I think the conclusion of five generations is fairly recent within the last 10 years and is based on isotope ratios found in the wings. The average is four generations and it does include the move to the northern part of the range in southern Canada. Each generation moves north as adults until the fall when they head south.
I used to work on a moth species called the black cutworm that is a pest of corn. It migrates in on warm fronts moving north east out of the southwest and Mexico in the spring. Some of the flights are so large, they can be observed with radar. What is interesting about some of these flights of migrating insects is if the air temp drops to 50 degrees F or lower, they will fold up their wings and drop out of the sky like it is raining insects.
We have a Bogong Moth that is a fairly famous migrator (not sure if that's a word) here, I'd never heard of them until one year they got blown off course by a freak weather system to the coast and we had millions of them. Making a cup of coffee at work was a nightmare, you'd usually end up with 2 or 3 doing back stroke in it.
The only time I've seen big swarms of bugs was out west in a locust plague. I rode a motorbike through a swarm of them for about 3 kilometres, I made it all the way through without head butting any then at the end of it I saw two fly up from the side of the road and got 2 direct hits on the visor.