Earwig question?

For the past several years I (along with many others) have observed thousands of European earwigs congregating on the summit of Mount Diablo in May through June…Mount Diablo is a State Park east of San Francisco in Northern California. There is a visitor center, parking lot, and observation deck at the summit where little critters fly, run, lurk, hide, and land on everybody and every thing during this time. Has any one else observed this kind of behavior. I have not seen them mating at this time but it could occur at night or in flight high in the air. I would appreciate any information to solve this mystery.

welcome to the forum.

here’s the first article that i found online that seems to describe a similar situation and provides many nuggets of information for possible further research: https://www.trustterminix.com/earwigs-gone-wild/ (similar story but with photos: https://www.mypmp.net/2018/10/03/earwig-invasion-mystery/). here’s another interesting article: https://www.applegater.org/pdf/2009/v02n04/v02n04p14.pdf. and here’s one more that describes an “explosive irruption” in Baja California, Mexico: https://www.thefreelibrary.com/Invasion+of+two+widely+separated+areas+of+Mexico+by+Forficula…-a0298614340.

i haven’t come across anything that suggests a specific reason why swarms occur, although the consensus seems to be that the swarms aren’t for mating. it seems like the more likely situation is just that there’s a population boom for some reason (maybe due a boom in their food source, or maybe they’re new in the area with no known predators or other control agents), and those new insects are just looking for better conditions (maybe because the original place has run out of food or is just too crowded, or maybe control organisms like nematodes are just springing to life, making the area inhospitable). it seems unclear why the earwigs may be attracted to a particular destination though…

it can get foggy at times in the Bay Area during the summer. i wonder if the extra humidity might offer earwigs the opportunity to swarm and migrate, and then maybe they stop up in the mountains because that’s as far as they can go before it dries up again or something like that? (just speculations…the main case in the first story i linked to also occurs at the top of a mountain.)

it might be interesting to dissect a few just to see if there is anything unusual going on inside of the insects. (that probably won’t lead anywhere i’m guessing, but it doesn’t hurt to check… well, it might hurt the dissected insects, but it will be for science.)

Hi! This seems to be published in the Spanish subsection of the forum. Should be moved to general, so more people can see it. I’m copying @tiwane ! Thanks

Thanks @roget, I moved the topic to Nature Talk, since it’s not directly about iNaturalist.

I’m not aware of any studies on this, but I did find another similar report that seems to indicate this might be a mating swarm.

“we observed an explosive irruption of F. auricularia on the Mexican island of Guadalupe (N 29° 07’, W 118° 32’; 3/4-V-2011), located 260 km off the coast of Baja California in the Pacific Ocean. Many thousands of flying individuals were seen in cypress (Cupressus guadalupensis S. Watson) groves and surrounding areas in the north of the island. The swarm appeared suddenly around 11:00 am (roughly the same time on both days) and stayed in the wind for a few hours. Individuals tended to land on trunks and building walls and to climb upwards, some times reaching concentrations of many hundreds of individuals at the tips of cypress stumps. They were commonly seen in the area for the following 2 wk, walking on the ground or on tree trunks, but very few were still flying around after the first 3 days of the irruption. Hundreds of dead carcasses (about 5 individuals per m2, quite evenly spread) were observed on the gravel-covered ground between the buildings of the biological station soon after the irruptions.”

Thank you…this has been on going for at least five years at the same location…fog bank rarely reaches over to mount Diablo this time of year…normally midday weather is above 75 degrees, some times breezy. I will check out your links and appreciate your work.


Thanks for the response. Sounds similar…I have never been at the park’s visitor center when this appears to start…I seem to always come in at the flying around individually or crawling around…many dead toward the end also, mostly swatted and stepped on. They are out and about for four to six weeks during the warm hours of the day…/I have never been around after 6:00 pm so I don’t know about night time activity but temperatures do drop down to the low 50s at night.