Estimating Flying Ant Day

If you follow TOS at all, you know I’m always checking out the wrack line. First thing I noticed May 8 was the unusual composition of the wrack; kind of a wrack line mystery. The fresh wrack was composed primarily of dead winged ants.

Winged ants in the wrack. See also bird tracks in the dray sand
Winged ants in the wrack; see also bird tracks in the dry sand

The photograph above was taken in the morning, May 8. The dark grainy material in the wrack are winged ants. You can also see lots of bird tracks in the dry sand. The ants were deposited in the previous night’s high tide, just after midnight, about nine hours before I took this photo. What’s going on? How did all those ants get out to sea?

Winged ants
Winged ants

Winged ants from many colonies synchronize their nuptial flights. In the United Kingdom these flights have given rise to the term Flying Ant Day.

Winged ants and bird tracks in the fresh wrack
Winged ants and bird tracks in the fresh wrack

The photo above captures about of foot of ground on the wrack line. Let’s say it’s representative. Let’s also say the ant zone in the wrack is twice this wide and a mile long. I know it’s at least a mile long because I hiked a mile and it was present the whole way and continued beyond my range of vision. There are about 100 ants in the photo so that’s 200 ants/ft x 5280 ft. My estimate is admittedly conservative but even so, that’s over a million ants. What’s your estimate?

Next I wanted to figure out how the ants got out to sea, and when. I learned that ant nuptial flights take place in the afternoon. That’s when winds are highest on the outer shores. In the table below I show the wind speed and direction recorded at a nearby weather station on the days preceding my discovery.

Slide1

You will notice a pretty strong east (offshore) wind on May 5. I think that’s that day of the nuptial flight. Normal springtime prevailing wind direction on the outer shores is WNW, which would push airborne ants inland, never to wind up in the wrack. If I’m right, about 80 hours elapsed between the nuptial flight and deposition of the carcasses in the wrack line. How far they were blown offshore before falling exhausted into the sea?

I explored one more piece of the puzzle. Did the fishes in the surf zone take advantage of the food source made available by this event? I was able to capture two male redtail surfperch, Amphistichus rhodoterus, but upon inspection the stomachs were nearly empty, containing only a few mole crab, Emerita analoga, parts. Still, I wonder. If food items pass thru the surfperch gut quickly, my timing could have been a bit off. If only I had collected some fish on May 6th…

My explanation of Flying Ant Day on the outer shores is open to alternative explanations, what’s yours? If there are any entomologists reading this post, any guesses about what kind of ants these might be?

If you want to see some other things I found in the wrack that day, go to Wrack Line 2013 and scroll to May 8.