i wouldnt say that b.t. and the rain are enemies,.
i just 3 days ago, found my first hornworms, and like you, we had continual rain,
during a dry spell. i got some of my plants sprayed,,,,,then it rained,, > it stopped and i sprayed the rest of them....< then it went to raining again,
damn: i thought, i just wasted that 2 gallons worth of poision,
.............................................................................................................
yesterday i was in the patch, supporting some plants, that the rain had BEAT DOWN.
and found dead, worms on many of the plants, so my theory is this,
them worms eat constantlly, so if you can get in a good spraying, with a little break from the rain, you'd be amazed at how many worms will get a good TASTE OF YOUR LEAF. and it's special sauce,
spray my brother,,,,,,let the gates to worm heaven open, and accept the souls of them poor critters, that thought you were growing them a buffet!!!!!!!!