Depending on how heavy your gypsy moth population is, there may be some caterpillars that start chewing your cannabis. However, last June when my outdoor autos were surrounded by a bad gypsy moth season, I noticed barely none on the cannabis. They can be particular about their preferred food source, so maybe you won't have a problem. This year I used the BT spray on some small trees (caterpillars still small at a young stage),and it was very effective! 😊