Good question! I was about to post the same!

I've applied it when silks first appear, and again a week later. There were virtually no earworms, none at all some years. Try to spray directly into hole where the silk emerges.

Oh, dear, it looks I missed the window of opportunity then. Some of the ears, the silks are already starting to turn brown. Just have to keep my fingers crossed now. But thanks for the info - I will be better prepared next year!
Edie

Thanks Zeedman! Mine are a little past that stage but only by about 5 days.

I think BT pretty ineffective. I've sprayed every other evening with little success. In the evening because it photo degrades. And I started as soon as silks emerged. That amounts to about 10 applications. Ears were still heavily damaged. We have very severe pressure here.

I've had very good luck with just before pollen shed, immediately after and then once about 1 week to 10 days later. Don't use old Bt, for sure. I use a hand pump sprayer and shoot the liquid down into the tip with a low pressure stream. Bt does photodegrade quickly so putting it on the silks outside the ear won't help as much unless your ears are well protected by a dense canopy. Just for grins I tried a bicolor SHII triple stacked GMO, RR, Bt sweetcorn this year called Obsession II, out of 120 ears I had 2 with 1 tiny live worm each and never sprayed.

Ya, that BT works great when there aren't any ear worms...;-)