Are you sure you want to use mineral oil? I wasn't aware that corn had problems with ear wax! I use Sevin shortly after bloom with excellent results.

They drown from the oil

Thanks. Never heard of that before.

If the oil coats the worm so it can't breathe, I would suspect it works on existing worms and wouldn't necessarily repel new arrivals, although they might detect the presence of the oil and stay away anyway.

I guess I would be concerned that once you got a high enough concentration of oil to suffocate the worms, you would also be suffocating and damaging the plant.

To tell when your corn is pollinating, watch the tassles. When they finish dropping their pollen, you are done with pollination and coming into corn ear worm season.