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While checking on my yard trees, I noticed a tiny swarm of gnats above one small maple that has a metal cup holder stake used to help train the trunk straighter. Approximate swarm size of 20 - 30 gnats and they hovered about 6 inches above the top of the cup holder.

I know there's studies on the phenomena of swarm behavior, and out of curiosity as to what they would do I put my hand a couple inches above the cup holder. The little swarm repeatedly moved straight upwards 3-4 inches then returned to the original spot as soon as I removed my hand. When I raised my hand up towards the swarm they would move higher and higher and also follow my hand back down as I lowered it. No matter how high I raised my hands if I removed it quickly they returned immediately to the original location. I checked back a half hour later and they are still there and acted the same way.

What might be the reason for that specific preferred location of the swarm? I has been cloudy and humid all morning and I wonder if the metal of the cup holder is changing the air temperature or some other quality that far above itself? I did also notice a stronger sweet smell of the clover in that location but do have clover all over the yard. My other trees, including a very similar sized young maple but without any support next to it, have no swarms so the metal cup holder perhaps is a correlating factor?

I tried adding a picture for reference but can't get it to upload.

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'Gnat' is the common name for a number of species of tiny flying insects in the Dipterid suborder Nematocera (https://entomology.ca.uky.edu/ef632), including Ceratopogonidae (biting midges), Chironomid midges (non-biting midges) and Simuliidae (blackflies). Like many other Diptera, a lot of these species mate aerially in swarms: the males gather using visual cues to determine swarm location. I'll try to add more detail to this later on why they go for specific locations, but in the meantime here are some papers on mating swarms in some of the groups normally called 'gnats':

General papers on swarming behaviour in Diptera:

  • Downes, J. A. 1969. The swarming and mating flight of Diptera. Annu. Rev. Entomol. 14: 271–298.

Swarming behaviour of the family Ceratopogonidae:

  • Downes, J. A. 1955. Observations on the swarming flight and mating of Culicoides (Diptera: Ceratopogonidae). Trans. R. Entomol. Soc. London 106: 213–236.
  • Campbell, M. M., and Kettle, D. S. 1979. Swarming of Culicoides brevitarsis Kieffer (Diptera: Ceratopogonidae) with reference to markers, swarm size, proximity of cattle, and weather. Australian. J. Zool. 27: 17–30
  • Zimmerman, R. H., Barker, S. J., and Turner, E. C. 1982. Swarming and mating behavior of a natural population of Culicoides variipennis (Diptera: Ceratopogonidae). J. Med. Entomol. 19: 151–156.
  • Blackwell, A., Mordue, A. J., Young, M. R., and Mordue, W. 1992. The swarming behavior of the scottish biting midge, Culicoides impunctatus (Diptera, Ceratopogonidae). Ecol. Entomol. 17: 319–325.

Swarming behaviour of the family Chironomidae:

  • Gibson, N. H. E. 1945. On the mating swarms of certain Chironomidae (Diptera). Trans. R. Entomol. Soc. London 95: 263–294.
  • Oliver, D. R. 1971. Life history of the Chironomidae. Annu. Rev. Entomol. 16: 211–230.
  • Fyodorova, M. V., and Azovsky, A. I. 2003. Interactions between swarming Chironomus annularius (Diptera: Chironomidae) males: role of acoustic behavior. J. Insect Behav. 16: 295–306.

Swarming behaviour of the family Simuliidae:

  • Moorhouse, D. E., and Colbo, M. H. 1973. On the swarming of Austrosimulium pestilens MacKerras and MacKerras (Diptera: Simuliidae). Australian J. Entomol. 12: 127–130
  • Hunter, D. M. 1979 Swarming, mating and resting behaviour of three species of black fly (Diptera: Simuliidae). Australian J. Entomol. 18: 1–6
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