From my childhood -and this would be around 1970 - I remember a huge and sudden infestation of ladybugs. For the probably two days it lasted, lawns were tinted red, the air - also out over the water - was visibly red-misted, you couldn't walk anywhere without stepping on the critters, you couldn't quite escape the buzzing and the sound of what sounded like munching, and everywhere was an iron-like smell, some what reminiscent of peas (I can't quite describe it, but to this day the right kind of odour will remind me of ladybugs).
And let me tell you: They bite! Walk outside uncovered, they would alight on your skin and start nibbling, not terribly painful, but enough so be unpleasant.
This was Northern Europe. I have seen swarms since then, but nothing remotely compared to that massive outbreak.
I witnessed similar in the mid 90s in NA. My suspicion was the agricultural industry (or it's associated regulatory bodies) unleashed the swarm. Unfortunately for my area, they were the non-native variety and since then, native lady bugs are completely wiped out. Haven't seen one in 2 decades.
There were a lot in the UK in 1976. We had a huge growth in aphids after a warm spring, the ladybirds followed on. I remember seeing a boat at our local reservoir that was covered in them, even the sails, it had turned from white to read.
Oh yes, the long, hot summer of '76. A glorrious time to be 16 and unexpectedly having the house to myself for several weeks.
But no ladybugs to speak of, over here across the North Sea from you.
Jellyfish, on the other hand. I remember sailing through shallow waters almost solid with them. Also a one-off, never seen anything like it since then.
This was Northern Europe. I have seen swarms since then, but nothing remotely compared to that massive outbreak.