This seems to be a good time of year for immigrant moths, even if few reach us here in Scotland. It did set me wondering, however, just how long a moth would have to stay airborne before it arrives in the UK. Does anyone know of any source of information about this?
Not quite the question you were asking but this page mentions that Painted Ladies can reach 30 mph in favourable conditions.
I think a lot of them get moved along with the wind so it probably depends a lot on the wind speed. Been some reasonably fast ones in recent weeks.
I would guess that when the wind speed gets above a certain level the turbulence is too much for moths and butterflies, and they settle on the ground for safety.
The Vestal seems to be a fair example. There are a considerable number of records of it from the south coasts of Devon and Cornwall. This is, roughly, 120 miles from the north of France. Even at 30mph (wind assisted or otherwise) that means a flight time - with no chance of a rest and re-fuel - of 4 hours: I suspect the actual time is normally well in excess of that figure. It’s an amazing achievement for a small insect, particularly one with a normally rather erratic flight.
Interesting question, to which I don’t know the answer: but we can maybe calculate some.
The “unassisted” flight speed on the average butterfly or moth is said to be around 10kph. However, they are known to seek out tail winds, and - according to various web sites - they can travel 250 - 500km in a 24 hour period. At least some species (e.g. Silver Y) have some sort of magnetic sense, and will only fly when tail-winds can provide a boost.
So I guess working out the distance they need to travel, and taking low and high values (say 10kph and 20kph), we can arrive at an estimate of the shortest flight times. Whether they stop to rest, feed, or await a favourable breeze adds a complication!