Why do scorpions glow under a black light?
Written by the Hydra Pest Control team, Arizona-licensed (license #9790)
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Short answer
Scorpions glow because their outer shell, called the cuticle, contains compounds that absorb ultraviolet (UV) light and give it back off as a bright cyan-green glow. Researchers have identified two of them: beta-carboline and 4-methyl-7-hydroxycoumarin. Why scorpions glow is still an open question. A leading idea, backed by lab studies, is that the glow helps scorpions sense light so they can find shade and shelter.
Arizona context
University of Arizona Cooperative Extension notes that scorpions glow brightly under UV or black light, and it recommends using a UV light to check inside your home for scorpions before bedtime. That matters in Arizona because the Arizona bark scorpion is the only scorpion of medical concern in the U.S. Bark scorpions move mostly at night, between about 7 and 11 p.m. and again from 3 a.m. to sunrise, and nighttime temperatures above 70 degrees are ideal for them.
How to use a black light safely
- Check inside your home with a UV flashlight before bedtime, including floors, walls, beds, sinks and tubs. Bark scorpions climb walls and can fall from ceilings
- Outdoors, look on summer evenings between about 8 and 11 p.m., and repeat the check several times during the summer
- Wear boots or closed shoes, never pick up a scorpion with your hands, and use long tongs if you need to move one
- Teach children not to touch scorpions and to tell an adult when they see one
- Keep shoes, towels and clothing off the floor so scorpions have fewer places to hide
What the research says about the glow
In a 2012 study published in Animal Behaviour, researchers found that desert grassland scorpions reacted strongly to UV light. Scorpions with their eyes covered were much less likely to react to cyan-green light than to UV light. The researchers proposed that the whole shell may work like a light collector, turning UV light into the cyan-green light that scorpion eyes are most sensitive to. That could help a scorpion tell when it is out in the open and move toward shelter.
A 2014 follow-up study in The Journal of Arachnology found scorpions reacted most to UV at light levels like those around sunset, and it notes that scorpions generally move away from light. Scientists have suggested other explanations too, and some think the glow may serve no purpose at all. No single explanation has been proven.
When professional service is warranted
If you keep finding scorpions inside, University of Arizona Cooperative Extension recommends calling a pest management professional experienced with scorpions, and it says pest-proofing your home is by far the most effective way to reduce scorpion contact and stings. For any sting, call Poison Control at 1-800-222-1222 (Arizona Poison and Drug Information Center). If a child 9 or younger, or under 70 pounds, is stung, call 911 or Poison Control, or go to the nearest hospital right away. This page is general information, not medical advice.
How Hydra handles it
Our Service Pros look for where scorpions are getting in and hiding around your home and treat those areas. To talk it through, call or text your local office to reach an Office Pro, or ask for a free custom quote.
Frequently asked
Will a black light find all the scorpions around my house?
No, that's mostly a myth. A black light only shows scorpions that are out in the open within its beam. During the day, Arizona bark scorpions hide in crevices, wall voids, hollow block walls, and under rocks, logs and tree bark, and around buildings they usually go unnoticed until they get inside. In the coldest months they cluster in sheltered, undisturbed spaces. Use a black light to spot and monitor scorpions, not to count every one.
What color do scorpions glow?
Under UV light, scorpions glow a bright cyan-green, a blue-green color. The glow comes from compounds in the outer layer of the shell.
What kind of black light works best for scorpions?
Studies report that scorpion shells glow most strongly under UV light at about 395 nanometers. If you shop for a UV flashlight, check the wavelength on the label.
Is a scorpion sting an emergency?
It can be. The Arizona bark scorpion is the only scorpion of medical concern in the U.S. Most healthy adults can manage a sting at home, but call Poison Control at 1-800-222-1222 after any sting for advice. If a child 9 or younger, or under 70 pounds, is stung, call 911 or Poison Control, or go to the nearest hospital right away. This is general information, not medical advice.
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References
- University of Arizona Cooperative Extension: Scorpions of the Desert Southwest United States (AZ1768)
- Gaffin et al. 2012, Animal Behaviour 83:429-436: Scorpion fluorescence and reaction to light
- Gaffin and Barker 2014, The Journal of Arachnology 42:111-118: Comparison of scorpion behavioral responses to UV under sunset and nighttime irradiances
- Arizona Poison and Drug Information Center: Scorpions
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