Napalm seems like a bit of a stretch.
My experience is that it is because of the high number of swallow bird houses.
If you drive around (I've driven most of the southern end of VN and Cambodia), the multi story, windowless buildings are everywhere and in towns with a lot of them, far fewer bugs. The Vice article quotes 4200 houses... my guess is that it is 10x higher than that.
I'm currently in Hoi An and there is a ton of bugs here and not as many bird houses.
http://vietnamnews.vn/sunday/features/216083/swallow-nest-fa...
https://munchies.vice.com/en_us/article/ev8da4/farming-edibl...