light-trapping lamps. Nota Lepidopterologica 40(1): 87-108. https://doi.org/10.3897/nl.40.11887
  • Price, Benjamin. “NightLife: A cheap, robust, LED based light trap for collecting aquatic insects in remote areas.” Biodiversity Data Journal 4 (2016).
  • Boyes, Douglas H., et al. “Is light pollution driving moth population declines? A review of causal mechanisms across the life cycle.” Insect Conservation and Diversity (2020).