Ephoron virgo (summer snow), Heesch, Netherlands
The day after my first moth night, I found a few lifeless moths on the floor. Which initially shocked me a little, worrying that I killed them with the light somehow. Imagine my far greater horror when finding this one day later, after two nights of running the light trap.
At the far end of our porch, near ground level, I found this cluster of 15 "moths" all stuck in cog webs, some still flapping their wings. They are being taken apart and eaten alive by several spiders. In this case I wish the photo wasn't sharp, but it is. It's a whopping 47MP of gore if you have the guts to zoom in.
I've been pondering for a few days now about what happened here. I didn't see any of these during the actual nightly traps. Why are they in our garden, in such numbers, and why would they specifically go to the absolute worst place to be: a dark corner full of spiders, collectively? I couldn't make sense of it.
I found some answers that may wash away some of the gore. Rather than disgusted, I now feel fortunate to have seen this.
This is Ephoron virgo, a species of mayfly declared extinct in the Netherlands in 1937, due to the pollution of our rivers. Before that low point, it was a well known natural phenomenon where on specific hot summer August nights, these mayflies would collectively rise from the rivers and descend into the cities in such huge numbers, that it literally appeared to be snowing.
A forgotten natural phenomenon for decades, until in 1991, only moments after water quality started to slowly improve again, a few larvae were found in the Rhine river. Still, nobody had any hope of this natural spectacle ever being restored to its former glory. Yet between 1992 and 2007, small snow returned, every year. Followed by 4 years of nothing, and a mere hundreds in 2012. 2015 marks the first reports of thousands again, followed by similar numbers in 2016:
The video shows an observation of thousands, imagine millions! Now let's discuss their life cycle. Only during the first two weeks of August (by exception late July), specifically on very hot and dry nights, there is a chance of "snow". 30 minutes after sunset (which is very late in the Netherlands, currently at 9:30 pm), males first emerge from the rivers, followed by the females a little later. Peak activity is reached 90 minutes after the first males emerge. They emerge to mate, as a final step in their life cycle. After the females deposit their eggs in the river, they have a few more hours to live. It is during this final flight that they show an extreme attraction to light, and can end up very far from the river they came from. Such as...our garden.
The next morning, every single one of them is dead, or in the process of dying. The only memory left are some wings in cogwebs (according to this article: https://www.naturetoday.com/intl/nl/nature-reports/message/?msg=22214).
So I have some answers. They were not attracted specifically to this spider area of the garden, it's much more likely they were dragged there whilst already dead or dying. As I found a few additional dead ones spread out across the garden, I think this theory makes sense. Plus, I have a photo of a spider actually dragging one:
Very likely, I did cause the initial attraction to our garden by the moth light, as I remember turning it on quite early on this second night. The part I don't yet understand is that these appear to be the sub imago, not the imago.
Altogether this is a pretty lucky find. It doesn't happen every year, is highly weather dependent. It depends on the exact time of year (that I didn't plan for at all) and with all those odds combined, they ended up at my light very far from the river, out of so many light sources in between.
So to end positively, these deaths were supposed to happen. In fact, they are a good sign as it means water quality is improving again. Let it snow.
Ephoron virgo is a mayfly in the Ephoron genus that occurs in the Palearctic realm area. It is known to cause the natural phenomenon of "summer snow", where swarms emerge from the water collectively, and after mating, descend as if it is snowing.