Honestly Bt isnāt very effective once you start seeing worms. Preflower theyāre silk together the growing tips and hide out where the spray wonāt penetrate. Then as the plant starts flowering theyāll 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 youāll see a little yellow sugar leaf on a bud and when you give it a little tug, the entire bud will disintegrate. (Itās 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. Iāll see dozens flittering around and mating, theyāll disappear for awhile, and then in a few weeks thereāll be another wave, which repeats all summer. I actually donāt think theyāre the source of the bud worms, but the Bt will seriously knock down their population. So by this time of the year Iāll only see a couple, where as if Iām not spraying each cycle will be thick. (And I know itās not just environmental as Iāll 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.