Honestly Bt isn’t very effective once you start seeing worms. Preflower they’re silk together the growing tips and hide out where the spray won’t penetrate. Then as the plant starts flowering they’ll move towards the center of the (tallest thickest) buds and once again hide out safe from the spray as they chomp away, shitting up a storm until one day you’ll see a little yellow sugar leaf on a bud and when you give it a little tug, the entire bud will disintegrate. (It’s amazing how perfect the bud will look, only be be all rotten up against the stem.) So you really need to start spraying in May/June, (about every 3 weeks) to knock down the population before they have a chance to get established.
I have a ton of Skipper butterflies that will start appearing in June and come in waves. I’ll see dozens flittering around and mating, they’ll disappear for awhile, and then in a few weeks there’ll be another wave, which repeats all summer. I actually don’t think they’re the source of the bud worms, but the Bt will seriously knock down their population. So by this time of the year I’ll only see a couple, where as if I’m not spraying each cycle will be thick. (And I know it’s not just environmental as I’ll see other grows within a mile of mine, and the bud worms will be all over the plants.)
Since you know that worms are a problem in your area, plan ahead and start spraying early in the season.
