[Odonata-l] dragonfly migration off the coast of China

[Dennis Paulson]

Here is an interesting article: Feng, Hong-Qiang, Kong-Ming Wu, Yun- 
Xia Ni, Deng-Fa-Cheng, and Yu-Yuan Guo. 2006. Nocturnal migration of  
dragonflies over the Bohai Sea in northern China. Ecological  
Entomology 31: 511-520.

The magnitude of migration of some dragonflies, in this case Pantala  
flavescens, is documented in this paper. Using a searchlight trap (I  
don't know how this works), researchers caught thousands of P.  
flavescens in two summers of research on a tiny island in the Bohai  
Sea. The island is halfway between two peninsulas that enclose a  
large gulf on the coast of China east of Beijing, very obvious on a  
world map. Most of the dragonflies were caught on relatively few  
nights in June, July, and August (a few as late as early October).  
The only other odonates caught were <100 Anax parthenope.

Interesting conclusions:

1) Migration can occur in prodigious numbers, as we know already. The  
trap captured 42,161 dragonflies, almost all P. flavescens, on one  
night in late July 2003. No other nights came anywhere close to this,  
but thousands were captured on several additional nights.

2) As the trap worked at night, the dragonflies were clearly  
migrating at that time. This is one of the more surprising findings.

3) They tended to migrate (or at least to be caught by the trap) on  
nights that were foggy and/or had slight winds. The prevalence of  
flight during foggy nights make it possible that these flights over  
the ocean were unusual and the result of the loss of visual  
orientation. Nevertheless, the island lies on a likely migration path.

4) Dragonflies spent the day on the island, presumably foraging, and  
left on the second or third night after their arrival.

5) Radar signatures gathered during the research showed that most  
dragonflies flew at 200-300 m and 500 m above the ocean, the flight  
altitude correlated with temperature inversions on those nights.

6) It was estimated that the dragonflies flew 9-10 hours at speeds of  
5-11 meters/second and thus might migrate 150-400 km in a single flight.

7) During late summer southbound flights, the dragonflies were able  
to orient in a southwest direction (toward their goal), no matter  
what the wind direction, thus were able to compensate for wind drift.

8) The dragonflies in question probably migrated north to and bred in  
paddyfields on the mainland to the north of the island, then their  
offspring were detected in southbound migration.

9) To me, one of the more interesting aspects of the paper is that  
the authors didn't mention seeing any evidence of diurnal migration,  
which should have been easy to observe!

Thanks to Bob Behrstock for sending me the article.
-----
Dennis Paulson
1724 NE 98 St.
Seattle, WA 98115
206-528-1382
dennispaulson at comcast.net

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